Pyton – PROGRAMMING LANGUAGE – GENERAL NOTIONS

Overview

G.I. JOE is the name of the most remarkable carrier pigeon and saved 1000 British soldiers in World War II. The little pigeon flew 20 miles, in as many minutes, back to the American base and the attack planes never took off.

British soldiers had already occupied the town of Colvi Vecchia, the Germans had retreated, and an air raid would have been devastating for the alliance. The Mayor of London later presented the Dove with the Dickin Medal for bravery.

Similar to animal training, computers are programmed. G.I. Joe orients himself according to the position of the Sun, the Earth’s magnetic field, or instincts, but the electronic devices are raw computing power that initially knows nothing (hardware).

The microprocessor (Central Processing Unit) is the brain of the computer and operates under the control of machine code instructions, incredible sequences of 0s and 1s that are hard to understand without advanced knowledge.

To use the computer, we use software, i.e. specialised programs that fall into four broad categories: operating systems, applications, compilers and interpreters.

For example, to develop an application we need to know a programming language such as C++, C#, Java, Python.

The programming language contains instructions that follow certain syntax rules and we can program the computer to perform a sequence of operations to obtain a result. Instructions are written in a language that is close to natural language, often English. This produces source code.

Depending on the language used, the source code is transformed into machine code using a compiler or an interpreter.

The compiler scans and analyses all the source code, then turns it all into machine code in the form of an executable program. Although it is faster, program errors are displayed at the end, making debugging slightly more difficult. Examples: C++, C#, Java, etc.

The interpreter transforms the code line by line into machine code, so there is no need for an executable file generated at the end. At the first error the analysis stops, making it easier to debug programs. Examples: Python, Pearl, JavaScript, etc.

As a programmer, you need to have deep analytical thinking, be thoughtful, persistent, creative and enjoy developing useful applications.

Algorithms

Algorithm definition

Vom porni de la un exemplu preluat din viaţa noastră.

For example, when you are doing your maths homework for the second day, you notice that you need another notebook and should go to the bookshop.

Arrange the list so that the order is logical.

Algorithm is a computer-implemented method of solving a problem.

The concept of an algorithm is not new. The term algorithm derives from the name of a Persian mathematician, Abu Ja’fat Mohammed ibn Musa al Khowarizmi, who wrote a book known by the Latin name “Liber algorithmi”.

            Mathematicians of the Middle Ages understood algorithm as a rule by which arithmetical calculations were made. Later, the term algorithm circulated in a restricted sense, exclusively in the field of mathematics. With the development of computers, the word algorithm took on a special meaning, so that today algorithmic thinking has been transformed from a tool specific to mathematics into a fundamental way of approaching problems in various fields.

            An algorithm is a method of solving problems of a particular type.

            To solve a problem means to obtain, for some input data, the result of the problem, output data.

   The algorithm consists of a sequence of operations that describe, step by step, how to obtain output data from the input data.

            Algorithms can be written to solve problems in any field of activity. For example, any cooking recipe can be considered an algorithm which, starting from raw materials, produces a finite sequence of operations to obtain the finished product.

Writing algorithms

Let’s imagine that we send a robot to buy the book from the bookstore. How does the robot buy the book? It blindly executes a string of actions that are called instructions, so the robot must be taught so that it knows the algorithm.

      Writing algorithms directly in a programming language (getting the program immediately) is advantageous in that we can check that the algorithm is correct by running the code. However, because the restrictions of the language must always be taken into account when writing, we can make mistakes.

Regardless of the form of writing, it is essential that an algorithm is thought out correctly, and that is not easy!

 What happens if the shop is closed?

Properties of algorithms

To be able to say that a series of actions forms an algorithm, we need three fundamental conditions: finiteness, generality and clarity.

FINITUDE

The algorithm must terminate after a finite number of steps, however many. If it had an infinite number of steps, our robot could go buy bread until its battery ran out… A stop condition could be when the battery level is below 15% or, more logically, when we already have the notebook, right?

GENERALITY

The algorithm must solve a whole category of problems, not just a particular one. Returning to the bread buying algorithm made above, it can be successfully applied to other products as well:

CLARITY

The algorithm must describe accurately and unambiguously the steps to be taken in solving the problem.

If we want to tell the robot to buy us a colourful t-shirt to our liking, … it has no idea! We are talking about Artificial Intelligence and Machine Learning already. It needs to know what patterns we like, what colours or stores we prefer, and so on, from previous experience, applying complex computing and analysis algorithms.

And the algorithms must be EFFICIENT. If we are in Piatra Neamt and we decide to go to Constanta, our route will certainly not include the city of Oradea, unless we take a relative from there with us.  Can you imagine what fantastic algorithms are developed for Google Maps or Waze to display the optimal route in real time?

The number of steps taken by an algorithm has to be as small as possible to say it is OPTIMAL, and there is complexity theory of algorithms that deals with this.

The “black box” model

A program quantifies a computational algorithm and is designed to take some input data (such as stimuli) which it processes and provides as output data (a response):

This is called the black-box model, which abstracts the primary abstraction of any “living” element around us. The car engine revs faster if the accelerator pedal is pressed harder; if we are cold, our hair muscle contracts and we get “goose bumps”; if we press a certain icon on our phone, the associated program opens, etc.

Input data is ‘read’ using a peripheral input device such as a keyboard, mouse, joystick, touch screen, etc.

After processing, the result is “displayed” on the screen or sent to another output device: screen, printer.

Applications

1. In everyday life we encounter algorithms all the time, for example: the algorithm to make a phone call, the algorithm to add two integers, etc. Give us other examples of algorithms!

2. Read about the scientist Abu Abdullah Muhammad bin Musa al-Khwarizmi on the Internet!

3. Describe the algorithm for the day’s History lesson and for the homework.

PYTHON LANGUAGE

Installing the Python programming environment

Go to the official Python language page, Downloads section, which has the URL below:https://www.python.org/downloads//

Press the appropriate button, save the file and open it:

After the file has been launched, a panel will appear where it is recommended to check the options below to automatically add the required system variables:

Start the installation process by pressing the:

( pressing the Install Now button will install Python for the current user only, and will be somewhat cumbersome).

Leave everything ticked in the first window:

In the second window check the Install for all users option:

Thus, the installation location will be in the location “C:\Program Files\Python38-32”, accessible to all users (in my case, version 3.8, 32-bit operating system).

Press the Install button. If everything was ok, then press Close.

The environment is successfully installed on your system, but by default you have no shortcut to Python on the Desktop, as you might expect. The operating system shown is Microsoft Windows, so press the Start button, then in the search bar type “IDLE”, then select:

IDLE stands for Integrated Development and Learning Environment.

The following window will open:

Above you have the command line, the console (Interactive Python Shell), and the current position is indicated by the three characters “>>>” and the cursor. Type “3 + 5” and press Enter,

Python will immediately display the result.

Quick access to Python

Open the folder where you installed Python (C:\Program Files\Python38-32) then the “Lib” folder and inside, “idlelib”, where you will find the location of the “idle.bat” file associated with IDLE. Create a Desktop shortcut to this program:

Vocabulary of language

 

The vocabulary of any programming language consists of:

·         set of characters;

·         identifiers;

·         separators;

·         comments.

Character set

The character set is the set of characters with which a Python program can be built, and is made up of:

upper and lower case letters of the English alphabet (A – Z, a – z);

digits of the base 10 numbering system (0 – 9);

special characters (+, -, *, /, =, ^, <, >, (, ), [, ], {, }, ., ,, :, ;, #, $, @, _ and blank (space)).

Unlike other programming languages, Python gives us direct character representation using the [Unicode] standard, specifically in [UTF-8] format. So, we can also enter characters that are not on the keyboard, but we can retrieve them from the Internet. That’s why we can insert special characters and even [emoji] or [smileys].

Example 1.

In Python we can also write like this, unlike other programming languages:

Greek = “αβγδεζηθικλμνξοπρςστυφχψ”

print(greek)

print(“î u h a t”)

print(“Hey! “)

Identifiers

By identifiers we mean a sequence of letters, digits or the special character “_”, provided that the first one is not a digit. Identifiers are used to associate names to variables, functions, etc.

Example 2. Look at the following variable declarations:

var1 = “mother”

var2 = “dad”

un_sir = “a family”

As counterexamples, we have “1var”, “sir&”. The first starts with a digit, and the second contains a special character.

A special category of identifiers is given by Python keywords (they have a well-defined meaning, are reserved and cannot be used in any other context). The full list is below:

False await else import pass

None break except in raise

True class finally is return

and continue for lambda try

as def from nonlocal while

assert del global not with

async elif if or yield

These must be written exactly as above to be interpreted correctly.

Python distinguishes between uppercase and lowercase letters. priNt(“Hello!”) gets a syntax error because the interpreter doesn’t know the priNt command, only the print command.

Separators

Definition. The simplest elements made up of characters with linguistic meaning are called lexical units.

These are separated from each other, as appropriate, by one or more spaces, the line-ending character or the “;” character, as already described.

Example 3. ab may mean the name of a variable, so we have one lexical unit, whereas a b contains two units…

Comments

Python encourages the insertion of comments into our code, as it is much easier to understand later. As you have seen, they can be inserted anywhere in the program, starting with the ten character (“#”) and continuing to the end of the line.

Example 4. Below is a comment written in Python:

#this is a comment

The Python language is very sensitive to syntax and is strongly oriented towards writing in a way that makes it easy to understand the code of another programmer to whom the program is written.

Writing codes

Definition 1. The syntax of the language is given by the totality of the rules of correct writing (in the sense of its acceptance by the translator program (interpreter, in the case of Python), which has the role of executing it.

Definition 2. The semantics of a language means the meaning of the correct syntactic constructions (what the instructions do, etc).

The Python language is very sensitive to syntax and is strongly oriented towards writing in a way that makes it easy to understand the code of another programmer to whom the program is written, so it is preferable to write a single instruction on each line. For example, on the same line we can’t write two instructions without separating them with “;”, and the Python interpreter lets us know immediately!

Furthermore, Python distinguishes between uppercase and lowercase letters.

Indentation is very important and we will discuss it further. For now, don’t leave blanks at the beginning of the line.

Semantically, the most difficult thing is that the program executes exactly what the maker intended, and checking for correctness is no easy task.

The Python language is interpreted, i.e. the code is executed line by line, unlike Pascal or C/C++, where you need a compiler to generate an executable file.

The console allows us to execute a Python command and get the effect immediately. The current position is indicated by the three characters “>>>” and the cursor (“|”).

Examples

EXERCISE 1         

Enter the command print(“Hello, Python!”) and press the Enter key. Note the quotation marks!

It is necessary to display the result using the print() function.

EXERCISE 2  

Enter the command 2+3*5-1 and press Enter.

The result will be displayed immediately after pressing the Enter key.

EXERCISE 3  

Create a new program to display your name.

EXERCISE 4  

The program below contains a number of errors. Your job is to correct it!

Comments

Python encourages the insertion of comments in code, as it is much easier to understand later.

They can be inserted anywhere in the program, start with the ten character (“#”) and continue to the end of the line, as you saw above.

Mathematical expressions

Programs use expressions to perform calculations.

Definition 1. An expression is a sequence of one or more operands linked by operators according to syntactic rules specific to the programming language.

For example, for the expression 3 + 4*2 , 3, 4 and 2 are operands and + and * are operators.

An incorrectly written expression will lead to an error in interpretation (this is called a syntax error):

During the execution of the Python program, expressions are evaluated (i.e. a result is calculated).

Arithmetic operators

Unary operators

Unary operators (+ and -) act on a single operand which is always to the right of the operator. The “+” returns the value of the operand and the “-” returns the value of the operand with the changed sign.

Examples: -23, +11

Binary operators

. Binary operators (+, -, *, /, // and %) act on two operands. One operand is always to the left of the operator and the other to the right of the operator.

Examples: 10+5, 5-2, 7*5, etc.

The division operator ( / )

The operator / has the meaning of division. Operands can be integer or real values, but the result will always be real.

Examples:

3/2 results in 1.5

4/2 results in 2.0

Integer division operator ( // )

The // operator (known in other languages as div) has the meaning of integer division (in English, floor division, because the result of division rounds down to an integer) – this is how you get the quotient.

Examples:

7//2, resulting in 3

14//2, resulting in 7

25//12, results in 2

The modulo operator ( % )

The % operator (known as modulo) has the meaning of the remainder of integer division.

Examples:

7%2, results in 1

14%2, results in 0

25%12, results in 1

Raising to power ( ** )

With the operator “**” you can raise a number to the power of an exponent.

Example:

2**3, yields 8

Brackets

Unlike mathematics, in Python we can only use round brackets. So instead of writing [2+3*(2+1)]*2, we write (2+3*(2+1))*2.

Exercises

Now you can practise the operators you learned earlier. First, run the following Python program:

Evaluation of expressions

It is essential to understand how to evaluate an expression, and for this we need some basics.

Priority (precedence) of operators. You are already familiar with this notion, knowing that it indicates the order in which operations are performed.

Operator associativity. The notion may be new to you, and is of two kinds: left-to-right and right-to-left. From the outset we state that operators with the same priority have the same associativity.

To understand the notion of associativity, we start from an expression in which operators are linked by operators with the same priority. If the associativity of the operators is from left to right, the first operation that is performed is the one corresponding to the first operator on the left, the second operation is the one corresponding to the second operator on the left, etc. Obviously, if the associativity is from right to left, the first operation that is performed is the one corresponding to the operator on the right, and so on.

For example, if we have the operation 7 * 2 // 4, we have left-to-right associativity, so 7*2 is performed first, then 14//4, the result is obviously 3. So here the operators have the same priority.

On the other hand, with the “**” operator, you can raise a number to the power of an exponent. In this case, associativity is from right to left. For 2 ** 3 ** 2, first 32 is performed, the result is 9. Then 29, which gets the value 512.

Notice that 2 ** 3 ** 2 is equivalent to 2 ** (3 ** 2).

If we use parentheses, the result differs, of course, because they have a higher priority than the “**” operator:

(2 ** 3) ** 2

The bracket is performed first, i.e. 23 = 8, then 82, the final result being 64.

The key is to understand how an expression evaluates, and for that we need some fundamentals.

Priority (precedence) of operators. You are already familiar with this notion, knowing that it indicates the order in which operations are performed.

Operator associativity. The notion may be new to you, and it is of two kinds: left-to-right and right-to-left. From the beginning we state that operators with the same priority have the same associativity.

To understand the notion of associativity, we start from an expression in which the operands are linked by operators with the same priority. If the associativity of the operators is from left to right, the first operation performed is the one corresponding to the first operator on the left, the second operation is the one corresponding to the second operator on the left, etc. Obviously, if the associativity is from right to left, the first operation performed is that of the operator on the right, and so on.

For example, if we have the operation 7 * 2 // 4, we have left-to-right associativity, so 7*2 is performed first, then 14//4, the result is obviously 3. So here the operators have the same precedence.

On the other hand, with the help of the “**” operator, you can raise a number to the power of an exponent. In this case, the associativity is from right to left. For 2 ** 3 ** 2, 32 is performed first, resulting in 9. Then 29, which results in 512.

Note that 2 ** 3 ** 2 is equivalent to 2 ** (3 ** 2).
If we use the parentheses, the result differs, of course, because they take precedence over the “**” operator:

(2 ** 3) ** 2

The parenthesis is performed first, i.e. 23 = 8, then 82, the final result being 64.

Variables and constants

Data can be constant (values do not change) and variable (values change)

Run the code below that reads an input date from the keyboard using the input() function, then displays it using the familiar print() function:

Exercises

Run each program below to learn more:

1.

2.

3.

Several ways of awarding

Test the following program which contains a number of assignments as well as some interesting drafting tricks:

Remark. When the print function receives multiple arguments enclosed in parentheses, separated by commas, they are displayed on the same line in order and separated by a space.

So far we have used assignment as an operator, giving a certain value to a variable. So the general form is:

v = expression

where v is the variable.

The execution principle is as follows:

   – evaluate the expression;

   – v is assigned the value obtained.

Multiple assignments of the form can also be performed:

v = v1 = v2 = … = vn = expression

where v, v1, v2, …, vn are variables.

Applications

1.Using the knowledge you have acquired in Physics, create a program to calculate the average speed of a mobile.

2.Using the knowledge acquired in Physics, create a program to calculate the density of a body.

3. Make a program to calculate your age.

4. Create a program to display the number of years since the birth of the poet Mihai Eminescu.

Quiz

Given the variables below, answer the following questions:

x = “ab”

y = “cd”

z = “ef”

n = 3

m = 2

1. What will the function print(x+y+z) display?

    a) ab cd ef

    b) abcdef

    c) a b c d e f

2. What will the function print(x,z,y) display ?

    a) ab cd ef

    b) abefcd

    c) ab ef cd

3. What will the function print(m**(1+n)) display ?

    a) 9

    b) 16

    c) 32

4. print(1 + m*n,m,n) ?

    a) 7 2 3

    b) 723

    c) 9 2 3

    d) 9 3 2

5. print(m*n-1) ?

    a) 7

    b) 4

    c) 5

Types of data

One of the advantages of using Python is that any variable can dynamically hold values of different types throughout a program.

Perform the following example:

The result is as follows:

What do we observe? Initially, the variable n held the numeric value 123, then a string “Python”, and finally, again a numeric value, this time real, 23.25. After each assignment we displayed what variable n holds.

Also, a first problem we identified was when we wanted to add two numbers entered from the keyboard. Implicitly, the input function takes the entered information and provides it as a string for manipulation in our code – 2 + 3 does not make 23!

In Python there are six standard main data types:

– Numeric

– Strings

– Lists

– Tuples

– Sets

– Dictionaries

Numeric data types

Numeric data types are int (signed integers), float (real numbers, with decimal places) and complex (complex numbers).

Examples:

int()	float()	complex()
11	0.0	2+3j
124	23.14	1-2j
-93	-173.955	3.14j

When defining a variable, we can use the function named a.î. to set the type of data to be retained.

Examples:

Example 1

REMARKS – Example 1

We used the int() function which took as argument the text string entered by the user. This converted it to an integer signed value that was held by the variable x, then similar for y. This time the result is mathematically correct and that’s what we wanted – adding the numbers together.

Warning. Since we have set the data type to int() for the two variables, x and y, if we try to enter the text Star (or ‘Star’ or ‘Star’) as a value for the first variable, we will now get an error:

ValueError: invalid literal for int() with base 10: ‘Star’ on line 1

x can only hold signed integers.

Example 2

REMARKS – Example 2

A real number (float) added to an integer (int) gives a real result (float).

For 5 and 6, we get 11.0.

For 5.23 and 2, we get 7.23.

For 2.128 and 4, we get 6.128.

Note that the default result rounds to the number of decimal places of the most detailed real number. If we enter an integer, only one decimal place is used, 0, which tells us the data type – float.

Example 3

REMARKS – Example 3

Suppose we want to divide it by 10 to 3. The result is a known 3.(3) and reads 3 period 3.

The default result displayed by Python is: 3.33333333333333333

If we use the function format (value,format) as above, we can display for example the result to 20 decimal places (f comes from the fractional part) and notice that the variable rez actually holds:

Computers retain real values using many decimal places, and the value displayed may be different from the true value of a variable.

What is displayed is not always retained!

For example, we can use the function round(number[,decimal]) to round a number. The default value of the decimal parameter is 0, so the function returns the nearest integer without imposing it.

Examples:

round(12.234567,3) gets 12.234

round(12.234567,1) gets 12.2

round(12.234567) gets 12

round(6.7543,1) gets 6.8

round(6.7543) gets 7

Strings

We have already used strings, which in English are called strings. To declare a variable that holds a string, we write the text directly in quotes or apostrophes. Explicit conversion to this type is done by the str() function.

Example

The result is:

We can insert strings that are on more than one line. In this case we use three apostrophes or three quotation marks:

Examples

 Test the code below in the editor:

str4 = ”’A text written

on two lines.”

str5 = ””Other written text

on two lines.””

print(str4)

print(str5)

Function type()

We can always find out the type of data held by a given variable at a given time using the type(variable) function.

The result is:

The type function in this case takes as argument the value of the variable x and returns as result the type of data retained.

Applications

1. Make a program that allows you to enter and display a question and answer about Animal World Diversity.

2. Make a program that allows you to enter and display the names of continents and the names of some countries on the continent.

Quiz

Answer all the questions below correctly:

1. What will the function print(x + ‘6’) display if x = 2 ?

    a) 26

    b) 8

    c) error

2. What will the function print(type(x)) display if x = ’81’ ?

    a) <class ‘int’>

    b) <class ‘str’>

    c) 81

    d) ’81’

3. What type of data will x hold if x = 10 + 23/5 – 1 ?

    a) int

    b) float

    c) str

4. If x = 11 and y = 14.25, what type will hold the variable z = x + y ?

    a) float

    b) int

5. Which data type holds the variable declared as: n = 100 % 9 ?

    a) int

    b) float

Structured programming

The theory of programming languages is vast, and one of the important concepts is structured programming.

We will learn how to program on fundamental structures.

Of particular note is the study published in 1966 by Corrado Böhm and Giuseppe Jacopini (“The Structured Programming Theorem”), followed by that of the Dutch scientist Edsger W. Dijkstra who rooted the notion of structured programming in 1968. A scientific heyday when you consider that Neil Armstrong walked on the moon the following year..

The principle is as follows:

Any algorithm that has inputs and outputs, i.e. a start and an end point, can be represented by a combination of three fundamental control structures called:

– sequence (linear structure)

– decision (alternative structure)

– cycle (repetitive structure)

Structured programming allows programs to be written in a natural language (called pseudocode), independent of practical language.

Programs are based on computational algorithms, which can be translated into logical schemes, such as the one below:

Linear structure

Linear structure, is a sequence of statements that is performed every time we run Python code, independent of the input values.

EXAMPLE

Suppose we want to make a program that reads three whole names from the keyboard and displays their arithmetic mean.

The calculation formula is simple. For a, b and c, the arithmetic mean is:

How do we think about the algorithm?

We first read the three values and hold them as integers in the three variables, a, b and c. We define a ma variable that will hold the arithmetic mean using the expression (a+b+c)/3.

Look at the logic diagram below. Suppose that the values 9, 10 and 8 have been entered: We write in the Python language

Applications

1.Modify the above algorithm a.î. to display the geometric mean of the three numbers, using the calculation formula:

2.Modify the above algorithm a.î. to display the geometric mean of four numbers.

 Elementary liniar algorithms

Interchanging the values of two variables

One of the classic problems that can be encountered when creating programs is the interchanging of the values of two variables. It may seem trivial, but there are many tricks!

EXAMPLE

Suppose we have two glasses, labelled A and B, each containing 70 and 40 ml of liquid respectively:

Etape

Steps

How do we interchange their content? We can use a third handling cup, called C, which is initially empty:

Stage 1. Pour the contents of A into C:

Stage 2. We then pour the contents of B into A:


Stage 3. Finally, we pour the contents of C into B:



Glass C is empty again.

The algorithm translated into Python is therefore as follows:

Warning. Unlike glasses, where we use a mechanical process, at the end of the program variable C will hold the last value, i.e. the one now held by B.

HOW CAN WE BE WRONG?

Simple. Consider the sequence below:

A = B #A retains 40

B = A #B will hold 40 again

The first assignment loses the content of A permanently…

ANOTHER METHOD

Whoever tells you that computer science doesn’t require mathematics is dead wrong. You can perform the interchange without another handle variable! Try the code below for example:

A = A + B #A retains 110

B = A – B #B will retain 70

A = A – B #A will hold 40

Inversion of digits of a number

A classic problem asks us to invert the digits of a natural number consisting of a fixed three digits, i.e. to find its flip.

Flipping is also called inverting or mirroring.

So we start with the number 725, held by the variable n:

n = 725

Steps

1.LAST NUMBER

Again we turn to mathematics, where we already know the subtraction theorem. If we divide 725 by 10, the remainder will be 5, exactly what we want:

So we can write as a first step:

c3 = n % 10

where c3 is a handle variable that will hold the last digit. The operator “%” (modulo) gives us the remainder of the division by 10.

2. PENULTIMATE DIGIT

First, we need 72, which is the quotient of 725 divided by 10

which we will use to find the remainder of the division by 10, i.e. 2:

So we define another intermediate variable, c2, which will hold the expression:

c2 = (n // 10) % 10

We read: the quotient of n divided by 10, all modulo 10.

The operator “//” represents the integer division (the quotient), already studied.

3.FIRST FIGURE

Using the operator “//” again, we immediately find the first digit:


So we will write it in the programme:

c1 = n // 100

where the variable c1 will hold the first digit, i.e. 7.

We read: the quotient of n divided by 100

4. CREATING THE INVERSE

We use multiplication and addition:

because c1 holds hundreds, c2 holds tens, and c3 holds units:

inverse = c3*100 + c2*10 + c1

At the end we display the value of the inverse variable.

Writing in Python

Applications

Make a programme showing the reverse of the year in which one of the Renaissance figures was born.

Quiz

1.         What will variable x retain after the following assignments?

x = 3

y = 1

x = x + x

y = x + y

x = y

    a) 9

    b) 8

    c) 7

    d) 4

2.         What is displayed after executing the sequence below?

a = 3

b = 4

c = 5

a = a + b – c

b = a – b + c

c = c – a + b

print(a,b,c)

    a) 3 4 5

    b) 5 4 3

    c) 2 3 6

    d) 6 5 9

3.         Evaluate the expression below:

(16/4+16%4)*5/2

    a) 10

    b) 10.0

    c) 20

    d) 12.5

4.         Evaluate the expression below:

5-2*3**2

    a) 81

    b) -31

    c) -13

    d) -31.0

5.         Daniela has x lei in her piggy bank. She has saved some money and asked her father to change it at the bank because she wants as many 100 lei notes as possible! Which statement gives us the quick result in Python? How many 100 lei notes can Daniela have?

    a) x/100

    b) x//100

    c) x**100

    d) x%100

6.         Eric and Leo are very good friends and are passionate about literature. They often chat on WhatsApp or by phone about the world of reading… Leo reads x pages a day, and Eric reads y, 3 more than his friend. After z days, how many pages have they read together?

    a) x + y*z

    b) (2*x + 3)*z

    c) (x + y)*z + 3

    d) x*z + 3*y

Alternative structure

So far we have used the linear sequence, i.e. a series of instructions that are carried out sequentially, in the order in which they are written.

EXAMPLE

We read from the keyboard an integer number held by the variable n. Let’s display whether or not it is greater than zero.

In practice, things are more serious. Depending on the data read from the keyboard, we can execute or not execute a block of instructions!

HOW DOES IT WORK?

The computer can be programmed to make decisions, but only as we teach it!

The execution is as follows:

Step 1. Evaluate the logical_expression.

Step 2. If the logical expression is True, then execute the instruction set instruction_1. Otherwise, if the value is False, execute instruction set_2.

Above, we tested whether the value held by the variable n is greater than zero or not. Depending on the result, we displayed the corresponding information!

When we used the if statement, after typing the mandatory colon character and pressing the Enter key to enter the associated (subordinate) case statement, the new line is indented with fixed 4 characters:

which automatically indicates that inside if we can write a block consisting of several instructions, … not just one.

Relational operators

All relational operators are binary (act on two operators). These are: > (greater), >= (greater or equal), < (less), <= (less or equal), == (equal) and != (different).

Operands can be variables or values of any learned type. Here we will study how relational operators act only on integer or real variables.

The result of applying a relational operator is always a logical value – True or False.

EXAMPLE

Analyse the result of the expressions below by running the program:

Boolean values

As you can see, of particular importance are the truth values True or False. These are called Boolean values, and the concept was first defined by a 19th century mathematician called George Boole.

These values are essential in the decisions that a program must make based on values entered by the user or determined in our code. The result of a logical evaluation is held in a data type called a bool.

Of course, Python is adaptive, so explicit bool(expression) conversion is not necessary.

Logical operators

Logical operators are the following: not, and, or.

With the exception of the not operator – which is unary (acts on a single operand), the rest of the logical operators are binary (acts on two operands). Operands can be of type logical, integer, real, etc. The result will always be a value of type logical – True or False.

EXAMPLE

Analyse the result of the logical expressions below by running the program:

REMARKS

The logical negation operator (not) acts as follows: if the operand is a value other than 0 times True, the result is False; in any other case, the result is True.

The logical and operator (and). How the result is obtained can be seen from the table below:

If both operands are different from False, the result is True; otherwise it is False.

The logical operator or (or). How the result is obtained can be seen from the table below:
And here the rule is simple: if one of the operands is True, the result is True, otherwise the result is False.

Exercises

1.Read two different integers from the keyboard. Show which one is larger.

SOLUTION

After reading the two numbers (variables n and m), the if statement is executed.

First, the logical expression is evaluated. In our example it is n>m. If the number held by variable n is greater than the number held by variable m, the logical expression takes the value True and prints n is greater than m. Conversely, the logical expression takes the value False and prints m is greater than n.

Since no further instruction follows after if, execution of the program is terminated.

2.Write a program that reads a natural number (command). If it is 0, read two integers a and b and print their sum; otherwise read two real numbers x and y and print their product.

SOLUTION

Analyse and run the program below:

3.Read an integer value. If it is even (divides exactly by 2) the message “I read an even number” will be printed. Otherwise, the program will not give any message.

RESOLUTION

The program is as follows:

4.
Read 4 real values a, b, c, d. Evaluate the expression:

RESOLUTION

Numerical example: let a=1, b=2, c=3, d=4.

We have c+d=7>0, it follows that it will print a+b=1+2=3.0.

The program is as follows:

5.

Read 3 whole numbers. How many does it look like?

SOLUTION

The number of even values will be held in the handle variable p. Test each number in turn, and if it is even, add 1 to p:

6.Read an integer x. Calculate the expression::

RESOLUTION

Let’s look at the program below:

Applications

1. Write a program that checks whether a colleague/user knows the multiplication table. The student is asked what 7 x 8 is, and after their answer the computer will respond with an appropriate message.

2. Read x, a real number. Evaluate the expression:

3. Read 4 whole numbers. Decide if they are distinct (i.e. no two are equal).

Hint. Compare the first number with all the others, the second with those that follow, the third with the fourth. If none of the comparisons show equality, print the appropriate message.

4. Read 3 whole numbers. Print, if any, the number which equals the sum of the other two. Pentru x și y citite, algoritmul afișează ‘ok’:

Quiz

1.Which of the following statements is true?

    a) The numbers are positive.

    b) The numbers have the opposite sign.

    c) The numbers read are not positive.

    d) At least one is positive.

2. What is the logical condition that completes the decision operation below a.î. to check whether the value of the integer variable x is a two-digit number?

If……:

print (”Has 2 digits!”)

Else:

print(”It does not consist of 2 digits!”)

  a) (x<100) and (x>10)

    b) (x<=100) or (10<=x)

    c) (x>9) and (x<=99)

    d) No correct answer

3. What value will be displayed if 7 is read?

   a)   2

   b)   25

   c)   12

   d)   9

4. What will be displayed if x=4?

   a)   5

   b)   6

   c)   7

   d)   8

5. For the above program, what is the range to which x, a.î. must belong? to display x+3?

   a)   (-∞,3)

   b)   (-∞,3]

   c)   (3,5]

   d)   [3,5)

Operations with strings

We’ve used strings before. You assign values simply by writing the text between apostrophes, quotation marks or a combination of three by three to write multi-line text.

The data type is str.

Python has the ability to adapt, so assignments without explicitly imposing the type automatically result in str objects.

We also know that the input() function reads a user-entered string from the keyboard and returns it as type str. The read data must eventually be explicitly converted to a data type for processing, otherwise we may have errors in interpretation:

Concatenation of strings

The “+” addition operator is used to join (concatenate) two or more strings.

For two numeric values, the “*” operator represents multiplication. In the case of str operators, it is used to multiply the value (text).

Examples

The “+” operator joins strings after the operation. We can play with spaces (blank character), with multiple strings, test!

Pay attention to the data type:



Both must be of type str, otherwise you get an error. Variable n holds an int object and m holds a str object. Use the conversion str(n)+m!

Access to characters

Of course it is important to be able to access one or more characters read or held by a str variable, and the Python language gives us an easy mechanism to use.

Take a look at the example below:


Consider a variable s1 that holds the string “character”.

The index of the first character is 0 and the last is 7, i.e. 8-1, where 8 is the number of characters contained in the object. Using a given index and the operators “[” and “]” together, we have access to characters:

EXAMPLE

We cannot update a certain value for a character because we will get an error from the interpreter:

însă putem actualiza întreaga valoare reținută de variabila s1 prin atribuire, precum știm.

Indice negativ. Știind lungimea șirului de caractere, putem accesa caracterele acestuia de la dreapta la stânga folosind index-urile negative. Astfel, ultimul caracter are indexul -1, penultimul, -2, ș.a.m.d.

Obviously, we get an error if we exceed the index – in this case -8, -9 does not exist!

String length

To get the length of a string, we use the len(variable_name) function.

Note again that indices start at 0 and stop at len()-1.

EXAMPLE

To get the length of a string, we use the len(variable_name) function.

Note again that indices start at 0 and stop at le

Belonging – in and not in

We can check whether a character or a substring is present in a string. Analyse the binary operators in and the not in group in the following program:

EXAMPLE

The string “Star” is found in sw, but “StarWars” is not, note the lack of space between the two words. Then it is not true that “Wars” is not a substring of the content of the str object.

As you can see, the result of the expressions is of type bool, i.e. a True or False value.

Applications

1.Make a program that reads a string of characters from the keyboard and displays whenever the letter “a” appears in the text.

2. Make a similar program that displays how many times each vowel of the English alphabet occurs in the text read and then at the end, the total of their occurrences. Note the details – the result is stylised a little below:

3.Check whether or not a read string is longer than 20.

Example. For “Eric is a smart boy in 6th grade.” True will be displayed.

4. Make a program to display information about the Enlightenment. Display the length of the text entered.

5.Create a program to calculate the volume of a body.

6. Make a program that allows you to enter the answers to 3 questions about the US Declaration of Independence.

7.Make a program to enter answers to 3 questions about transportation in the Middle Ages. After entering each answer, display one of the messages: Correct, Wrong.

8.Make a program that allows you to enter answers to 4 questions about general features of the landscape. Each answer will be marked with 4 points.  Display the final score.

9. Make a program to display a message: “Isosceles triangle,” Equilateral triangle”, “Any triangle” depending on the values entered from the keyboard for the sides.

Quiz

1. For string s1, what does print(s1[2]) display?

s1 = ‘Cleopatra’

    a) l

    b) e

    c) o

2. For the string s2, what does print(s2*3) display?

s2 = ’12’

    a) 36

    b) 12 12 12

    c) 121212

    d) s6 🙂

3. For the strings s3 and s4, what does print(s3+s4+1) display?

s3 = ’46’

s4 = ’53’

    a) 100

    b) 46531

    c) 46 53 1

    d) Error

4. What will be displayed by print(len(s5)-3), where s5 is the string below?

s5 = ‘I’m going to Greece for the summer!’

    a) 19

    b) 20

    c) 21

    d) 22

5. Let the string s6 below be. What does print(‘ ar’ not in s6) display?

s6 = ‘Mary is a doctor.’

    a) True

    b) False