Loops in Python

loops are a powerful tools used in various problems, that's why we focus on this course in how to use them in Python.

In the other programming languages like C/C++ and Java we found three different kinds of loops, In Python, there are only two ways to execute loops. Although all methods offer similar basic functionality, their syntax and time for checking conditions differ.

In python, the while loop is used to execute a block of statements repeatedly until a given condition is met. And when the condition becomes false, the line immediately after the loop in the program is executed.

Syntax

    while condition/expression:
        instructions

All instructions indented by the same number of character spaces and are considered to be part of a single block of code. Python uses indentation as a method of grouping statements.

Example

    nb = 1
    while nb <= 5:
        print(nb)
        nb += 1

Else

while loop executes the block until a condition is met. When the condition becomes false, the instruction immediately after the loop is executed.

The else clause is only executed when your while condition becomes false. If you exit the loop or if an exception is thrown, it will not be executed.

Syntax

    while condition/expression:
        # Execute theses instructions
    else:
        # Execute theses instructions

Example 1

    nb = 1
    while nb <= 5:
        print(nb)
        nb += 1
    else:
        print("bye bye")

Example 2

    nb = 1
    while nb <= 5:
        print(nb)
        nb += 1
        break
    else:
        print("bye bye")

The for loop is used for sequential traverses. For example: browse a list, a chain, an array, etc.

In Python, there is no for style (i = 0; i <n; i ++). There is a "for in" loop similar to foreach in other programming languages.

Syntax

    for iterator in sequence:
        instructions

Example 1

L = ['Lundi', 'Mardi', 'Mercredi', 'Jeudi', 'Vendredi', 'Samed', 'Dimanche']
for jour in L:
    print(jour)

Example 2

L = ('Lundi', 'Mardi', 'Mercredi', 'Jeudi', 'Vendredi', 'Samed', 'Dimanche')
for jour in L:
    print(jour)

Example 3

D = {}
D['nom'] = 'ESSADDOUKI'
D['prenom'] = 'Mostafa'
D['Annee'] = 2019
for cle in D:
    print(cle, ' : ', D[cle])

Exemple 4

# range(fin)
for i in range(5):
    print(i, end="  ")
print()

#range(debut, fin)
for i in range(2, 5):
    print(i, end="  ")
print()

#range(debut, fin, pas)
for i in range(1, 11, 2):
    print(i, end="  ")
print()

Example 5

L = ('Lundi', 'Mardi', 'Mercredi', 'Jeudi', 'Vendredi', 'Samed', 'Dimanche')
for i in range(len(L)):
    print(L[i])

Example 6

msg = 'Bonjour'
for lettre in msg:
    print(lettre)
        
print('-------------')
for i in range(len(msg)):
    print(i, ': ', msg[i])

Else

The else block will be executed immediately after the block is executed. If you exit the loop or if an exception is thrown, it will not be executed.

for i in range(5):
    print(i)
else:
    print('bye bye')
    
print('------------------')
for i in range(5):
    if i == 2:
        break
    print(i)
else:
    print('bye bye')

Python allows you to use a loop in another loop (nested loops).

Syntax

for iterator1 in sequence1:
    for iterator2 in sequence2:
        instructions

while condition1:
    while condition2:
        instructions

Example 1

for i in range(2):
    for j in range(3):
        print(i, ' - ', j)

Example 2

matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9], [10, 11, 12]]
n = len(matrix) # nombre de lignes
m = len(matrix[0]) # nombre de colonnes

for i in range(n): # pour chaque ligne
    print("row ", i, " of the matrix : ", end=" ")
    for j in range(m):# for each column of row i
        print(matrix[i][j], end=' ')
    print("")

The loop control instructions modify the normal execution of the sequence.

It returns control to the start of the loop.

L = ['Lundi', 'Mardi', 'Mercredi', 'Jeudi', 'Vendredi', 'Samed', 'Dimanche']

print("here are the days of the week except Thursday and Friday in French")

for i in range(len(L)):
    if i == 3 or i == 4:
        continue
        
    print(L[i], end=" ")

It ends the current loop and resumes execution at the next instruction after the loop

L = ['Lundi', 'Mardi', 'Mercredi', 'Jeudi', 'Vendredi', 'Samed', 'Dimanche']

# leave if day is Thursday
for i in range(len(L)):
    if i == 3:
        break
        
    print(L[i], end=" ")
        
print("\nIt's Done")

We use the pass statement to write empty loops. Pass is also used for empty control statements, functions, and classes.

L = ['Lundi', 'Mardi', 'Mercredi', 'Jeudi', 'Vendredi', 'Samed', 'Dimanche']
for jour in L:
    pass
        
print("the last day of the week in french is ", jour)

class EmptyClass:
    pass