1. About Loops in Python

Loops are algorithmic tools that allow us to execute a block of code several times, i.e. to execute code "in a loop" as long as a given condition is verified.
When coding, we will often have to execute the same code several times. Using a loop allows us to write code that needs to be executed multiple times only once.
We will thus be able to use loops to browse the values ​​of a list variable or to display a sequence of numbers etc.
For this purpose, the Python language offers us two loops:

  1. The while loop
  2. The for loop

The general operation of loops will always be the same: we set a condition which will generally be linked to the value of a variable and we execute the code of the loop as long as the condition is verified.

2. The For loop in Python ...

The for loop in Python, allows to execute repeated instructions.


for counter in range(start_counter, end_counter):

Example. display the first 10 numbers

for i in range (1,11):
#prints the first 10 numbers 1, 2, ..., 10


Note that in the loop for i in range(1, n) the last one which is n is not included! This means that the loop stops at order n-1.

3. The repetitive structure While

The while structure is used to execute a set of statements as long as a condition is fulfilled and execution stops when the condition is no longer satisfied.


while (condition):
       intructions ...

Example. display of the first 10 integers with the while loop

i = 1
while(i <= 10):
    print (i)
    i = i + 1

4. Stopping a loop with break instruction

Sometimes we need to stop a loop when a certain result is reached. To immediately stop a loop, you can use the break keyword.
Example let's have fun looking for the first number greater than 10 in a list:

Exemple (searching the first number greater than 10)

L = [ 3 , 7 , 11 , 23, 31 , 20 , 27]
for x in L:
    # we test if x > 10
    if x > 10:
        print("The first number greater than 10 is : ", x)
        # on quitte la boucle
# output: The first number greater than 10 is :  11


Younes Derfoufi

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