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:
- The while loop
- 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): instructions...
Example. display the first 10 numbers
for i in range (1,11): print(i) #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 """ output: 1 2 3 4 5 6 7 8 9 10 """
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 break # output: The first number greater than 10 is : 11