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LESSON 12
PAGE 3 / 3
The for and while Statements
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General Form of while Loop

Just like the for statement, the while statement is iterative and controls a set of instructions. The general form is as follows:

while true_logical_expression:
    subordinate_instructions


The execution principle is:

Step 1. Evaluate the logical expression. If it evaluates to True, proceed to Step 2, otherwise the execution of the statement ends.
Step 2. Execute the set of instructions, then return to Step 1.

EXAMPLE

We read a natural number, stored in a. Calculate the sum of the first a natural numbers.
Editor - lesson_12.py
       
Console/Output done
DETAILS

The initial value of the variable i is 1. As long as i is less than or equal to the input a, the compound statement (subordinate to the while statement) is executed. This includes the statement where the value of i is added to the value stored in total and the statement where 1 is added to the value stored in i (i.e., it is incremented).

Example for a = 5:


Execute the simulation

Once again, pay attention to the indentation of the subordinate instructions...

NOTE

It is recommended to use the while statement when two conditions are met simultaneously:

• the set of instructions is executed (as many times as necessary) only if a certain condition is met;
• at the moment of entering the repetitive sequence, it is not known how many times it will be executed.

In the case of the problem above, we could have also used for:

a = int(input("a="))
suma = 0
for i in range(1, a + 1):
    suma += i
print("The sum is", suma)


The call range(a + 1) was another option, but is it necessary to add the first value, 0? It would have been an extra step... so we need to be optimal!
The lesson is now over.
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