Lesson 6
While loop

1. While loop

while loop repeats the sequence of actions many times until some condition evaluates to False. The condition is given before the loop body and is checked before each execution of the loop body. Typically, the while loop is used when it is impossible to determine the exact number of loop iterations in advance.

The syntax of the while loop in the simplest case looks like this:

while some condition:
    a block of statements

Python firstly checks the condition. If it is False, then the loop is terminated and control is passed to the next statement after the while loop body. If the condition is True, then the loop body is executed, and then the condition is checked again. This continues while the condition is True. Once the condition becomes False, the loop terminates and control is passed to the next statement after the loop.

For example, the following program fragment prints the squares of all integers from 1 to 10. Here one can replace the "while" loop by the for ... in range(...) loop:

1
4
9
16
25
36
49
64
81
100

i = 1
while i <= 10:
    print(i ** 2)
    i += 1

In this example, the variable i inside the loop iterates from 1 to 10. Such a variable whose value changes with each new loop iteration is called a counter. Note that after executing this fragment the value of the variable i is defined and is equal to 11, because when i == 11 the condition i <= 10 is False for the first time.

Here is another example use of the while loop to determine the number of digits of an integer n:

5678

n = int(input())
length = 0
while n > 0:
    n //= 10  # this is equivalent to n = n // 10
    length += 1
print(length)  # 4

On each iteration we cut the last digit of the number using integer division by 10 (n //= 10). In the variable length we count how many times we did that.

In Python there is another, easier way to solve this problem: length = len(str(i)).

2. Loop control flow: else

One can write an else: statement after a loop body which is executed once after the end of the loop:

None

i = 1
while i <= 10:
    print(i)
    i += 1
else:
    print('Loop ended, i =', i)

At the first glance, this statement doesn't seem to have sense, because the else: statement body can just be put after the end of the loop. "else" statement after a loop only has sense when used in combination with the instruction break. If during the execution of the loop Python interpreter encounters break, it immediately stops the loop execution and exits out of it. In this case, the else: branch is not executed. So, break is used to abort the loop execution during the middle of any iteration.

Here is a Black Jack-like example: a program that reads numbers and sums it until the total gets greater or equal to 21. The input sequence ends with 0 for the program to be able to stop even if the total sum of all numbers is less than 21.

Let's see how it behaves on the different inputs.

Version 1. The loop is exited normally after checking the condition, so the "else" branch is executed.

2
4
7
0

total_sum = 0
a = int(input())
while a != 0:
    total_sum += a
    if total_sum >= 21:
        print('Total sum is', total_sum)
        break
    a = int(input())
else:
    print('Total sum is less than 21 and is equal to', total_sum, '.')

Version 2. The loop is aborted by break, so the "else" branch is skipped.

9
9
5
4
0

total_sum = 0
a = int(input())
while a != 0:
    total_sum += a
    if total_sum >= 21:
        print('Total sum is', total_sum)
        break
    a = int(input())
else:
    print('Total sum is less than 21 and is equal to', total_sum, '.')

"Else" branch can also be used with the "for" loop. Let's look at the example when a program reads 5 integers but stops right when the first negative integer is met.

Version 1. The loop is exited normally, so the "else" branch is executed.

3
6
2
4
5

for i in range(5):
    a = int(input())
    if a < 0:
        print('Met a negative number', a)
        break
else:
    print('No negative numbers met')

Version 2. The loop is aborted, so the "else" branch isn't executed.

3
6
-2
4
5

for i in range(5):
    a = int(input())
    if a < 0:
        print('Met a negative number', a)
        break
else:
    print('No negative numbers met')

3. Loop control flow: continue

Another instruction used to control the loop execution is continue. If Python interpreter meets continue somewhere in the middle of the loop iteration, it skips all the remaining instructions and proceeds to the next iteration.

None

for num in range(2, 10):
    if num % 2 == 0:
        print("Found an even number", num)
        continue
    print("Found a number", num)

If the break and continue are placed inside several nested loops, they affect only the execution of the innermost one. Let's look at rather silly example to demonstrate it:

None

for i in range(3):
    for j in range(5):
        if j > i:
            # breaks only the for on line 2
            break
        print(i, j)

The instructions break and continue are discouraged, if you can implement your idea without using them. Here is a typical example of a bad usage of the break: this code counts the number of digits in an integer.

567

n = int(input())
length = 0
while True:
    length += 1
    n //= 10
    if n == 0:
        break
print('Length is', length)

It's cleaner and easier-to-read to rewrite this loop with a meaningful loop condition:

567

n = int(input())
length = 0
while n != 0:
    length += 1
    n //= 10
print('Length is', length)

4. Multiple assignment

In Python it is possible for a single assignment statement to change the value of several variables. Let's see:

None

a, b = 0, 1

The effect demonstrated above code can be written as:

None

a = 0
b = 1

The difference between the two versions is that multiple assignment changes the values of two variables simultaneously.

Multiple assignment is useful when you need to exchange the values of two variables. In older programming languages without the support of multiple assignment this can be done using the auxiliary variable:

None

a = 1
b = 2
tmp = a
a = b
b = tmp
print(a, b)
# 2 1

In Python, the same swap can be written in one line:

None

a = 1
b = 2
a, b = b, a
print(a, b)
# 2 1

The left-hand side of "=" should have a comma-separated list of variable names. The right-hand side can be any expressions, separated by commas. The left-hand side and the right-hand side lists should be of equal length.