13. Iteration

The three basic control structures are:

1. Sequence (steps in order)

2. Selection (branching using if … elif … else)

3. Iteration (repetition or looping using while or for)

13.1. Loops

Loops are very useful structures which enable repeating of a certain block of code several times over.

There are two types of loops:

for loops, that keep count of the number of times a block of code is executed, and

while loops which perform an action until a specified condition is no longer true.

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13.2. For loops with the range function

Use the range() function to loop through a set of code a specified number of times.

The range() function returns a sequence of numbers, starting from 0 by default, and increments by 1 (by default), and ends at a specified number.

range(8) means the values 0 to 7 (8 not included).

for number in range(8):
    print(number, end = ' ')
# 0 1 2 3 4 5 6 7

range(2, 8) means values from 2 to 8 but not including 8.

for number in range(2,8):
    print(number, end = ' ')
# 2 3 4 5 6 7

The range() function defaults to increment the sequence by 1, however it is possible to specify the increment value by adding a third parameter.

range(1, 15, 3) means values from 1 up to but not including 15, in steps of 3.

for number in range(1, 15, 3):
    print(number, end = ' ')
# 1 4 7 10 13

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13.3. Nested For loops

In the example below, x and y are assigned the values by the range function:

for x in range(2, 5):
    for y in range(3, 6):
        print(x, y)

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13.4. For loops with sequences

A for-loop can be used to iterate over a sequence that is either a string, a list, a tuple, , a set or a dictionary.

The for-loop can execute a set of statements, once for each item in a sequence.

for letter in 'Common Operating Machine Particularly Used for Technical, Education and Research':
    print(letter, end = '-')

for network in ['DNS','VPN','PoP','VoIP']:
    print(network, end = ' ' )

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13.5. While True loops

One of the most common things you might want to do with a while loop is to do something forever. Here is an example of some code to repeat forever:

while True:
    print("Hello..."")

This code will repeatedly print the text Hello....

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13.6. While loops

The while loop requires relevant variables to be ready, and incremented in the loop to avoid it running for ever.

In the example below, the variable num is increased by one each loop.

The loops stops when num gets to 5.

num = 0
while num < 5:
    num += 1
    print(num)

In the example below, the variable num is decreased by one each loop.

The loops stops when num gets to 0.

num = 5
while num > 0:
    print(num)
    num -= 1

In the example below, a final clock of code can run as part of the else statement.

The loops stops when num gets to 0, then the else block is run.

num = 5
while num > 0:
    print(num)
    num -= 1
else:
    print("Blastoff")

The break statement stops the loop even if the while condition is true:

Example. Exit the loop when i is 3:

i = 1
while i < 6:
    print(i)
    if i == 3:
        break
    i += 1

The continue statement stops the current iteration (the current run through the loop), and continues with the next iteration of the loop.

Example. Continue to the next iteration if i is 3:

i = 0
while i < 6:
    i += 1
    if i == 3:
        continue
    print(i)