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3.6. The "for" loop

Sometimes, we want to repeatively execute some piece of code. To do this, we use loops. The next few sections of the guide will cover how loops work in Python.

In this section, we'll learn about the for loop.

Understanding for loop

A for loop is used to iterate through an iterable. The syntax for this loop is like so:

for variable in iterable:
    # ... code to execute here ...

For example, here's a really basic for-loop that prints the first 10 numbers starting from 0 and ending at 9.

for num in range(10):
    print(num)

0
1
2
3
4
5
6
7
8
9

Don't worry if you don't understand what is happening here as we'll go through that in a minute.

If you're a beginner, you might find the terms "iterating", "iteration" and "iterable" confusing so lets see what each of these terms exactly mean.

Basic loop terminologies

There are some terms that are used when talking about loops, or repetition in general. It is important that you know the meaning of these terms as these terms are used frequently in programming.

These terms are explained in simple words below. Note that these are not technical definitions.

  • Iterating means going through or processing the elements present in an iterable.

  • Iterable could be a string of characters, range of numbers (as shown above), some data structure, or any other sequence that could have many elements.

Iterating over range of numbers

The most basic and common use of a for loop is iterating over a range of numbers. To do so, we use the range() function.

We've already seen an example above that uses this range() function but let us understand it more deeply.

1
2
for num in range(10):
    print(num)

0
1
2
3
4
5
6
7
8
9

To shed some light on the syntax,

  • range(10) is the iterable here that can iterate from 0 to 9.
  • num is the variable which is assigned the number being processed on each iteration.
  • print(num) is the body of loop which is executed on each iteration (loop body could be multiple lines of code too)

When we execute above code, we're basically telling Python to go through the items in the range(10) iterable (numbers from 0 to 9) and for each item, assign the item to num variable and execute the body of loop (line 2).

Each execution of the loop body (line 2) is referred to as a single iteration and when we say "first iteration", we're referring to first execution of the loop, when num is set to 0. There are a total of 10 iterations i.e the loop body is executed ten times (from num=0 to num=9).

Manipulating range() function

The range() function can be used in a number of ways to modify the range of numbers that are iterated through.

  • range(end) iterates from 0 to end - 1 (upper bound, end, is exclusive)

    for num in range(10):
    print(num)

    0
1
2
3
4
5
6
7
8
9

  • range(start, end) iterates from start to end - 1 (upper bound, end, is exclusive)

    for num in range(1, 11):
    print(num)

    1
2
3
4
5
6
7
8
9
10

    Tip

    range(10) and range(0, 10) are both equivalent. In former, the start is automatically defaulted to 0.

  • range(start, end, step) iterates from start to end - 1 with an increment of step on each iteration

    On every iteration, step is added to previous number. By default (when no step is provided), the step is set to 1.

    for num in range(0, 10, 2):
    print(num)

    0
2
4
6
8

    It is also possible to move backwards (descending range) by providing step as -1

    for num in range(10, 0, -1):
    print(num)

    10
9
8
7
6
5
4
3
2
1

Exclusive Upper Bound

Remember! the upper bound or end, in range() is always exclusive. This means:

  • range(10) iterates from 0 to 9.
  • range(1, 10) iterates from 1 to 9.
  • range(0, 10, 2) has elements 0, 2, 4, 6, 8 while range(0, 12, 2) has elements 0, 2, 4, 6, 8, 10.

Repeatedly executing a code

If you repeatedly want to execute same block of code, you can do this using the same range() approach:

for num in range(10):  # execute 10 times
    print('Hello World')

Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World

Iterating over string characters

If we have a string, we can use a for loop to iterate over the characters of that string.

for char in 'London':
    print(char)

L
o
n
d
o
n

word = input('Enter a word: ')

for char in word:
    print(char)

Enter a word: Hello
H
e
l
l
o

Exercise

We'll learn about more iterables that for loop can iterate over later in the guide such as data structures but for now, here's a simple task for you:

Exercise

Take two numbers as input and output the times table of first number from 1 to second number.

You must use a for loop for this problem.

Example output: 

Enter a number: 2
Enter the end point: 12

2 * 1 = 2
2 * 2 = 4
2 * 3 = 6
2 * 4 = 8
2 * 5 = 10
2 * 6 = 12
2 * 7 = 14
2 * 8 = 16
2 * 9 = 18
2 * 10 = 20
2 * 11 = 22
2 * 12 = 24

Highlighted lines are the input.

num = int(input('Enter a number: '))
end = int(input('Enter the end point: '))

for x in range(1, end + 1):  # (1)!
    result = num * x
    print(num, '*', x, '=', result)  # num * x = result
  1. The upper bound, end, in range(start, end) is exclusive so we add 1 to the end variable to include end in our output.