For loops

We learned that a while loop is a great tool when you know the stopping condition.

In this section we will explore a second tool for repeating code: the for loop. A for loop is a good fit when we want to run a block of code a definite number of times, or when we want to iterate over list of things.

for Loops

for loops step through a list of items in order. Each iteration will assign the next item to the loop variable, then execute the loop body.

The syntax of a for loop starts with the for keyword, and has an indented loop body. A for loop has a variable name, the in keyword, and a list of things to loop through.

for variable in items:
	loop body to execute
For loop flowchart

Iteration gif

image credit: https://www.dataquest.io/blog/python-list-tutorial

The items you iterate through can be any sequence of values. For example, you can loop through a list of numbers, or a string of characters. The sequence of values can be written explicitly, or generated with a function like range().

Example of iterating through a list of numbers:

for i in [2,4,5,6] :
	print(i)

output:

2
4
5
6

Example of iterating through a string:

for letter in "Hello" :
	print(letter)

output:

H
e
l
l
o

Example of iterating through a range of numbers:

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

output:

1
2
3
4
5
6
7
8
9
10

Watch the following video for an introduction to for loops with examples:

Comparing For and While loops

Let's take a look at an example of a for loop:

for i in [5, 4, 3, 2, 1] :
	print(i)
print('Blastoff!')

This for loop will have the same output as the while loop we saw in the previous section:

5
4
3
2
1
Blastoff!

Let’s compare the for loop with the while loop :

for i in [5, 4, 3, 2, 1] :
	print(i)
print('Blastoff!')

n = 5
while n > 0:
    print(n)
    n = n - 1
print('Blastoff!')

🤔 Compare the two code examples above (the for loop and the while loop).

What do you notice about them?

while vs. for

Similarities:

  • loop keyword, then something, then :
  • loop body is indented

Differences:

  • variable n created before the while loop, variable i created as part of the for loop
  • while loop changes the variable with n = n - 1, for loop variable changes automatically
  • for loop has to write out exactly what numbers to loop through

for loop iteration variable

The initial statement in the for loop is:

for i in [5,4,3,2,1]:

In this code, the loop creates a new variable i. The value of i will change in each iteration of the loop, to take on the value of each item in the list. In this example, i will take on successive values of 5, 4, 3, 2, and 1.

As you can see, for loops offer a more direct syntax than while loops, because you can explicitly declare the values of the iteration variable.

We’ll cover the list syntax [5, 4, 3, 2, 1] in more detail later in the course. For now, you can use it to write for loops, without knowing exactly what it means. You can put any values inside the [], and the loop variable will be assigned to each value in turn.

Practice