3.9 For Loops

In Python, we will often want to perform an action more than once. For example, if we have a list, we might want to do the same operatation on every item within the list.

One way to do this is to use a for loop, which is structured like this:

for <temporary_variable> in <thing to loop through>:
    {do something}

For example:

stations = ["Oddity", "Londres", "Cherry", "Swedish"]

for i in stations:
  print(i)

## Oddity
## Londres
## Cherry
## Swedish

3.9.1 For loop walkthrough

Here is how the loop works:

for i in stations:

In this line, we:

Define the temporary variable i. The name of this variable is arbitrary.
We say that we are looping through stations.

In the first iteration of the for loop, i takes on the value of the first item in stations ("Oddity"). Now we perform all of the indented code, which here is just a print() statement.

In the second iteration of the for loop, we set the value of i to the second entry in stations ("Londres"). We execute all of the indented code, printing out "Londres".

We continue on doing this until there is nothing left in stations.

In this example, there was a single line in the body of the loop, but a for loop can be arbitrarily long. Here is a longer example:

my_list = [1, 4, 6, 9, 10, 2]

for i in my_list:
  i = i + 3
  i = i ** 2
  print(i)

## 16
## 49
## 81
## 144
## 169
## 25

3.9.2 Repeating an action `n` times

We can also use a for loop to perform an action a set number of times, even when we don’t have a list to loop through. To do so, we can use the range() function. We’ll use this function a ton throughout the course.

range(<n>), where n is a single integer, generates a sequence of numbers from 0 to n (not including n itself). So to run a function 3 times, we would provide the for loop with range(3) (which generates the list [0, 1, 2]).

for i in range(3):
  print(i)

## 0
## 1
## 2

So far, the body of our for loop has always referenced the temporary variable i. We can also use a for loop to run a block of code repeatedly without actually manipulating i:

for i in range(3):
  print("All work and no play")

## All work and no play
## All work and no play
## All work and no play

3.9 For Loops

3.9.1 For loop walkthrough

3.9.2 Repeating an action n times

3.9.2 Repeating an action `n` times