For loops

Learning Objectives

• Understand how for loops can allow us to perform the same operations on different data

Introduction to the For Loop

A for loop in Python, is good at going through elements of a list one by one. Let's take an initial array.

zero_to_three = [0, 1, 2, 3]

Now to print the elements of a list, we could do the following:

print(zero_to_three[0])
print(zero_to_three[1])
print(zero_to_three[2])

Press shift + enter

So in the code above, we increased the index by one each time, going from the 0 to 1 to 2. A for loop is great at going through sequential elements in a list, like 0 through 2. For example:

for i in [0, 1, 2]:
    print(i + 5)

Again, press shift + enter

So above, our expression prints three times -- once for each element in our list. The first time it starts with the number 0, as that is the first element in the array. Then it goes forward to the second element and then the third. So we can use the for loop to operate on the numbers zero through two, and the i represents a successive element in our list each time.

Pay careful attention to the syntax. What is that colon at the end of the first line? Essentially, Python needs to know when the body of the loop begins and when it ends. So we mark the beginning of the loop's body with a colon, :, and then indent each successive line of the loop. (If you press enter after the colon, the next line will automatically indent). To end the body of the loop, we simply unindent.

for i in [0, 1, 2]:
    print(i + 5)
print(10)

Just like any other variable, we can call the i whatever we like. Below, we call it number.

for number in [0, 1, 2]:
    print(number + 5)

Whichever word we place after the for keyword will be the name of our loop variable to reference later on. If we are inconsistent, we receive an error.

for number in [0, 1, 2]:
    print(what + 5)

Press shift + enter

Using list elements as indices

In the above section we iterated through a list of successive numbers. But we can also use our for loop to access successive elements of a separate list, like so.

countries = ['Croatia', 'USA', 'Argentina']
for i in [0, 1, 2]:
    print(countries[i])

So notice what happened there. Just like we did previously, our loop variable, i, is an increasing number for each iteration. Because these successive numbers are also successive indices of our countries list, we can use them to access and then operate on the elements of the countries.

for i in [0, 1, 2]:
    print(i)
    print(countries[i])

Of course, this does not work if we have more number of iterations does not match up with the size of our list.

for i in [0, 1, 2, 3]:
    print(i)
    print(countries[i])

So it would be nice to perform some calculation to ensure that this is the case. Let's do it. We can use the len function to calculate the size of our list.

len(countries)

3

Then we can turn this length into a successive list of elements with the following.

First, create a range object:

range(0, len(countries))

range(0, 3)

And then convert this into a list:

list(range(0, len(countries)))

[0, 1, 2]

Note that the range object is marking the starting and ending point, and excluding the end. So this works perfectly:

for i in list(range(0, len(countries))):
    print(countries[i])

Croatia
USA
Argentina

And as we add or subtract countries, we will still be iterating through our list elements.

countries = ['Croatia', 'USA', 'Argentina']
countries.append('Mexico')
for i in list(range(0, len(countries))):
    print(countries[i])

Croatia
USA
Argentina
Mexico

Iterating through different datatypes

So far our loop variable has always been an element of a list that is a number. However, our loop variable can represent any data type. For example, let's have the loop variable represent each of the countries directly:

countries = ['Croatia', 'USA', 'Argentina']
for i in countries:
    print(i)

Croatia
USA
Argentina

So now i points to each element of the countries list. We previously used i since it was equal to the index of a list. However, here our loop variable will equal an individual country. Might as well be expressive:

for country in countries:
    print(country)

Croatia
USA
Argentina

This is a standard pattern. The variable name pointing to a list is plural, and to refer to a singular element as a loop variable, use the singular version. So if we were printing out a list of our friends names, we would write it as the following:

friends = ['Bob', 'Sally', 'Fred']
for friend in friends:
    print(friend)

Bob
Sally
Fred

And there we are printing out a list of friends.

Summary

In this section, we saw how we can use loops to iterate through various elements. We started with iterating through a list of numbers, and performed the same operation on each number. Then we saw how we can loop through the numbers and have each number be used to access a successive element from a separate list, like countries. We showed that to ensure that our list of numbers matched the indices of a separate list, we could use the expression, for element in list(range(0, len(list))). Finally, we saw that we can also just iterate through the list of elements directly as in the expression, for friend in friends:.