CSE 110: Introduction to Programming

Learning Activity (2 of 3): For Loops

Overview

In this lesson, you will continue working with loops and learning how to solve more complex problems.

Preparation Material

In the previous lesson, you learned about while loops. In this lesson, you will build on that foundation to learn about another kind of loop called a for loop.

Watch the following videos which discuss both kinds of loops:

• Direct Link: Loops (5 mins)

• Direct Link: Demo: Loops (3 mins)

As shown, there are a few different ways you can tell the computer to loop through code over and over again.

Looping through a list

One example of looping, is to loop, or iterate, through each item of a collection or a list, one at a time. You will learn about lists in more detail in future lessons, but the following shows an example of looping through a list of words to print them on the screen one at a time:

items = ["crayon", "scissors", "paper", "glitter glue", "markers", "pens"]

for item in items:
    print(f"The item is: {item}")

The output of this code is:

The item is: crayon
The item is: scissors
The item is: paper
The item is: glitter glue
The item is: markers
The item is: pens    

Just as with if statements, the colon : at the end of your for statement tells the computer that there is a block of code coming. Then, you indent all the code that is in the block. The code that is indented is the code that will be run on each item.

When looping through each item, you can do much more with it than simply printing it out. You could include an if statement, to check if it meets certain conditions, and then use it in a math expression, or anything else that you've learned how to do in your programs.

Looping through numbers

It's very common in programming to want to loop through a series of numbers. You could create a list of numbers, just like the list of words in the last example, like this:

numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

But this could get really long if you wanted to work with a large amount of numbers, so there is an easier way, using range.

You can create the same list of numbers like this:

numbers = range(10)

This creates a list of numbers for you that starts at 0 and goes up until (but not including) 10.

You could assign this list to a variable like numbers as in the example above, and then loop through each number in the list of numbers, but it is more common to just put the range code right in your for loop like this:

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

The output of this code is:

0
1
2
3
4
5
6
7
8
9

The range function gives you other options like starting at a different number, or counting by 3's, or even going backwards. In all of these cases, the idea of looping through the values is the same. You start with the first one and then continue on, one by one, through the list.

To start with a different number, you provide two values to the range function. If you provide two numbers, it will start with the first one and continue up until (but not including) the second one.

If you provide three values to the range function, it will use the third number as the "step size" or in other words the number to count by.

# This loop will start with 100, and go up to, but not including 200
for i in range(100, 200):
    print(i)

# This loop will do the same thing, but this time, it will count by 10's
for i in range(100, 200, 10):
    print(i)

Loops within loops

A loop will blindly repeat any code that is in its body. That code could include if statement, mathematics, new variable definitions, or anything else—including other loops!

The following example displays the numbers 0-4:

for i in range(5):
    print(i)

Output:

0
1
2
3
4

We can add a second, inner loop, that for each one of those displays the numbers 10-12 (with two - characters in front to make it easier to visualize).

for i in range(5):
    print(i)
    for j in range(10, 13):
        print(f"--{j}")

Output:

0
--10
--11
--12
1
--10
--11
--12
2
--10
--11
--12
3
--10
--11
--12
4
--10
--11
--12

Notice how the inner loop is run every time the outer loop displays it's number. Inner loops can be very helpful in iterating through a two-dimensional structure, such as the pixels in an image.

Activity Instructions

Write a Python Program that does each of the following:

1. Given the following list of items (copy and paste this line into your program):

   
   colors = ["red", "blue", "green", "yellow"]
   

   Use a for loop to iterate through this list one by one and display each item on its own line as follows:

   
   red
   blue
   green
   yellow
   

2. Use a for loop to display the numbers 1–8, one number on each line as follows:

1
2
3
4
5
6
7
8

3. Use a for loop to display the even numbers (hint: count by two) 2–20, one number on each line as follows:

2
4
6
8
10
12
14
16
18
20

Sample Solution

When your program is finished, please view a sample solution of this program to compare your approach to that one.

You should work to complete this checkpoint program first, without looking at the sample solution. However, if you have worked on it for at least an hour and are still having problems, you may feel free to use the sample solution to help you finish your program.

• Sample solution

Testing Procedure

Verify that your program works correctly by following each step in this testing procedure:

1. Verify that the colors display as desired.

2. Verify that your output for part 2 does not include 0.

3. Verify that your output for part 2 does not include 9.

4. Verify that your output for part 3 does not include any odd numbers.

5. Verify that your output for part 3 starts at 2 and ends at 20.

Submission

When you have completed all of the learning activities for this week, you will return to Canvas and submit the associated quiz there.

Up Next

• Learning Activity (3 of 3): Looping through Strings

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