Counting with a For Loop¶

As you saw in Counting with a While Loop, a while loop can be used to to make something happen an exact number of times.

However, this isn’t our best choice. while loops are designed to keep going as long as something is true. But if we know in advance how many times we want to do something, Python has a special kind of loop designed just for that: the for loop.

Type in the following code, and get it to run. Then answer the questions down below.

Name the file: counting_for.py

print("Type in a message, and I'll display it five times.")

message = input("Message: ")

for n in range(0, 5, 1):
    print(f"{n}. {message}")

for loops are best when we know in advance how many times we want to do something.

  • Do this ten times.

  • Do this five times.

  • Pick a random number, and do it that many times.

  • Take this list of items, and do it one time for each item in the list.

On the other hand, while loops are best for repeating as long as something is true:

  • Keep going as long as the player’s health is greater than 0.

  • Keep going as long as they haven’t guessed it.

  • Keep going as long as they keep typing in a negative number.

  • Keep going as long as they haven’t typed in a zero.

What You Should See¶

Type in a message, and I'll display it five times.
Message: Hello, Goodbye!
0. Hello, Goodbye!
1. Hello, Goodbye!
2. Hello, Goodbye!
3. Hello, Goodbye!
4. Hello, Goodbye!

What You Should Do on Your Own¶

Assignments turned in without these things will receive no credit.

  1. What happens when you change the loop variable n to some other name?(Then change it back.) Why do you suppose I chose to name this particular loop variable “n”?

  2. How do the first two arguments (0, 5) given to the range function effect the loop? Change them and experiment. Change it back.

  3. What do you suppose the third number given to the range function is for? Change it to 2 and see. Change it back.

  4. What happens when you call the range function with only one number? i.e. range(7)?

  5. What happens when you call the range function with only two numbers? i.e. range(3, 9)?

  6. Change the code so that the loop repeats ten times instead of five.

  7. See if you can change the for loop so that the message starts at 2 and counts by twos, like so:

    Type in a message, and I'll display it ten times.
Message: qwerty
2. qwerty
4. qwerty
6. qwerty
8. qwerty
10. qwerty
12. qwerty
14. qwerty
16. qwerty
18. qwerty
20. qwerty

©2021 Daniel Gallo

This assignment is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.

Creative Commons License

Adapted for Python from Graham Mitchell’s Programming By Doing