Looping Fun

File: looping_fun.py

These are practice programs to help you get familiar with loops in Python. They are ordered from easiest to hardest.

Loop Refresher

Recall the general structure of a for loop:

for i in range(15):
    # some code...

The loop above runs for 15 iterations. You can parse the meaning of a for loop from the first line:

• Create a counter variable i
• Every iteration of the loop, assign i to be the next number in the range 0 to 15, exclusive. In other words, the first loop iteration, i is 0, the next time i is 1, and so on, and the last iteration, i is 14.
• For each value of i, run the code inside the loop. While running the loop code, you can use the value of i. So for example, the first time the loop runs, the value of i is zero.
• Note that even if you change i within the loop body, i will still be the next value in the range the next time through the loop. For example:

for i in range(10):
    i += 2
    print(i)

The first time through this loop, i will be 0, and it will print out 2. However, the second time through the loop, i will be 1. Think about it as each loop iteration, regardless of what i currently is, i is re-assigned the next value in the specified range.

That being said, it may be easier to follow if you create another variable instead of changing the loop variable:

for i in range(10):
    j = i + 2 # does not change the value of i
    print(j)

The following exercises are all about using the for loop counter i.

Tip: when approaching looping problems, think about how a value of the loop counter can tell you what to do. For instance, in problem 1 below, we might consider, "when i is 0, I want to print 1; when i is 1, I want to print 3, when i is 2, I want to print 5, etc.".

Looping Fun 1

Try to produce the following output with a for loop. Print the first ten odd numbers, starting from 1.

Print the first ten odd numbers, starting from 1.
1
3
5
7
9
11
13
15
17
19

Extension: Now try to print the first ten even numbers, starting from 0.

Looping Fun 2

Try to produce the following output with a for loop. Print the first ten even numbers in decreasing order.

Print the first ten numbers in decreasing order, starting from 20 and ending with 2.
20
18
16
14
12
10
8
6
4
2

Can you solve this problem without changing the loop condition from the previous problem? That is, start from the following for loop code:

for i in range(10):
    # some code...

How would you solve the problem by using the following loop?

for i in range(20):
    # some code...

Looping Fun 3

Use a for loop, but print out everything on a single line. Using print() automatically adds a line break/new line after what is printed. To tell it not to do this, add end='' right before the closing parentheses. For instance, print("some text or variables") adds a line break/new line, whereas print("some text or variables", end='') does not.

Print out the squares, starting from 1.

Print the first ten square numbers on a single line, starting from 1.
1, 4, 9, 16, 25, 36, 49, 64, 81, 100

Try avoiding a comma at the end of the last square!

Additional Practice: while Loops

Want more practice with while loops? Do all the above exercises with a while loop instead of a for loop. Good luck!