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Exercise #24: Every 15 Minutes

Clocks have an unusual counting system compared to the normal decimal number system we’re familiar with. Instead of beginning at 0 and going to 1, 2, and so on forever, clocks start at 12 and go on to 1, 2, and so on up to 11. Then it loops back to 12 again. (Clocks are quite odd if you think about it: 12 am comes before 11 am and 12 pm comes before 11 pm.) This is a bit more complicated than simply writing a program that counts upward. This exercise requires using nested for loops to loop over the minutes, the hours, and the am and pm halve of the day.

Exercise Description

Write a program that displays the time for every 15 minute interval from 12:00 am to 11:45 pm. Your solution should produce the following output:

12:00 am

12:15 am

12:30 am

12:45 am

1:00 am

1:15 am


11:30 pm

11:45 pm

There are 96 lines in the full output.

Try to write a solution based on the information in this description. If you still have trouble solving this exercise, read the Solution Design and Special Cases and Gotchas sections for additional hints.

Prerequisite concepts: for loops, lists, nested loops, string concatenation

Solution Design

This solution requires the use of three nested for loops. The outermost for loop iterates over 'am' and 'pm'. The second for loop iterates over twelve hours, starting with '12', then '1', then '2', and so on until '11'. The third for loop iterates over the minutes in 15-minute increments: '00', '15', '30', and '45'. Note that the hours and minutes values are strings, not integers, because we need to concatenate them into our final string, like: '12' + ':' + '00' + ' ' + 'am' evaluates to '12:00 am'

You’re used to for loops iterating over a range of integers from the range() function. But Python’s for loops can iterate over lists of any values. For example, enter the following into the interactive shell:

>>> for i in ['Alice', 'Bob', 'Carol']:

...     print('Hello ' + i)


Hello Alice

Hello Bob

Hello Carol

What a for loop does is iterate over a sequence of values. The following interactive shell example is the equivalent for i in range(4):

>>> for i in [0, 1, 2, 3]:

...     print(i)






In this case, we explicitly typed out the integers to iterate in a list rather than use the more convenient range(4). But they produce identical results. And explicitly typing out the integers in a list becomes prohibitively long for large ranges such as range(1000).

Special Cases and Gotchas

The order of the nested for loops is important. You want the innermost for loop to iterate over minutes, the next innermost to iterate over hours, and the outermost for loop to iterate over 'am' and 'pm'.

Now try to write a solution based on the information in the previous sections. If you still have trouble solving this exercise, read the Solution Template section for additional hints.

Solution Template

Try to first write a solution from scratch. But if you have difficulty, you can use the following partial program as a starting place. Copy the following code from and paste it into your code editor. Replace the underscores with code to make a working program:

# Loop over am and pm:

for meridiem in [____, 'pm']:

    # Loop over every hour:

    for hour in [____, '1', '2', '3', '4', '5', '6', '7', '8', '9', '10', '11']:

        # Loop over every 15 minutes:

        for minutes in ['00', ____, ____, '45']:

            # Print the time:

            print(____ + ':' + ____ + ' ' + ____)

The complete solution for this exercise is given in Appendix A and You can view each step of this program as it runs under a debugger at

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