Computing
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What do I need to Learn?

I need to learn how to understand and know how to use definite count loops in a computer program.

Key Terms

While loop Repeat Until loop Iteration Iterator Counter Repeat Condition Infinite loop

Iteration Using definite count loops

There are two major types of loops in Python, for loops and while loops. If you want to repeat a certain number of times (definite count), you will normally use a for loop.

For example, a for loop can be used to print all student records since the computer knows exactly how many students there are.

For Loop Syntax


for variable in range(number):
    indented code goes here...

Using the range function

For loops will iterate through a sequence of numbers using the "range" function. The range function allows you to specify up to three values

range(start value, stop value, step value)

  • The 'start value' is the value the for loop will start at (default value is 0)
  • The 'stop value' is the value the for loop will stop at ( it will never include the stop value so if 5 is used it will stop when it gets to 5 )
  • The 'stop value' is the only value that must always be delcared
  • The 'step value' is the size of the increments for each loop, for example if the step value was 2 then the loop variable(iterator) would increase in value each time by 2 - (default value is 1)

Below is an example of a for loop using the range function with just the stop value set to 5 so it will repeat the code inside the loop 5 times

Click here to read more...

Printing a range of numbers

You can use the range function in a for loop to print out numbers within a range you decide by setting both the start value and the stop value.

For example:

  • If we wanted our loop to sequence through the values 3 to 7 one at a time we would make our start value = 3 and our stop value = 8
  • Why 8 and not 7? Well because our range will stop when it gets to the stop value

    Python example:

    Coding task

    Use the code window below

    1. See if you can print out the range of numbers from 2 to 10.

    Extension Task

    2. Now ask the the user to enter two numbers and print out the range between those two numbers

    Using the step value

    The final parameter in our range function allows us to choose the size of the increment in our loop

    For example if we set the step value = 2

  • our loop would increase by 2 each time see examplke below
  • print out all of the odd numbers from 1 to that number
  • Coding task

    Use the code window below

    1. See if you can print out all the positive even numbers up to and including 10.

    Extension Task

    2. Now ask the the user to enter three numbers and print out the range between those two numbers using the step value they have entered.

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