For loops


Martin McBride, 2017-02-03
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A for loop is a form of iteration, often used as an alternative to a while loop.

Quite often, we need to loop for a specific number of times. For example, if you wanted to print the 7 times table you would need to loop 12 times.

You can do this with a while loop and a counter variable, but you end up writing similar code every time you want to do this type of loop. It is such a common thing to do that most languages have a special way of doing it - the for loop.

For loops

A for loop looks something like this:

FOR i = 0 TO 4
    PRINT(i)
ENDFOR

The first line declares a variable i, and sets the loop to run from 0 to 4. So the loop will print:

0
1
2
3
4

The first number is called the start value (or initial value), and the second number if called the end value.

{{% orange-note %}} Different languages have different ways of defining a for loop. Often, as shown, the loop count starts at 0. If we want the loop to execute n times, it will count from 0 to n - 1. {{% /orange-note %}}

Of course, you can use variables for the start and end values:

FOR i = a TO b
    PRINT(i)
ENDFOR

where a and b are variables that have been initialised in code before the start of the loop.

Using a step value

By default the loop counters changes by 1 each time. A step value allows the loop count to change by a different value:

FOR i = 0 TO 4 STEP 2
    PRINT(i)
ENDFOR

Starting at 0, the variable i steps by 2 each time until it reaches 4. This prints:

0
2
4

The step can be negative, causing the loop to count down:

FOR i = 10 TO 5 STEP -1
    PRINT(i)
ENDFOR

This prints:

10
9
8
7
6
5

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