Sometimes when might want to execute a block of code an indefinite number of times, as long as some condition is met. While Loops are designed exactly for this purpose.
Here is the format:
while { condition } do {
block of code
}
First Grace checks the condition. If the condition is false, Grace does nothing.
If the condition is true, Grace
executes the block of code after the do. Then it loops back to the top, checking
the condition again.
If the condition is false, it does nothing and exits the loop. If the condition
is true, it executes the
block of code after the do. Then it loops back to the top, checking the …
Notice that the condition is in braces, not parenthesis. Why is this? Because
it is essential to the behaviour of the while loop that the condition is re-evaluated
every iteration. If it were in parenthesis, it would be evaluated once before the
start of the loop, which means that the loop would either not execute at all (if
the condition were false), or would execute forever (if it were true).
Using braces means that the while loop gets a block of code,
rather than a simple Boolean value, and can re-evaluate that block on each iteration.
Don’t worry if you didn’t quite follow that explanation: just remember to enclose
your conditions in { braces }.
Below is an example of a while loop that prints out numbers less than 30, counting by threes. Feel free to explore it and change it to see the result.
Sometimes you want to execute the body of the loop at least once. Then, depending
on the condition, you want to decide whether or not to execute it again.
This can be accomplished with the do(_)while(_) loop:
do {
block of code
} while { condition }
The block of code and the condition play the same roles as in the while(_)do(_) loop,
but here the block is executed first, and the condition checked after the
block has been executed.
Want to know more? Read The Dangers of While Loops