Introduction to Swift | Control Statements | Repetition with while

A for statement includes a mechanism that changes on each iteration. There is also a simpler loop statement that only checks a condition to decide whether to repeat a process. This is the while statement.

There are two ways to write a while loop, depending on where the condition appears. The latter is also called a do while statement.

Basic form of while

while condition {
    ... repeated processing ...
}

Basic form of do while

do {
    ... repeated processing ...
} while condition

Write the condition after while. If the condition is true, the loop runs. When it becomes false, execution exits the loop.

Why are there two forms for such a simple operation? The difference is when the condition is checked. The first form, with while at the beginning, checks the condition before executing the block inside {}.

The second form, with while at the end, executes the block first and checks the condition afterward. As a result, the block runs at least once even if the condition is initially false.

Compare the following simple loops.

var n:Int = 0
 
while 10 > n++ {
    "index:" + String(n)
}
 
var m:Int = 0
do {
    "index:" + String(m)
} while 10 > m++

The first loop produces values in the form index:1 through index:10. The second loop produces index:0 through index:9. Because the variable is incremented with ++ in the condition, the values differ depending on whether the condition appears before or after the block.

As this example shows, the two forms behave slightly differently in subtle cases. As a general rule, use while by default and reserve do while for situations that require it.