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Loops

docs/design/control_flow/loops.md

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Loops

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Table of contents

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Overview

Carbon provides loops using the while and for statements. Within a loop, the break and continue statements can be used for flow control.

Details

while

while statements loop for as long as the passed expression returns True. Syntax is:

while ( boolean expression ) { statements }

For example, this prints 0, 1, 2, then Done!:

carbon
var x: Int = 0;
while (x < 3) {
  Print(x);
  ++x;
}
Print("Done!");

for

for statements support range-based looping, typically over containers. Syntax is:

for ( var declaration in expression ) { statements }

For example, this prints all names in names:

carbon
for (var name: String in names) {
  Print(name);
}

PrintNames() prints each String in the names List in iteration order.

TODO: Add semantics discussion from #1885: ranged-based for for user-defined types.

break

The break statement immediately ends a while or for loop. Execution will resume at the end of the loop's scope. Syntax is:

break;

For example, this processes steps until a manual step is hit (if no manual step is hit, all steps are processed):

carbon
for (var step: Step in steps) {
  if (step.IsManual()) {
    Print("Reached manual step!");
    break;
  }
  step.Process();
}

continue

The continue statement immediately goes to the next loop of a while or for. In a while, execution continues with the while expression. Syntax is:

continue;

For example, this prints all non-empty lines of a file, using continue to skip empty lines:

carbon
var f: File = OpenFile(path);
while (!f.EOF()) {
  var line: String = f.ReadLine();
  if (line.IsEmpty()) {
    continue;
  }
  Print(line);
}

Alternatives considered

References