loops.md

Loops

Table of contents

• Overview
• Details
  • while
  • for
  • break
  • continue
• Alternatives considered
• References

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!:

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:

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):

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:

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

Alternatives considered

• Non-C++ syntax
• Initializing variables in the while
• for:
  • Include semisemi for loops
  • Multi-variable bindings
  • : versus in
• Optional braces
• Optional parentheses

References

• Proposal #340: while
• Proposal #353: for
• Proposal #618: var ordering
• Proposal #623: Require braces