Does the break statement break out of multiple loops?-CodePudding

If I have a while loop (a) inside another while loop (b) and I use the break statement while in the 'b' loop, does it also break out of the 'a' loop or do I stay in the 'a' loop?

CodePudding user response：

break; only breaks from the innermost loop or switch body it appears in.

If you intend to break from nested loops, you might consider moving the nested loops to a separate function and using return to exit the function from any point inside its body.

Example:

    ...
    int matrix[ROWS][COLS];
    int value;
    ...
    int found = 0;
    for (int row = 0; row < ROWS; row  ) {
        for (int col = 0; col < COLS; col  ) {
            if (matrix[row][col] == value) {
                found = 1;
                break;
            }
        }
        if (found)
            break;
    }
    ...

Can be simplified as:

int hasvalue(int matrix[ROWS][COLS], int value) {
    for (int row = 0; row < ROWS; row  ) {
        for (int col = 0; col < COLS; col  ) {
            if (matrix[row][col] == value)
                return 1;
        }
    }
    return 0;
}

    ...
    int matrix[ROWS][COLS];
    int value;
    ...
    found = hasvalue(matrix, value);
    ...

CodePudding user response：

Does the break statement break out of multiple loops?

No:

A break statement terminates execution of the smallest enclosing switch or iteration statement.

CodePudding user response：

Documentation on the break statement:

Causes the enclosing for, while or do-while loop or switch statement to terminate.

The enclosing statement, singular.

CodePudding user response：

It breaks out of the inner most loop that contains it

https://beginnersbook.com/2014/01/c-break-statement/

  for(int i = 0; i < 10; i  ){
      int x = foop();
      nargel();
      for (int j = 0; j < 20; j  ){
         int k = fringle(i,j);
         if(are_we_done_yet(k))
            break; <<<<<<<<<<<<<<====== this goes
      }
      blink(); <<<<<<<<<<<<===== here
   }

some languages allow you to say which loop to break out of by giving the for a label, but not c. Java for example https://docs.oracle.com/javase/tutorial/java/nutsandbolts/branch.html

CodePudding user response：

The keyword break works only on the current loop. You can't break the outmost loop from any enclosed loop with a single break, you'll need to set a flag in order to break at start of each loop when it becomes non-null. It becomes way more tricky if you also use continue and multiple break/continue in the same loop level, but it can be done.

Example:

int must_break = 0 ;
...
...
for (int l1=0;l1<LIMIT1;l1  ) {
  ...
  ...
  for (int l2=0;l2<LIMIT2;l2   {
    ...
    ...
    for (int l3=0;l3<LIMIT3;l3   {
      ...
      ...
      if (INTERNAL_BREAK_CONDITION) {
        // This flag will cause all outer loops to break instantly in cascade.
        must_break = 1 ;
        break ;   // Break L3
      }
    }
    // Must be placed just after the possibly breaking loop.
    if (must_break)   // Break L2
      break ;
  }
  // Must be placed just after the possibly breaking loop.
  if (must_break)   // Break L1
    break ;
}
// Here, you can test: if "must_break" is non-null, then a break occurred.
if (must_break)
  printf("Break detected!\n");

Or you can buy yourself a warm place in Hell and use a goto to break all loops... But obviously, it is very badly considered in most programming languages but Assembler. In C, outside low-level drivers / kernel modules, it's usually a "no-go" whatever happens. But it works... It's not maintainable, but it works.