break and continue

[Sunjay Varma]

To implement break and continue, add a step to the desugaring process which replaces break and continue statements with the appropriate dummy variables and if statements.

From the Brainfuck algorithms wiki:

break and continue

To implement break and continue statements in loops, consider that the following two pieces of pseudocode are functionally equivalent:

while (foo) {
 if (bar == foo) {
  if (x > 2) {
   break;
  }
  else {
   // do stuff
  }
  // do stuff
 }
 // update foo for the next iteration
}

// Equivalent without break statement:
while (foo) {
 shouldBreak = false
 if (bar == foo) {
  if (x > 2) {
   shouldBreak = true
  }
  else {
   // do stuff
  }
  
  // don't evaluate any more code in the loop after breaking
  if (!shouldBreak) {
   // do stuff
  }
 }
 if (shouldBreak) {
  // so that the loop stops
  foo = 0
 }
 else {
  // update foo for the next iteration
 }
}

Notice that we need to guard all code after the break statement in the loop to prevent it from running. We don't need to guard in the else statement immediately after the break statement because that will never run after the break statement has run.

This approach allows us to implement break and continue statements in brainfuck despite the lack of sophisticated jump instructions. All we're doing is combining the concept of an if statement (defined below) with the while loop we just defined and applying it here.

Implementing a continue statement is the same thing except you never guard the loop updating code:

while (foo) {
 if (bar == foo) {
  if (x > 2) {
   continue;
  }
  else {
   // do stuff
  }
  // do stuff
 }
 // update foo for the next iteration
}

// Equivalent without continue statement:
while (foo) {
 shouldContinue = false
 if (bar == foo) {
  if (x > 2) {
   shouldContinue = true
  }
  else {
   // do stuff
  }
  
  // don't evaluate any more code in the loop after continuing
  if (!shouldContinue) {
   // do stuff
  }
 }
 
 // This code stays the same after a continue because we still want to move on to the next iteration of the loop
 // update foo for the next iteration
}

To implement both break and continue, you can compose the concepts here and make any combination you want. You can consider break and continue statements to be "sugar" that needs to be "desugared" in your brainfuck code.