Return Outside Loop

A return statement can be used both inside or outside a loop. In fact, returning early from within a loop when a certain condition is met is often the most efficient and readable way to implement a function.

However, it is important that all possible execution paths of the function result in a return. If the compiler cannot guarantee this (for example if a return only appears inside a loop or conditional block without a fallback) it will throw an error. This does not mean that return statements are not allowed inside loops. Instead, it highlights the need for a fallback return to handle cases where the loop or condition is never executed, ensuring that the function always returns a value.

Inefficient code:

float firstNegative(float[] array) {
  float a = 0; // variable to store the first negative value
  for (int i = 0; i < array.length; i++) {
    // check if value is negative and no other has been found
    if (array[i] < 0 && a == 0) {
      a = array[i];
    }
  }
  return a; // return the stored value after the loop completed
}

Improved code:

float firstNegative(float[] array) {
  for (int i = 0; i < array.length; i++) {
    if (array[i] < 0) {
      return array[i]; // return immediately when a negative value is found
    }
  }
  return 0; // fallback return if no negative value was found
}

1. Using a break instead of a direct return.

float firstNegative(float[] a) {
  float first = 0;
  for (int i = 0; i < a.length; i++) {
    if (a[i] < 0) {
      first = a[i];
      break; // used instead of directly returning
    }
  }
  return first;
}

2. Creating temporary variables to store the result and returning them after the loop.

float firstNegative(float[] array) {
  float a = 0; // variable to store the first negative value
  for (int i = 0; i < array.length; i++) {
    // check if value is negative and no other has been found
    if (array[i] < 0 && a == 0) {
      a = array[i];
    }
  }
  return a; // return the stored value after the loop completed
}

Many educational examples place the return statement outside of loops, such as when calculating sums, leading students to overgeneralize this pattern.

Compilers produce errors when a return is exclusively inside a loop without a fallback outside, which students may misinterpret as indicating that return is not allowed within loops, rather than understanding that all code paths must return a value.

Helpful Exercises:

Write a function that returns the first even number in an array. If no even number is found, the function should return -1.

Create two versions of a function that checks if a value exists in an array.

Version 1: The function should terminate early using return as soon as the value is found.

Version 2: The function should use a variable and return the result only at the end of the function. Which version is more efficient and why? Which version do you prefer, and why?

Peer Instruction Quizzes:

boolean contains(int[] array, int value) {
    for (int i = 0; i < array.length; i++) {
        if (array[i] == value) {
            return true;
        }
    }
}

boolean firstNegative(int[] numbers) {
    int found = 0;
    for (int i = 0; i < numbers.length; i++) {
        if (found == 0 && numbers[i] <= 0) {
            found = numbers[i];
        }
    }
    return found;
}