使用递归在 Javascript 中对一组数字求和

Question

Why isn't this working? I'm getting a stack too deep error:

var countRecursion = function(array) {
  var sum = 0
  var count = 0

  sum += array[count]
  count ++

  if (count < array.length) {
    countRecursion(array);
  } else {
    return sum
  }

}

Answer 1

You made a mistake and reset sum and counter inside the recursive block. I simply moved them outside.

var countRecursion = function(array) {

  sum += array[count]
  count ++

  if (count < array.length) {
    countRecursion(array);
  } else {
    return sum
  }

}

  var sum = 0
  var count = 0


countRecursion([1,2,3]);

alert(sum);

This code is not recursive but iterative. I am not 100% sure if that's what you really wanted. But since you mentioned it, some people down voted my answer since I only fixed your code, but didn't made it recursive I guess. For completeness, here is recursive version of your code:

var countRecursion = function(array, ind) {


  if (ind < array.length) {
    return array[ind] + countRecursion(array, ind + 1);
  } else {
    return 0;
  }

}

  var sum = 0
  var count = 0


sum = sum + countRecursion([1,2,3, 5, 6, 7], count);

alert(sum);

Answer 2

For recursion: pass data up, return data down.

The original code has a different count variable, being a local variable defined in the function, that is initial set to 0. As such the base case is never reached and the function recurses until the exception is thrown.

In addition to using a variable from an outer scope (or another side-effect) this can also be addressed by by following the recommendation on how to handle recursion, eg.

var countRecursion = function(array, index) {
  index = index || 0; // Default to 0 when not specified

  if (index >= array.length) {
     // Base case
     return 0;
  }

  // Recurrence case - add the result to the sum of the current item.
  // The recursive function is supplied the next index so it will eventually terminate.
  return array[index] + countRecursion(array, index + 1);    
}

Answer 3

I see what you're thinking.

The issue with your code is that everytime you call countRecursion , count goes back to 0 (since it's initialized to 0 within your function body). This is making countRecursion execute infinitely many times, as you're always coming back to count = 0 and checking the first term. You can solve this by either:

1. Initializing count outside the function body, that way when you do count++ , it increases and doesn't get reset to 0.
2. Passing count along with array as a parameter. That way, the first time you call the function, you say countRecursion(array, 0) to initialize count for you.

Note that you have to do the same for sum , else that will also revert to zero always.

Finally, (and this doesn't have to do with the stack error) you have to actually call return countRecursion(array) to actually move up the stack (at least that's how it is in C++ and what not - pretty sure it applies to javascript too though).

Answer 4

Array sum using recursive method

 var countRecursion = function(arr, current_index) { if(arr.length === current_index) return 0; current_index = current_index || 0; return countRecursion(arr, current_index+1) + arr[current_index]; } document.body.innerHTML = countRecursion([1,2,3,4,5, 6])