Why does my function always return 0 instead of returning the sum result?

Question

I just started learning C++, now I'm making a simple array sum function.

Why is my code output always 0? Does it mean that my function returns "0"?

If I put cout in the function, it shows the right sum result.

#include <iostream>
using namespace std;

int ArraySum(int arr[], int size){
  int sum=0;
  for(int i=0 ; i<size; i++){
      cin >> arr[i];
      sum +=arr[i];
  }   
  return sum;
}
   
int main()
{
    int n, sum;
      cin >>n;
   int arr[n];
     ArraySum(arr, n);
   cout << sum;
    return 0;
}

`ArraySum(arr, n);` -> `sum = ArraySum(arr, n);`. Trying to print the value of an uninitialized variable is [*Undefined Behavior*](https://en.wikipedia.org/wiki/Undefined_behavior). — Ruks, Sep 23 '21 at 02:20
`int arr[n];` is invalid in C++. It's just an extension in some compilers. Use `std::vector` instead — phuclv, Sep 23 '21 at 02:25

score 2 · Accepted Answer · edited Sep 23 '21 at 02:23

2

You are not assigning the return value to the sum when it is returned.

You have 2 options:

pass a pointer to the sum, and dereference it inside ArraySum()
assign the value that is returned by ArraySum() to the sum int.

edited Sep 23 '21 at 02:23

Remy Lebeau

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answered Sep 23 '21 at 02:17

Martin Savov

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i use the second option you gave because i haven't learned pointer. it works perfectly. thanks for the answer, its very helpful – Fawwaz Anrico Purnomo Sep 23 '21 at 02:24

Nikos C. · Answer 2 · 2021-09-23T02:54:34.210

You forgot to assign ArraySum()'s return value to main()'s sum. As a result, main()'s sum is still uninitialized when you print it.

Change this:

ArraySum(arr, n);

to this:

sum = ArraySum(arr, n);

As a side note, you should know that this is not actually valid, standard C++:

int arr[n];

This is a "variable-length array" ("VLA" for short.) This works on some C++ compilers because they support VLAs as an extension. But relying on compiler-specific extensions is not recommended if you want your code to be portable to any other C++ compiler.

Use std::vector instead. This also has the benefit of not needing to manually keep track of the size. vector knows its own size.

So, use this instead:

#include <iostream>
#include <vector>

// Note the '&' here. That means a reference of the vector is passed,
// not a copy, so the original vector defined in main() is used when
// assigning the values we read with `cin`.
int ArraySum(std::vector<int>& arr)
{
    int sum = 0;
    for (int i = 0; i < arr.size(); i++) {
        std::cin >> arr[i];
        sum += arr[i];
    }
    return sum;
}
   
int main()
{
    int n;
    int sum;

    std::cin >> n;
    std::vector<int> arr(n);
    sum = ArraySum(arr);
    std::cout << sum;
}

(Also note that there is no need to return 0 at the end of main(). It happens automatically. Keep in mind that this is only true for main(). No other function that returns a value is allowed to omit the return statement. main() is a special case in this regard. But it doesn't hurt to return 0 anyway though. It's up to preference.)

"*Also note that in C++, **unlike C**, there is no need to return 0 at the end of main(). It happens automatically.*" - Actually, you can [omit `return 0` since C99 as well](https://stackoverflow.com/a/4138846/8423261). — Ruks, Sep 23 '21 at 02:40

Why does my function always return 0 instead of returning the sum result?

2 Answers2