Recursive sum of an array in C [duplicate]

问题

Hello I'm learning recursion in C and I am trying to find the sum of the elements.

This is my main:

int main()
{


    int arr[] = {1,2,3,4,5};
    int sum;
    sum = arr_sum(arr,4);
    printf("\nsum is:%d",sum);



    return 0;

}

And my recursive function:

//n is the last index of the array

int arr_sum( int arr[], int n )
{ // must be recursive

    int sum = 0;
    //base case:
    if (n < 0) {
        return sum;
    } else{
        sum = sum + arr[n];
    }
    //make problem smaller
    arr_sum(arr,n-1);
}

The output is:

sum is :0

回答1:

You could add a third argument, which is the running total calculated so far (start it as 0).

When you recursively call the function, pass the running total.

int arr_sum( int arr[], int n, int sum )
{ // must be recursive

    if (n < 0) {
         return sum;
    }

    sum += arr[n];

    return arr_sum(arr, --n, sum);
}

Alternatively, you change it to not require passing the sum variable like so.

int arr_sum( int arr[], int n )
{ // must be recursive

    if (n < 0) {
         return sum;
    }

    return arr[n] + arr_sum(arr, n - 1);
}

In this way, it is similar to finding a number in the Fibonacci sequence.

回答2:

Try this for your recursive function:

int arr_sum( int arr[], int n ) { 
  if (n < 0) {
    //base case:
    return 0;
  } else{
    return arr[n] + arr_sum(arr, n-1);
  }
}

you need to add your n-th case to your n-1 case until you get to the base case.

回答3:

Try this modified version of your program and work out on pen/paper the way it flows through. Hope it helps.

#include <stdio.h>

//n is the last index of the array
int
arr_sum(int arr[], int n )
{
    //base case:
    if (n == 0) {
        return arr[0];
    }

    return (arr[n] + arr_sum(arr,n-1));
}

int
main(void)
{
    int arr[] = {1,2,3,4,5};
    int sum;

    sum = arr_sum(arr,4);
    printf("\nsum is:%d\n",sum);

    return 0;
}

回答4:

You are not returning any thing from else part.You also have return from that. like.

return arr_sum(arr,n-1)+arr[n];

so this call the function again and again until n will be zero.

you got my point?

回答5:

The problem with your code is that everytime when your recursive function calls, it initializes the sum as 0.

Declare sum outside the recursive method that will solve the problem.

来源：https://stackoverflow.com/questions/15801957/recursive-sum-of-an-array-in-c

标签

arrays

recursion

sum