Unhandled Exception while checking values of an array in C

Question

I have an assignment to write a recursive program that gets an array (lets call it arr1), the size of the array (n), and a value(s). The program will check if there are two numbers in the array so that their summary is s. example if the array is {1,3,2,0,5} and s=7, then the function will print "yes" because 5+2=7. if the array is {1,3,2,0,5} and s=9, then the function will print "no". no summary of a pair is equal to 9.

My algorithm works like this: i calculate the summary of the last spot in the array (arr1[n-1]), with every other spot. if i find a couple that their sum is s, great, print yes and leave. if i dont find, then i do the same, but instead of arr1[n-1] i check arr1[n-2]. i delete the last spot.

Here's my code:

#include <stdio.h>
#include <conio.h>
#include <stdlib.h>
void input_array_dyn(int* a, int n);
void rec(int* a,int n, int s);
void main()
{
    int n=0,s=0;
    int* a;
    printf("Enter value S\n");
    scanf("%d",&s);
    printf("Enter the size of the array\n");
    scanf("%d",&n);
    a=(int*)calloc(n,sizeof(int));
    printf("Enter %d values for the array\n",n);
    input_array_dyn(a,n);
    rec(a,n,s);
    free(a);
    getch();
}
void input_array_dyn(int* a,int n)
{
    int i=0;
    for(i=0;i<n;i++)
        scanf("%d",a[i]);
}
void rec(int* a,int n, int s)
{
    int i;
    if(n==1)
    {
        printf("There are no 2 number whos summary yields S\n"); 
        return;
    }
    for(i=0;i<n-1;i++)
    {
        if(a[n-1]+a[i]==s)
        {
            printf("There are two numbers that give s\n");
            return;
        }
    }
    rec(a,n-1,s);
}

I get an error saying: "Unhandled exception at 0x5846e30e (msvcr100d.dll) in Test.exe: 0xC0000005: Access violation writing location 0x00000000."

also: Does anyone have a better idea for an algorithm to do this? :)

score 2 · Accepted Answer · answered Feb 04 '13 at 18:53

2

In input_array_dyn():

scanf("%d", a[i]);

You've forgotten a &.

answered Feb 04 '13 at 18:53

md5

23,373
3
44
93

That solved it, thank you :) Can you comment on the algorithm? I'm quite inexperienced, this is the best way i could think of. – Oria Gruber Feb 04 '13 at 18:55
@OriaGruber Some comments: for a local array, `calloc()` and `free()` is absolutely an overkill, better just use an automatic array (if you can use C99 VLAs). Also, you **must not** cast the return value of `malloc()`, `calloc()` and `realloc()` - it's dangerous. Coding style: you should use whitespaces. – Feb 04 '13 at 18:59
I'm sorry relatively new at this. Can you please explain what do you mean by whitespaces? and why not cast return value of calloc / malloc? In all the lectures the teacher shows it like that. so i figured its mandatory. – Oria Gruber Feb 04 '13 at 19:04
And also really, is there a better algorithm to do what I need in this program? I'd like to know. a recursive algorithm. – Oria Gruber Feb 04 '13 at 19:04
@OriaGruber - There are lots of questions about why not to cast the return from malloc and friends. See [here](http://stackoverflow.com/questions/1565496/specifically-whats-dangerous-about-casting-the-result-of-malloc) for one such link. Whitespace means spaces, tabs, new lines, pretty much like it sounds (empty space). – Mike Feb 04 '13 at 19:07
@OriaGruber: The cast is unnecessary as of the 1989 standard, and under that version the cast can mask an error if you forget to `#include ` or otherwise don't have a declaration for `malloc` in scope. Prior to the 1989 standard, `malloc` returned `char *` instead of `void *`, and under those early versions a cast *was* required. That hasn't been true for over 20 years now, but the practice persists. – John Bode Feb 04 '13 at 19:18

Unhandled Exception while checking values of an array in C

1 Answers1