Pass array from a function to main

Question

How do I pass an array from a function to main in C?

#include<stdio.h>
int merge(int a[],int b[])
{
    int i=0;
    int j=0;    
    int k=0;
    int c[17];
    while(i<10&&j<8)
    {
        if(a[i]<b[j])
        {

            c[k]=a[i];
            i++;
        }
        else
        {
            c[k]=b[j];
            j++;
        }
        k++;
    }
    while(i<10)
    {
        c[k]=a[i];
        i++;
        k++;
    }
    while(j<10)
    {
        c[k]=b[j];
        j++;
        k++;
    }
    return c;
}

Also. Your question did not make sense. You want that your function return an array? — André Puel, Feb 10 '13 at 22:23
sir my moto is to combine 2 arrays and return the new array to the main function.. — SID_J, Feb 10 '13 at 22:25
i hv made the indentations proper but somhow the codes are not readabe propperly on the site .. — SID_J, Feb 10 '13 at 22:27
You can pass pointers around quite easily. How is your attempt failing? — John Dvorak, Feb 10 '13 at 22:27
I guess the `merge` method will _always_ overrun the `c` array, causing undefined behavior. — John Dvorak, Feb 10 '13 at 22:28
Also, you are returning a pointer to something allocated on the stack. That's bad. — John Dvorak, Feb 10 '13 at 22:30
1. Your array is local to the function it is declared in and references to it are invalid once the function returns. 2. You have declared the function to return an int, yet you are attempting to return a pointer to an int (you cannot return arrays from functions, they degrade to pointers). 3. Your loops are unsafe, you have no idea how large the input arrays are. 4. This is a very confused question in general and your description is lacking. — Ed S., Feb 10 '13 at 22:31

score 2 · Answer 1 · answered Feb 10 '13 at 22:35

2

One solution is to take three array parameters:

void merge(int a[], int b[], int c[]);

Then you can merge a and b and put the result into c. The person who calls the function is responsible for creating the array used as c to be large enough.

answered Feb 10 '13 at 22:35

Code-Apprentice

81,660
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score 1 · Answer 2 · answered Feb 10 '13 at 22:34

1

If you want your function to return the array there are 2 ways:

The function has to return a pointer to the first item in the array (actually the array itself is a pointer in C). In this case your function must be declared as int *merge(int a[], int b[]).
You have to pass the array to the function that will be modified by the function itself. Here's how you can do this: int merge(int A[], int B[], int C[]).

answered Feb 10 '13 at 22:34

siannone

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No, no, no. Arrays are not pointers. They *decay* to pointers when passed around, but those are different datatypes. – StoryTeller - Unslander Monica Feb 10 '13 at 22:36
@StoryTeller Good to know, I tought that it was a pointer to the first element of the array. Thank you :) – siannone Feb 10 '13 at 22:37
I'm not sure if you are being sarcastic or not. But the definitive way to see the difference is to declare an array an print it's `sizeof`. Than do the same with a pointer. http://ideone.com/FR9win – StoryTeller - Unslander Monica Feb 10 '13 at 22:42
You might also want to add a warning to option 1 that the returned pointer cannot point to a local variable. – Code-Apprentice Feb 10 '13 at 22:48
@StoryTeller I'm not being sarcastic but my professor said that arrays are basically pointers. – siannone Feb 10 '13 at 22:48
@ShadowBroker, that's alright than. Guess you also learned that you shouldn't *always* listen to your professors :) – StoryTeller - Unslander Monica Feb 10 '13 at 22:50
1

@StoryTeller you're right i guess :) Also found more informations here: http://stackoverflow.com/questions/4607128/in-c-are-arrays-pointers-or-used-as-pointers – siannone Feb 10 '13 at 22:51

DigitalRoss · Accepted Answer · 2013-02-10T22:48:45.623

Here are three ideas...

You can't return arrays in C but you can return a struct that contains an array, so you could do that.
If you just say int x[3]; return x; that will return the address of x. That's OK if it's static or malloc-ed but will lose horribly if it's an ordinary auto-storage-class local. So whatever you do, you should malloc(3) the object in the function, or at least make it static.
You could pass the new array as a parameter, thus avoiding the issue of returning it.

Pass array from a function to main

3 Answers3