How to "return" an array from a function to main in c

Question

I want to pass an arrays index from my function to main. How can I do that? For example:

void randomfunction()
{
    int i;
    char k[20][10];
    for (i=0;i<20;i++)
        strcpy(k[i], "BA");
}
int main(int argc, char *argv[])
{
    int i; for (i=0;i<20;i++) printf("%s",k[i]);
    return 0;
}

I know that for you this is very simple but I've found similar topics and they were too complicated for me. I just want to pass the k array to main. Of course my purpose is not to fill it with "BA" strings...

Firstly, your code won't compile, is that code meant to be part of `randomfunction`? Second, what do you mean by "pass the k array to main`? Show an example of how `randomfunction` is called and `k` is used in `main`. — etheranger, May 04 '14 at 04:13
Better. It's still not entirely clear (but we can guess) when `randomfunction` is called. Normally "passing a value from `a` to `b`" means that `a` calls `b`. It would of course be a nonsensical thing for `randomfunction` to call `main`, but given the naivety of the question you could have been trying to do that, and would need a different answer. — etheranger, May 04 '14 at 04:33

HelloWorld123456789 · Accepted Answer · 2014-05-04T04:14:58.300

2

You want to allocate the memory dynamically. Like this:

char** randomfunction() {    
    char **k=malloc(sizeof(char*)*20);
    int i;

    for (i=0;i<20;i++)
        k[i]=malloc(sizeof(char)*10);

    //populate the array

    return k;    
}

int main() {
    char** arr;
    int i;

    arr=randomfunction();

    //do you job

    //now de-allocate the array
    for (i=0;i<20;i++)
        free(arr[i]);        
    free(arr);

    return 0;
}

See about malloc and free.

edited May 04 '14 at 04:14

answered May 04 '14 at 04:08

HelloWorld123456789

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what is free(arr) doing? – GeorgeDavidKing May 04 '14 at 04:14
@GeorgeDavidKing It returns the memory you allocated with `malloc()` to the system. – EMBLEM May 04 '14 at 04:14
1

@GeorgeDavidKing `malloc` allocates memory in the heap and `free` releases that memory. – HelloWorld123456789 May 04 '14 at 04:27
@GeorgeDavidKing The `k` array is returned into the `arr` array. `arr` points to the same location as `k` pointed to. So `free`ing `arr` frees the space allocated for `k`. Read this: http://stackoverflow.com/questions/1957099/how-do-free-and-malloc-work-in-c – HelloWorld123456789 May 04 '14 at 04:59
@GeorgeDavidKing It seems that your basics about pointers are not clear. You can try a good book and read it. It will help you in the long run. – HelloWorld123456789 May 04 '14 at 05:01
@Rikayan Bandyopadhyay can you please suggest me one? – GeorgeDavidKing May 04 '14 at 05:20

score 2 · Answer 2 · answered May 04 '14 at 04:46

2

Here is another option. This works because structs can be copied around , unlike arrays.

typedef struct 
{
    char arr[20][10];
} MyArray;

MyArray random_function(void)
{
    MyArray k;
    for (i=0;i<sizeof k.arr / sizeof k.arr[0];i++)
       strcpy(k.arr[i], "BA");
    return k;
}

int main()
{
     MyArray k = random_function();
}

answered May 04 '14 at 04:46

M.M

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I like this option. +1. But it maybe unnecessary to create structs every time you want to return an array. – HelloWorld123456789 May 04 '14 at 04:47

score 1 · Answer 3 · answered May 04 '14 at 04:16

1

Simplest way:

void randomfunction(char k[][10])
{
    // do stuff
}

int main()
{
    char arr[20][10];
    randomfunction(arr);
}

If randomfunction needs to know the dimension 20, you can pass it as another argument. (It doesn't work to put it in the [] , for historial reasons).

answered May 04 '14 at 04:16

M.M

138,810
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Read the question. There is a word in quotes. – HelloWorld123456789 May 04 '14 at 04:17
@RikayanBandyopadhyay output parameters are a perfectly good way to "return" values. – etheranger May 04 '14 at 04:23
@etheranger You still don't get it. The question says `"return" an array`. – HelloWorld123456789 May 04 '14 at 04:26
1

@RikayanBandyopadhyay You still don't get it. You can (and often should) return data through pointer parameters. Yes, it's not syntactically the same as a literal `return` statement, but that's not what this question is about. – etheranger May 04 '14 at 04:35

How to "return" an array from a function to main in c

3 Answers3