Passing pointers of arrays in C

Question

So I have some code that looks like this:

int a[10];
a = arrayGen(a,9);

and the arrayGen function looks like this:

int* arrayGen(int arrAddr[], int maxNum)
{
    int counter=0;
    while(arrAddr[counter] != '\0') {
        arrAddr[counter] = gen(maxNum);
        counter++;
    }
    return arrAddr;
}

Right now the compilier tells me "warning: passing argument 1 of ‘arrayGen’ makes integer from pointer without a cast"

My thinking is that I pass 'a', a pointer to a[0], then since the array is already created I can just fill in values for a[n] until I a[n] == '\0'. I think my error is that arrayGen is written to take in an array, not a pointer to one. If that's true I'm not sure how to proceed, do I write values to addresses until the contents of one address is '\0'?

Answer 1

The basic magic here is this identity in C:

*(a+i) == a[i]

Okay, now I'll make this be readable English.

Here's the issue: An array name isn't an lvalue; it can't be assigned to. So the line you have with

a = arrayGen(...)

is the problem. See this example:

int main() {
    int a[10];

    a = arrayGen(a,9);

    return 0;
}

which gives the compilation error:

gcc -o foo foo.c
foo.c: In function 'main':
foo.c:21: error: incompatible types in assignment

Compilation exited abnormally with code 1 at Sun Feb  1 20:05:37

You need to have a pointer, which is an lvalue, to which to assign the results.

This code, for example:

int main() {
    int a[10];
    int * ip;

    /* a = arrayGen(a,9);  */
    ip = a ; /* or &a[0] */
    ip = arrayGen(ip,9);

    return 0;
}

compiles fine:

gcc -o foo foo.c

Compilation finished at Sun Feb  1 20:09:28

Note that because of the identity at top, you can treat ip as an array if you like, as in this code:

int main() {
    int a[10];
    int * ip;
    int ix ;

    /* a = arrayGen(a,9);  */
    ip = a ; /* or &a[0] */
    ip = arrayGen(ip,9);

    for(ix=0; ix < 9; ix++)
        ip[ix] = 42 ;

    return 0;
}

---

Full example code

Just for completeness here's my full example:

int gen(int max){
    return 42;
}

int* arrayGen(int arrAddr[], int maxNum)
{
    int counter=0;
    while(arrAddr[counter] != '\0') {
        arrAddr[counter] = gen(maxNum);
        counter++;
    }
    return arrAddr;
}

int main() {
    int a[10];
    int * ip;
    int ix ;

    /* a = arrayGen(a,9);  */
    ip = a ; /* or &a[0] */
    ip = arrayGen(ip,9);

    for(ix=0; ix < 9; ix++)
        ip[ix] = 42 ;

    return 0;
}