user2809114 user2809114 - 2 months ago 18
C Question

Pass Array by Reference in C

I am trying to pass my array by reference so that I can get around the fact that I cant return an array from a function. I believe I am following the examples that I have seen correctly. Whats going wrong, though? I am expecting the output to be the numbers 0 - 9, but rather I am getting something strange.

Currently my program does not do what is says. It doesn't do any calculations, its only currently testing the pass by reference. I want to make sure that I know how to manipulate the array before I start trying any calculations.

What I am trying to do: create an array of the numbers 0-9, pass it by reference into a function, print each element within the function.

The actual program will create an empty array, send it to the function, then the function will fill the array with prime numbers.

#include <stdio.h>

void primesUpTo(int max,int *resPt);
void main()
{
const int MAX = 10;
int primes[MAX];

/*TEST*/for(int i = 0; i < MAX;primes[i] = ++i)

primesUpTo(MAX,&primes);
}

/* Primes up to function
* Fills array that *resPt points to with primes from 0 to 'max'
* *resPt: pointer to first element of result array
* max: int, highest prime number in result array
*
* *resPt must be passed as reference
*/
void primesUpTo(int max, int *resPt)
{
/*TEST*/for(int i = 0; i < max; i++)
printf("\nTEST: %d",*(resPt + i));
}


My compile error and then output:

steve@steve-VirtualBox:~/C_Programs/CIS/hw_2/ex_3$ gcc -o test printPrimes.c
printPrimes.c: In function ‘main’:
printPrimes.c:11:17: warning: passing argument 2 of ‘primesUpTo’ from incompatible pointer type [-Wincompatible-pointer-types]
primesUpTo(MAX,&primes);
^
printPrimes.c:3:6: note: expected ‘int *’ but argument is of type ‘int (*)[(sizetype)MAX]’
void primesUpTo(int max,int *resPt);
^
steve@steve-VirtualBox:~/C_Programs/CIS/hw_2/ex_3$ ./test

TEST: 1835627636
TEST: 1600061541
TEST: 1869833334
TEST: 1952802655
TEST: 0
TEST: 0
TEST: 0
TEST: 0
TEST: 1
TEST: 0
TEST: 1835627636
TEST: 1
TEST: 1869833334
TEST: 1952802655
TEST: 0
TEST: 0
TEST: 0
TEST: 0
TEST: 1
TEST: 0
TEST: 1835627636
TEST: 1
TEST: 2
TEST: 1952802655
TEST: 0
TEST: 0
TEST: 0
TEST: 0
TEST: 1
TEST: 0
TEST: 1835627636
TEST: 1
TEST: 2
TEST: 3
TEST: 0
TEST: 0
TEST: 0
TEST: 0
TEST: 1
TEST: 0
TEST: 1835627636
TEST: 1
TEST: 2
TEST: 3
TEST: 4
TEST: 0
TEST: 0
TEST: 0
TEST: 1
TEST: 0
TEST: 1835627636
TEST: 1
TEST: 2
TEST: 3
TEST: 4
TEST: 5
TEST: 0
TEST: 0
TEST: 1
TEST: 0
TEST: 1835627636
TEST: 1
TEST: 2
TEST: 3
TEST: 4
TEST: 5
TEST: 6
TEST: 0
TEST: 1
TEST: 0
TEST: 1835627636
TEST: 1
TEST: 2
TEST: 3
TEST: 4
TEST: 5
TEST: 6
TEST: 7
TEST: 1
TEST: 0
TEST: 1835627636
TEST: 1
TEST: 2
TEST: 3
TEST: 4
TEST: 5
TEST: 6
TEST: 7
TEST: 8
TEST: 0
TEST: 1835627636
TEST: 1
TEST: 2
TEST: 3
TEST: 4
TEST: 5
TEST: 6
TEST: 7
TEST: 8
TEST: 9

Answer

Arrays are always passed by reference in C. The name of the array is pointer to the first element of it. So, you just do this :-

void function (int arr[]){
// Some Code.....
}
int main(){
// Some Code...
int name[5];
function(name);
// Some Code...
}

And that would work, you can modify the values of elements in the array and the changes would be reflected in the calling function.

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