Passing array of struct as a function parameter

Hello,

I have a structure and an multidimensional array of that structure. How can I access (read and write) that array's element's fields (without defining it as a global variable) inside a function when the array is declared in main() ?

Here's how I tried it (but failed):

Code:

---

#include <stdio.h>

typedef struct {
    double xy;
    char *message;
} twin;

void print(twin *table, char *m, int i)
{
    (*table[i][1]).message != m ?  <----- error: subscripted value is neither array nor pointer
        printf("\nX = &#37;f Y = %s", *table[i][0].xy, *table[i][1].message): <----- same here (twice)
        printf("\nX = %f Y = %f", *table[i][0].xy, *table[i][1].xy);  <----- same here (twice)
    return;   
}

int main()
{   
    int i, n;
    double x, a, h;   
    char v[] = "empty";
    ...
    twin table[n-1][1];
    ...
    print(&table, &v, 1);
 
  return 0;
}

---

Best wishes,
Desmond5

Why are you dereferencing table in print()?

Hint: Try not doing that.

You are passing a 2D array of structs as a simple pointer [although the second dimension is only "1" wide, looking at the code, you probably actually want to make it 2].

Here's one way to make what you currently have work, however:

Code:

---

void print(twin (*table)[2], char *m, int i)
{
    table[i][0].message != m ? 
        printf("\nX = %f Y = %s", table[i][0].xy, table[i][1].message):
        printf("\nX = %f Y = %f", table[i][0].xy, table[i][1].xy); 
    return;   
}

---

I'm not entirely sure that a terniary operator is the right solution here, and you probably want to use strcmp() rather than != when comparing text strings. [Unless you rely on the address of strings with equal contents to also always be the SAME string - that is, having the same address].

--
Mats

Thanks, I tried to do what you recommended and came up with this:

Code:

---

#include <stdio.h>
#include <string.h>

typedef struct {
    double xy;
    char *message;
} twin;

void print(twin (*table)[2], char *m, int i)
{
    strcmp(table[i][1].message, m) ?
        printf("\nX = &#37;f Y = %s", table[i][0].xy, table[i][1].message):
        printf("\nX = %f Y = %f", table[i][0].xy, table[i][1].xy); 
    return;   
}

int main()
{   
    char v[] = "empty";
   
    int i = 0, n = 10;     
    double somevalue = 2.0;
    twin table[n][2];
     
    for (i = 0; i < n; i++)
    {
        // input some data for the example to actually work
        table[i][0].xy = somevalue; 
        table[i][1].xy = somevalue * 2;       
        print(&table, &v, i);  //<-- [Warning] passing arg 1 of `print' from incompatible pointer type     
                              //    [Warning] passing arg 2 of `print' from incompatible pointer type
    }
 
  return 0;
}

---

But it crashes right after I run it. What could be wrong ? If I comment out the first two lines of the print function it seems to work, so there's something I'm not seeing there..

I'm no sure I understand "twin (*table)[2]". I seems to declare that there are only 2 elements in the array not n*2 as I intended to ?

I use the struct to have a element in the array (table) that can be either a double or string. I store calculations in the array. On some terms, if the function fails to calculate, I instead want to store the error message why the calculation failed in the array.

Best wishes, Desmond5

When you create an element of your struct, are you sure the member message will always be NULL and not pointing to some random location? :)

Code:

---

print(table, &v, i);

---

Remove & before table. Arrays are automatically passed as pointers w/o the need of &.