Attempt to access elements of a 2d struct array failing-CodePudding

    typedef struct{
        unsigned long a;
        unsigned long b;
        unsigned long c;
    } mini_struct;    


    struct ministruct** build_2Dstruct(unsigned long x, unsigned long y){
        double x_squared = pow(2, x);
        struct ministruct** temp = (mini_struct**)malloc(x*sizeof(mini_struct*));
        for(int i = 0; i < x_squared; i  ){
            temp[i] = (mini_struct*)malloc(y*sizeof(mini_struct));
            for(int j = 0; j < y; j  ){
                temp[i][j].a = 0;
                etc....
            }
        }
        return temp;
    }

In the code above I am trying to create a 2D array of ministructs**, with the whole struct being made out of 2^x ministructs*, and each ministruct* has y amount of ministructs.

aka: x = 2, y = 2,

[[struct, struct], [struct, struct], [struct, struct], [struct, struct]]

However, for some reason when I try to access the second element or index 1 of the struct inside each struct*, it says there is an error: "expression must be pointer to complete object".

I just do not understand why the code is not allowing me to access each individual element of the elements of the array?

Thanks

CodePudding user response：

You are trying to make an x by y array of structs. So:

// create array of x pointers
mini_struct **temp = malloc(x*sizeof(mini_struct*));
for (int i=0; i<x; i  ) {
    // to array of y structs
    temp[i] = malloc(y*sizeof(mini_struct));
    for (int j=0; j < y; j  ) {
        temp[i][j].a = 0;
        ... etc.

CodePudding user response：

Question is incomplete so I will be making asumptions.

You seem to be wanting to allocate a 2D array of structs and initialize all members to 0. Here is a possible solution:

#include <stdio.h>
#include <math.h>
#include <stdlib.h>

typedef struct mini_struct{
    unsigned long a;
    unsigned long b;
    unsigned long c;
} mini_struct;

struct mini_struct** build_2Dstruct(unsigned long x, unsigned long y){
    double x_squared = pow(x, 2);
    mini_struct **temp = (mini_struct **) malloc(x_squared * sizeof(mini_struct*));
    
    for(int i = 0; i < x_squared; i  ){
        temp[i] = (mini_struct *) calloc(y, sizeof(mini_struct));
    }

    return temp;
}

int main () {
    int x = 3;
    int y = 4;

    mini_struct **struct2D = build_2Dstruct(x, y);

    int x_squared = pow(x,2);
    for (int i = 0; i < x_squared;   i) {
        for (int j = 0; j < y;   j) {
            printf("Value of data stored at struct[%d][%d] is: %d\n", i, j, struct2D[i][j]);
        }
    }
    
    for (int i = 0; i < x_squared;   i) {
            free(struct2D[i]);
        }
    free(struct2D);

}

As you can see, this contains the whole program, not just the snippet you showed. In this case, a main function would have been useful so that we don't have to guess what you want to do. My solution creates the 2D array with all elements initialized to 0 (you can use calloc to do that, no need for a second for loop).

Another important point is that, because the function returns a newly heap allocated 2D array, you need to free it to avoid a memory leak (end of main function).

CodePudding user response：

You allocate x pointers to mini_struct:

mini_struct **temp = (mini_struct **) malloc(x_squared * sizeof(mini_struct*));

But then when you initialize them:

    for(int i = 0; i < x_squared; i  ){
        temp[i] = (mini_struct *) calloc(y, sizeof(mini_struct));
    }

You index temp based on upto x_squared.

Consider if x is 2. You would allocate temp to be an array of two pointers to mini_struct. But then your for loop would attempt to initialize four elements in temp.