Thread: dynamic array in STRUCT

Hi everyone,

I am new to structs but I need them for some complex problem where the struct not simply contains arrays or pointers, but also arrays of pointers as shown below.

What I can't figure out is how to dynamically allocate the array of pointers defined in the struct.

Can anyone help me with this issue?
thank you in advance. I can write my code without structs, but I rather use them to make it more readable and efficient.

Thank you in advance

Code:

#define PROB_ENTRIES 10
#define MAX_INPUT_ENTRIES 40

typedef struct
{

	char *problem[];
	float PROBVars[MAX_INPUT_ENTRIES]
	
} input;



main()
{
...

char *problem[PROB_ENTRIES];
	for(i=0; i<PROB_ENTRIES; i++)
		problem[i] = (char*) malloc(32 * sizeof(char *));
		
...
}

Originally Posted by GL.Sam

Add a semicolon.


Remove an asterisk.

Better now?

btw, don't use structs for such a simple case. I don't even understand why are you declaring *problem[] over again.

Hi Sam, thanks for replying. I sent just a simple psuedo-code as example but I think I didn't explain myself correctly.

The struct that I want to use is actually going to contain many elements (arrays of loats, arrays of pointers, strings, constants, etc.), but most of the arrays and arrays of pointers that I want to use inside the struct are supposed to be dynamic.

What I cannot do correctly is dynamically allocate them and I don't know whether that step must be done inside the struct or in the main.c

Thank you again

Hi Sam, thank youi very much! This helps a lot

All the best,
cfd

Originally Posted by GL.Sam

No problem. Uh, and don't forget about the free()! I myself did.

sure; about details I'll take care of them. I'm new to structs but still proficient in C

thank you again

Originally Posted by zacs7

Just 2 tips:
* Drop the sizeof(char) since it's always 1.
* Drop the cast on malloc()

Hi Zacs, thanks for the hints

Best,
cfd

Originally Posted by Salem

> Drop the sizeof(char) since it's always 1.
Writing
p = malloc ( sizeof(*p) * numRequired );
will save you from doing something like

int *p = malloc( 10 );
in the belief that you've allocated 10 int's.

Better still, because the sizeof uses the pointer variable itself for working out the size, you don't have to look back to make sure that the declaration type, and the type in the sizeof agree with one another. Change one, and the other is instantly correct!

Choose one idiom, that always works.

Optional programming, like
- leaving braces off single-statements inside a control (for, if etc)
- sizeof(char) being 1 for malloc
- optional ( ) in complex arithmetic or boolean expressions
will, sooner or later, mutate from"simple" code into something not so simple, and the optional becomes necessary. It might be months before the bug is noticed, and weeks more before it's finally tracked down and fixed.

Hi Salem,

great hints; thanks so much

Best regards,
cfd

Originally Posted by GL.Sam

Don't ever try to allocate anything being in a struct block. And if you use array notation [] in there, it shouldn't be empty. It goes smt like following:

Code:

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

#define PROB_ENTRIES 10


struct input
{

	char *problem[PROB_ENTRIES];
	
};


int main(void)
{
        struct input huh;
	int i;

	for (i = 0; i < PROB_ENTRIES; i++)
	{
		huh.problem[i] = (char*) malloc(32 * sizeof(char));
        	strcpy(huh.problem[i], "What?");
		printf("%s\n", huh.problem[i]);
	}
	
        return 0;

}

Hi again Sam,

I wrote a *.h file that contains my struct, and the definition of the function that uses the struct elements, and the corresponding READ_INPUT.c file and main calling READ_INPUT()

1) READ_INPUT.h:

Code:

//Struct for the Boundary condition codes and values:
typedef struct struct_bc
{
    unsigned int nbdy_nodes;	// Number of boundary nodes
    unsigned int *NODE;			// Node index number
    unsigned int *BDY_CODE;		// Array of the boundary codes
    float *BDY_VALUES;			// Array of boundary values: size [nbdy_nodes]x[nvariables]

}bc ;	//bc as Boundary Conditions	

void READ_BC(char *bdy_file, bc *BDYCS);

2) READ_INPUT.c:

Code:

void READ_BC(char *bdy_file, bc *BDYCS)
{
	FILE *bdyf_ID;
	char header[16];
	
	BDYCS.NODE = (unsigned int*) malloc(10 * sizeof(unsigned int *));

	free(BDYCS.NODE);
	
return;
}

but I get the following error in compilation:

Code:

READ_INPUT.c: In function ‘READ_BC’:
READ_INPUT.c:127: error: request for member ‘NODE’ in something not a structure or union
READ_INPUT.c:151: error: request for member ‘NODE’ in something not a structure or union
make: *** [main.o] Error 1

what am I doing wrong in using the structure arrays to allocate?

thanks a lot

Use:

Code:

   BDYCS->NODE = (unsigned int*) malloc(10 * sizeof(unsigned int *));

Originally Posted by slingerland3g

Use:

Code:

   BDYCS->NODE = (unsigned int*) malloc(10 * sizeof(unsigned int *));

Thank you