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Question

Hi I have a few doubts about how to access the members of a struct, in case that the struct is recived by reference by some function.

here is a program that generates a random "image" and have 2 examples of my problems:

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

#define WIDTH 16
#define HEIGHT 8

typedef struct
{
    int width;   
    int height; 
    int maxvalue;
    char *pixels;
} image;


void setimagemaxvalue (image *img)
{

    scanf ("%d", &((*img).maxvalue); //I know is wrong 

}

void createimage (image *img) //just creates a random matrix of pixels
{
        (*img).pixels=malloc((img->width)*(img->height));

        int i,j;

    char *tmp;
    tmp=(*img).pixels;

    for (i=0;i<(img->height);i++)
        {
        for (j=0;j<(img->width);j++)
        {
            *(tmp+(i*(img->width)+j))=(char)(rand()%(img->maxvalue));
        }

    }   

}



int main ()
{
    srand(time(NULL));
    image img;

    img.width=WIDTH;
    img.height=HEIGHT;

    setimagemaxvalue(&img);
    createimage (&img);

    return 0;
}

My questions are:

1) What should be the parameter send to the scanf function? (forget about the buffer cleaning issue, and yes, I know that I can just send the maxvalue member of the struct to the "setimagemaxvalue" function, but this is just an example, I want to know how to do it considering that it's a member of a struct.

2) How could I write the function "createimage" without using the tmp variable (there is the pointer img referencing the pointer pixels and it's referencing the data).

Answer 1

1) About the parameter of scanf

scanf ("%d", &((*img).maxvalue));

is right way to read the value. Alternatively you can also use

scanf ("%d", &(img->maxvalue));

Similarly you can replace (*img).pixels with img->pixels if you think later is more readable.

2) You can replace all occurances of tmp with img->pixels

Answer 2

You start by writing: 'received by reference.' , but you are actually passing you struct by pointer. Others have showed that you can use -> which is the correct way to address members when using pointer.

But I think in your case (looking at main) you would be better of passing by reference instead. eg

void setimagemaxvalue (image &img)
{
    scanf ("%d", &(img.maxvalue);
}

that way you can use . instead of -> and in your main it would look like this:

image img;
....

setimagemaxvalue( img );

No ampersand in front of img

Best regards Asger-P