access variable element of array in C

Question

im new to programming so this may have a simple solution. Heres what im trying to do.

My program loads a .bmp image and then takes the width and height to find out how many pixels the image has. Then it uses calloc() to make three arrays for the RGB values of each pixel, theres the array for RED, BLUE, GREEN. I have used a while() loop to take the first pixel and put the RGB values in the first element of their respective arrays. Then it moves on and does the same for the second pixel, third, fourth... This is where my issue is, it doesnt seem to want to put the values in the arrays. I did it like this,

while(temp=fgetc(fp) != NULL)
{
BLUE[current_pixel]=temp;

temp=fgetc(fp);
GREEN[current_pixel]=temp;

temp=fgetc(fp);
RED[current_pixel]=temp;

current_pixel++;
}

current_pixel is a variable that keeps track of what pixel Im looking at right now.

So I guess what I really want to know is why can't i do BLUE[current_pixel]=temp; I don't recieve any errors when compiling and I used a printf() statement to check where the problem is.

I have tried BLUE[1]=temp and it works fine, but thats no good for my program because I can't move on to the next pixel to save values.

Thank you in advance for any help!

EDIT: I still cant get it working so im just going to post the entire program here.

#include <stdio.h>
#include <stdlib.h>
int main()
{
FILE*fp;
int current_pixel=0;
int temp=0;
int paddingremoved=0;
int cycle=0;
int *RED;
int *BLUE;
int *GREEN;
int imgstrt=0;
int width=0;
int height=0;
int padding=0;

fp=fopen("C:\\Users\\Jason\\Documents\\test.bmp","rb");

if(fp==NULL)
{
printf("Error: File could not be opened");
getchar();
return(0);
}


fseek(fp,10,SEEK_SET);
fread(&imgstrt,1,1,fp);
printf("Image Starts At:%d\n",imgstrt);

fseek(fp,18,SEEK_SET);
fread(&width,4,1,fp);
printf("Image Width:%d\n",width);

fseek(fp,22,SEEK_SET);
fread(&height,4,1,fp);  
printf("Image Height:%d\n",height);

padding=(4 -(width*3)%4)%4;
printf("Padding:%d\n",padding);
getchar();

RED = (int*)calloc(height*width+1,sizeof(int));
GREEN = (int*)calloc(height*width+1,sizeof(int));
BLUE = (int*)calloc(height*width+1,sizeof(int));

if(RED == NULL)
{printf("Red Allocation Faliure\n");}
else{printf("Red Allocation Successful\n");}

if(GREEN == NULL)
{printf(" Green Allocation Faliure\n");}
else{printf("Green Allocation Successful\n");}

if(BLUE == NULL)
{printf("Blue Allocation Faliure\n");}
else{printf("Blue Allocation Successful\n");}

fseek(fp,54,SEEK_SET);
/*---------------------Main Loop--------------------------------*/
while((temp = fgetc(fp)) != EOF)
{
    BLUE[current_pixel]=temp;
    temp=fgetc(fp);
    GREEN[current_pixel]=temp;
    temp=fgetc(fp);
    RED[current_pixel]=temp;
    cycle++;
    current_pixel++;

    printf("[%d,%d,%d::%d] ",RED[current_pixel],GREEN[current_pixel],BLUE[current_pixel],current_pixel);
/*------------------------------------------------------------------*/

    /*---------------------Padding Remover-----------------------*/
    if(cycle==width)
    {printf("\n");
    while (paddingremoved!=padding)
    {fgetc(fp); 
                paddingremoved++;}
    cycle=0;
    paddingremoved=0;}
    /*-----------------------------------------------------------*/
}

getchar();
free(RED);
free(BLUE);
free(GREEN);
return(0);
}

what is the initial value of current_pixel? (presumably zero, but not shown) — Mitch Wheat
– Mitch Wheat, Commented Dec 9, 2012 at 0:44
Wherever the problem is, it's in the code you're not showing us. — Brian Roach
– Brian Roach, Commented Dec 9, 2012 at 0:46
Oh wow ... wasn't even looking at the LOOP. I presumed that was working. Was completely hung up on current_pixel being garbage; good catch. — Brian Roach
– Brian Roach, Commented Dec 9, 2012 at 0:59

zch · Accepted Answer · 2012-12-09 00:54:27Z

4

Due to C operator precendence, your while condition reads as:

temp=(fgetc(fp) != NULL)

and not

(temp=fgetc(fp)) != NULL

Use parens. Also, fgetc return EOF on error/finish, not NULL.

answered Dec 9, 2012 at 0:54

zch

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Comments

Nik Bougalis · Accepted Answer · 2012-12-09 01:19:02Z

1

Your code always sets every BLUE pixel to 1, because of operator precedence. The while() loop condition compares the result of fgetc(fp) with NULL, and then sets temp to the result of that comparison (which is 0 or 1). So every BLUE pixel will be set to 1.

Try this instead:

while((temp = fgetc(fp)) != EOF)

edited Dec 9, 2012 at 1:19

answered Dec 9, 2012 at 0:56

Nik Bougalis

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