C: Comparing two bytes to see if one is within a certain range of the other

Question

I am writing code that compares 2 bytes which represent integers. I want to see if byte R is with +-10 of G. The problem I am having with the code is with the comparison in the if-statment near the end. The bytes never come out as being out of range, even when they should. I am sure the problem comes from how I am adding/subtracting the error_range, but I don't know any other way to do it.

I first considered converting the bytes into integers but I cannot find any help with that online. If that would work better than what I am doing here, please tell me how to do it.

Any help is appreciated!

const char ERROR_RANGE = 0x1010; //warning: overflow in implicit constant conversion
char R, G; /2 separate bytes
char buffer; //enough space for 1 byte

image = fopen(fileName,"r"); //open file

fread(&buffer, 1, 1, image); //read 1 byte  
memcpy (&R,&buffer,1); //store it as R

fread(&buffer, 1, 1, image); //read 1 byte   
memcpy (&G,&buffer,1); //store it as G

fclose(image);

if((R >= (G + ERROR_RANGE)) && (R <= (G - ERROR_RANGE)))
{
    printf("Outside of range!\n");
}

Thanks.

Answer 1

Your problem is because your test says:

if (R is greater than or equal to G + ERROR) AND (R is less than or equal to G - ERROR)

it can't be both.

Replace the && with || in the first instance.

A better test would be:

if (the difference of R and G is greater than ERROR)

which translates to:

if (abs(R - G) > ERROR_RANGE)
{
    printf("Error");
}

Answer 2

First problem is that you're using &&, not ||. R isn't going to be both too high and too low.

Second, are you sure that R and G will be within reasonable bounds? If char on your system is unsigned, then G - ERROR_RANGE may well be a large number if G is small, not a small one. You're probably best off with something like if (abs(R - G) <= ERROR_RANGE).

Answer 3

Is there some reason you couldn't use something like:

if (abs(R-G) > ERROR_RANGE)
    // ...

Answer 4

Here's my stab at it, UNTESTED:

unsigned char buffer[2];
int r, g, diff;

image = fopen(fileName, "r");
fread(&buffer, 2, 1, image);

r = buffer[0];
g = buffer[1];
dif = r - g;

if (abs(dif) > 10) { printf... }

It uses the abs function to make the difference always positive... easier to compare that way. A slight performance improvement from reading both of the first 2 bytes in one go. Assigning the (unsigned) chars to ints will make them easily comparable.

Answer 5

Aside from what's already been pointed out...

The value 0x1010 is too large to fit in a byte, and two bytes can never be different by more than 255 (0xff). Did you mean 1010 binary (== 10 decimal == 0xA hex)?

Also, there's no need to read and copy, you can just read into your variables directly:

fread(&R, 1, 1, image); //read 1 byte  
fread(&G, 1, 1, image); //read 1 byte