converting from int to float in C changes value

Question

Hi I'm trying to convert from an int to a float in C and for some reason the cast changes the value and I'm not sure why. So:

fprintf (stderr, "%d,%d\n", rgbValues->green, (float)rgbValues->green);

produces two different numbers. Note that rgbValues->green is an int.

Any idea why this is happening?

Thanks

Answer 1

You have to say that in your format string. Use:

fprintf(stderr, "%d,%f\n", rgbValues->green, (float)rgbValues->green);
                     ^

instead of:

fprintf(stderr, "%d,%d\n", rgbValues->green, (float)rgbValues->green);
                     ^

Note the change from d to f (the circumflex ^ isn't part of the code, just an indicator as to where to look).

Answer 2

Expanding on @AraK's answer, you're casting once to float, and then printf is interpreting that bit pattern as an int in the format specifier string. It's doing exactly what you're telling it to do :)

If you want to only cast to float - do this:

// format specifier is %d for int, %f for float
// show the cast value of rgbValues->green as well as its "real" value
fprintf(stderr, "%d,%f\n", rgbValues->green, (float)rgbValues->green);

Edit - wording based on comments

Answer 3

The %d format specifier says to printf, "take the next 4 bytes off of the stack, interpret them as an integer, and print out that integer." Since you're actually passing a float as a parameter, the bytes of the float (which are stored in IEEE-754 format) are getting misinterpreted as an integer, hence the different values. (Actually, the float is getting converted to a double due to argument promotion within variadic functions, and it's the first 4 bytes of that promoted double that are getting interpreted as an integer.)

The correct solution is to use one of the %e, %f, or %g format specifiers instead of %d when printing out a float. %e says, "take the next 8 bytes off the stack, interpret them as a double, and print it out using scientific (exponential) notation" %f prints out using a fixed-point format, and %g prints out whichever would be shorter of %e or %f.

fprintf(stderr, "%d,%f\n", rgbValues->green, (float)rgbValues->green);