unions?
atszyman said:
Ok, things make a little more sense now but if I modify the first while loop to :
while ((float)eps != 0.0)
{
printf("After step %d, eps equals : %x\n", step, (float)eps);
eps = eps/2.0;
step+=1;
}
I get these as the hex values out:
After step 1, eps equals : 3ff00000
After step 2, eps equals : 3fe00000
After step 3, eps equals : 3fd00000
After step 4, eps equals : 3fc00000
After step 5, eps equals : 3fb00000
After step 6, eps equals : 3fa00000
.
.
.
As you can see the exponent bits which should occupy bits 30:23 are actually occupying 30:20 which matches for double precision. Is there any way to get the single precision values out?
Thanks again, in advance, for answering what will probably turn out to be another blatantly obvious question.
Ok, this took a few minutes. I haven't yet figured out why, but the printf you're doing with the cast to the float is not working. Your float variables are fine as they are - the problem in your program is only with the printf where the float still gets promoted to double.
When you see tricky problems like this, the first thing you ought to do is turn on all warnings during compilation - this is done by using the '-Wall' command line option in gcc (which I presume you're using) - for example, 'gcc -Wall test.c'. If you do this, you see a warning about "format %x expects 'unsigned int', but argument 3 has type 'double'" - this occurs with the (float) cast in the printf!
When you wish to interpret bit patterns in hexadecimal (or byte format), it is better to match the basic integer type - what I mean by this is that when you attempt to use a '%x' for printf, the best argument would be an unsigned int (as the warning above indicated). Now the question is, how would you get a floating point number in an unsigned int without truncation?
Use a union!
Declare something like
union floatint
{
float f;
unsigned int i;
};
and declare a variable like
union floatint fi;
Before your printf, do this:
fi.f = eps;
Here, you're assigning the value of eps into the float variable f. Since f is part of a union, it can also be reinterpreted as an int using the member called i. In a union, all variables share the same memory storage - so the most common way of using unions is to assign values to some members and extract the values from the other members to interpret the same set of bits in a different way.
In your printf, use the int part of this union instead of using eps, like:
printf("step %d, eps = %x\n",step,fi.i);
Viola! You have good results now!
Note: Although this has temporarily solved your problem, I would recommend digging into the manual for gcc to find out why the automatic promotion occurs even with a cast.
