Why is the output of this not what it should be?

Question

So I had a assignment to calculate 10^100, had to use the pow() function in result. So here's the code:

#include <stdio.h>
#include <math.h>
int main()
{
    long double result;
    result = pow(10, 100);
    printf("10^100 = %Lf", result);
}

If I assign pow(10, 100) to result, the result displayed is 100000000000000001590289110975991804683608085639452813897813275577478387721703810608. But, if I write the code like this:

#include <stdio.h>
#include <math.h>
int main()
{
    printf("10^100 = %Lf", pow(10, 100));
}

I get what I'm supposed to get i.e. 10000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.000000.

So someone please explain why this is happening and how to avoid this?

Answer 1

It seems that in the latter case, the compiler used constant folding to replace the printf with an exact string. But in the former case it used the closest result presentable in long double, and that's the best you can get.

Notice that you should use powl, not pow for the long double result. In the latter case it is not only should but must, as %Lf will expect a long double, not double in the arguments.

To have more precise output at runtime, you need to use an arbitrary precision math library or build one yourself.