unusual output from pow

Question

The following C code

int main(){
    int n=10;    
    int t1=pow(10,2);
    int t2=pow(n,2);
    int t3=2*pow(n,2);
    printf("%d\n",t1);
    printf("%d\n",t2);
    printf("%d\n",t3);
    return (0);
}

gives the following output

100
99
199

I am using a devcpp compiler. It does not make any sense, right? Any ideas? (That pow(10,2) is maybe something like 99.9999 does not explain the first output. Moreover, I got the same output even if I include math.h)

Answer 1

You are using a poor-quality math library. A good math library returns exact results for values that are exactly representable.

Generally, math library routines must be approximations both because floating-point formats cannot exactly represent the exact mathematical results and because computing the various functions is difficult. However, for pow, there are a limited number of results that are exactly representable, such as 10². A good math library will ensure that these results are returned correctly. The library you are using fails to do that.

Answer 2

Store the result computations as doubles. Print as double, using %f instead of %d. You will see that the 99 is really more like 99.999997, and this should make more sense.

In general, when working with any floating point math, you should assume results will be approximate; that is, a little off in either direction. So when you want exact results - like you did here - you're going to have trouble.

You should always understand the return type of functions before you use them. See, e.g. cplusplus.com:

double pow (double base, double exponent); /* C90 */

From other answers I understand there are situations when you can expect pow or other floating-point math to be precise. Once you understand the necessary imprecision that plagues floating point math, please consult these.