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c++c++14pow

Power function not working as expected

int n;
n = 2;
int d;
d = pow(10,n);
cout<<d<<endl;

It displays 99 instead of 100. I have indcluded math.h

Solution

  • Quite possibly a rounding issue.

    Some implementations of std::pow have special cases for integral arguments. Others just rely on basic math. For instance, pow(x,y) == pow(2, y*log2(x)). The typical reason why an implementation would use this rule is it represents floating-point numbers internally in base 2. But log2(x) may suffer from rounding issues.

    In your case, log2(10) would be 3.3219280948873623478703194294894. That means that pow(10.0, 2.0) can be calculated behind the scenes as pow2(6.6438561897747246957406388589788) = 64 * pow2(0.6438561897747246957406388589788). It's quite possible that the latter term is rounded down, to a value just below the correct 1.5625.