Comparison Efficiency

Question

What is generally faster:

if (num >= 10)

or:

if (!(num < 10))

Answer 1

In general any speed difference won't matter a great deal, but they don't necessarily mean exactly the same thing.

In many languages, comparing the floating point value NaN returns false for all comparisons, so if num = NaN, the first is false and the second true.

#include 
#include 

int main ( ) {
    using namespace std;

    double num = numeric_limits::quiet_NaN();

    cout << boolalpha;
    cout << "( num >= 10 )      " << ( num >= 10 ) << endl;
    cout << "( ! ( num < 10 ) ) " << ( ! ( num < 10 ) ) << endl;

    cout << endl;
}

outputs

( num >= 10 )      false
( ! ( num < 10 ) ) true

So the compiler can use a single instruction to compare num and the value 10 in the first case, but in the second may issue a second instruction to invert the result of the comparison. ( or it may just use a branch if zero rather than branch if non-zero, you can't say in general )

Other languages and compilers will vary, and for types where they really have the same semantics the code emitted might well be identical.