How to use Math.Pow with integers in Golang

I keep getting the error "cannot use a (type int) as type float64 in argument to math.Pow, cannot use x (type int) as type float64 in argument to math.Pow, invalid operation: math.Pow(a, x) % n (mismatched types float64 and int)"

func pPrime(n int) bool { var nm1 int = n - 1 var x int = nm1/2 a := 1; for a < n { if (math.Pow(a, x)) % n == nm1 { return true } } return false } 

4 Answers

func powInt(x, y int) int { return int(math.Pow(float64(x), float64(y))) } 

In case you have to reuse it and keep it a little more clean.

If your inputs are int and the output is always expected to be int, then you're dealing with 32-bit numbers. It's more efficient to write your own function to handle this multiplication rather than using math.Pow. math.Pow, as mentioned in the other answers, expects 64-bit values.

Here's a Benchmark comparison for 15^15 (which approaches the upper limits for 32-bit representation):

// IntPow calculates n to the mth power. Since the result is an int, it is assumed that m is a positive power func IntPow(n, m int) int { if m == 0 { return 1 } result := n for i := 2; i <= m; i++ { result *= n } return result } // MathPow calculates n to the mth power with the math.Pow() function func MathPow(n, m int) int { return int(math.Pow(float64(n), float64(m))) } 

The result:

go test -cpu=1 -bench=. goos: darwin goarch: amd64 pkg: pow BenchmarkIntPow15 195415786 6.06 ns/op BenchmarkMathPow15 40776524 27.8 ns/op 

I believe the best solution is that you should write your own function similar to IntPow(m, n int) shown above. My benchmarks show that it runs more than 4x faster on a single CPU core compared to using math.Pow.

Since nobody mentioned an efficient way (logarithmic) to do Pow(x, n) for integers x and n is as follows if you want to implement it yourself:

// Assumption: n >= 0 func PowInts(x, n int) int { if n == 0 { return 1 } if n == 1 { return x } y := PowInts(x, n/2) if n % 2 == 0 { return y*y } return x*y*y } 

If you want the exact exponentiation of integers, use (*big.Int).Exp. You're likely to overflow int64 pretty quickly with powers larger than two.