How to convert float to complex?

With the very simple code :

package main
import (
    "fmt"
	"math"
    "math/cmplx"
)
func sqrt(x float64) string {
	if x < 0 {
		return fmt.Sprint(cmplx.Sqrt(complex128(x)))
	}
	return fmt.Sprint(math.Sqrt(x))
}
func main() {
	fmt.Println(sqrt(2), sqrt(-4))
}

I get the following error message :

main.go:11: cannot convert x (type float64) to type complex128

I tried different ways, but couldn't find out how to convert a float64 to complex128 (just to be able to use cmplx.Sqrt() function on a negative number).

Which is the correct way to handle this ?

You don't really want to convert a float64 to complex128 but rather you want to construct a complex128 value where you specify the real part.

For that can use the builtin complex() function:

func complex(r, i FloatType) ComplexType

Using it your sqrt() function:

func sqrt(x float64) string {
	if x < 0 {
		return fmt.Sprint(cmplx.Sqrt(complex(x, 0)))
	}
	return fmt.Sprint(math.Sqrt(x))
}

Try it on the Go Playground.

Note:

You can calculate the square root of a negative float number without using complex numbers: it will be a complex value whose real part is 0 and imaginary part is math.Sqrt(-x)i (so the result: (0+math.Sqrt(-x)i)):

func sqrt2(x float64) string {
	if x < 0 {
		return fmt.Sprintf("(0+%.15fi)", math.Sqrt(-x))
	}
	return fmt.Sprint(math.Sqrt(x))
}