Constant truncated to integer

问题:

The following GO program gives the error:

./fft.go:13: constant -6.28319 truncated to integer
./fft.go:13: cannot use -7 * k / N (type int) as type float64 in assignment

Program:

package main

import (
    "math"
    "fmt"
)

func main() {
    fmt.Println("Hello world ",math.E)

    var k, N int = 1, 10
    var ans float64 = 0
    var c float64 = (-2.0 * math.Pi * k) / N
    x := make([]float64,N)
    for i := 0; i < len(x); i++ {
        x[i] = 1
    }
    ans = 0
    for i := 0; i < N; i++ {
        ans += x[i] * math.E
    }
    fmt.Println(ans)
}

Why cant I use an int in a type of float64 ?

回答1:

Replace

var c float64 = (-2.0 * math.Pi * k) / N

by

var c float64 = (-2.0 * math.Pi * float64(k)) / float64(N)

To quote the spec:

Conversions are required when different numeric types are mixed in an expression or assignment. For instance, int32 and int are not the same type even though they may have the same size on a particular architecture.

Go uses static typing and doesn't automatically convert between numeric types. The reason is probably to avoid some errors. For instance, what value and what type should float64(2.5) * int(2) yield? Should the result be int(5)? int(4)? float64(5.0)? In Go, this isn't an issue. The Go FAQ has more to say on this.

---

@jnml points out that, in this case, the following is enough:

var c float64 = -2 * math.Pi / float64(N)