Float64类型在Golang中打印为int

Question

Surprisingly I couldn't find anyone else having this same issue; I tried simply initializing a float64 in Go and printing it, then attempting a string conversion and printing that. Neither output was accurate.

I've attempted this with many fractions, including those which don't resolve to repeating decimals, as well as simply writing out the float and printing (e.g. num := 1.5 then fmt.Println(num) gives output 1).

package main

import (
    "fmt"
    "strconv"
)

func main() {
    var num float64
    num = 5/3
    fmt.Printf("%v
", num)
    numString := strconv.FormatFloat(num, 'f', -1, 64)
    fmt.Println(numString)
}

Expected:

// Output:
1.66
1.66

Actual:

// Output:
1
1

Answer 1

The Go Programming Language Specification

Integer literals

An integer literal is a sequence of digits representing an integer constant.

Floating-point literals

A floating-point literal is a decimal representation of a floating-point constant. It has an integer part, a decimal point, a fractional part, and an exponent part. The integer and fractional part comprise decimal digits; the exponent part is an e or E followed by an optionally signed decimal exponent. One of the integer part or the fractional part may be elided; one of the decimal point or the exponent may be elided.

Arithmetic operators

For two integer values x and y, the integer quotient q = x / y and remainder r = x % y satisfy the following relationships:

x = q*y + r  and  |r| < |y|

with x / y truncated towards zero.

---

You wrote, using integer literals and arithmetic (x / y truncates towards zero):

package main

import (
    "fmt"
    "strconv"
)

func main() {
    var num float64
    num = 5 / 3 // float64(int(5)/int(3))
    fmt.Printf("%v
", num)
    numString := strconv.FormatFloat(num, 'f', -1, 64)
    fmt.Println(numString)
}

Playground: https://play.golang.org/p/PBqSbpHvuSL

Output:

1
1

---

You should write, using floating-point literals and arithmetic:

package main

import (
    "fmt"
    "strconv"
)

func main() {
    var num float64
    num = 5.0 / 3.0 // float64(float64(5.0) / float64 (3.0))
    fmt.Printf("%v
", num)
    numString := strconv.FormatFloat(num, 'f', -1, 64)
    fmt.Println(numString)
}

Playground: https://play.golang.org/p/Hp1nac358HK

Output:

1.6666666666666667
1.6666666666666667