来自同一个Go程序的不同输出

Question

Here is my Go code: http://play.golang.org/p/CDUagFZ-rk

package main

import "fmt"

func main() {
    var max int = 0
    for i := 0; i < 1000000; i++ {
        var len int = GetCollatzSeqLen(i)
        if len > max {
            max = len
        }
    }

    fmt.Println(max)

}

func GetCollatzSeqLen(n int) int {
    var len int = 1
    for n > 1 {
        len++
        if n%2 == 0 {
            n = n / 2
        } else {
            n = 3*n + 1
        }
    }
    return len

}

On my local machine, when I run the program, I get 525 as the output. When I run it on the Go Playground, the output is 476.

I am wondering what's different.

Answer 1

It's because of the implementation-specific size of int, 32 or 64 bits. Use int64 for consistent results. For example,

package main

import "fmt"

func main() {
    var max int64 = 0
    for i := int64(0); i < 1000000; i++ {
        var len int64 = GetCollatzSeqLen(i)
        if len > max {
            max = len
        }
    }

    fmt.Println(max)

}

func GetCollatzSeqLen(n int64) int64 {
    var len int64 = 1
    for n > 1 {
        len++
        if n%2 == 0 {
            n = n / 2
        } else {
            n = 3*n + 1
        }
    }
    return len

}

Output:

525

Playground: http://play.golang.org/p/0Cdic16edP

---

The Go Programming Language Specification

Numeric types

 int32       the set of all signed 32-bit integers (-2147483648 to 2147483647)
 int64       the set of all signed 64-bit integers (-9223372036854775808 to 9223372036854775807) 

The value of an n-bit integer is n bits wide and represented using two's complement arithmetic.

There is also a set of predeclared numeric types with implementation-specific sizes:

 uint     either 32 or 64 bits
 int      same size as uint 

To see the implementation-specific size of int, run this program.

package main

import (
    "fmt"
    "runtime"
    "strconv"
)

func main() {
    fmt.Println(
        "For "+runtime.GOARCH+" the implementation-specific size of int is",
        strconv.IntSize, "bits.",
    )
}

Output:

For amd64 the implementation-specific size of int is 64 bits.

On Go Playground: http://play.golang.org/p/7O6dEdgDNd

For amd64p32 the implementation-specific size of int is 32 bits.