如果y超过64，为什么pow函数返回0？

Question

Why is the result of pow zero if y is greater than 64?

package main

import (
    "fmt"
)

func pow(x uint64, y uint64) uint64 {
    if y > 64 {
        return 0
    }
    var result uint64 = 1
    var counter uint64
    var previous uint64
    for y > 0 {
        previous = result
        result = result * x
        y = y - 1
        counter++
        if result == 0 {
            return previous
        }
    }
    return result
}

func main() {
    result1 := pow(2, 64)
    fmt.Println(result1)
    result2 := pow(2, 32)
    fmt.Println(result2)
    result3 := pow(2, 3)
    fmt.Println(result3)
}

I just realized that it is because it is in base 2. What can you say about this? (I am still new to programming and with golang.)

Answer 1

Your pow shouldn't be based on what is y. It would be less for bigger number.

Use this:

`

// Assuming that b will never be 0
func mult(a, b uint64) (uint64, bool) {
    result := a * b
    return result, (result/b == a)
}

func pow(x uint64, y uint64) uint64 {
if y == 0 {
    return 1
}
if x == 0 {
    return 0
}
var result uint64 = 1
var counter uint64
var previous uint64
var ok bool
for y > 0 {
    previous = result
    result, ok = mult(result, x)
    if !ok {
        return 0
    }

    y = y - 1
    counter++
    if result == 0 {
        return previous
    }
}
    return result
}

Old answer before clarification:

Actually it should be 63. It is because uint64 can have at max can have largest number as 2 ^ (64) -1 (2 raise to the power 64 minux 1). So maximum number that can be generated for 2 power is 2 ^ 63.

You can confirm that by running your code. It would give you same result as for all number greater that 63 if you remove the restriction of y > 64 . (9223372036854775808 or 2^63). That restriction should be y > 63.