如何在for循环初始化中声明uint8，因为for循环初始化中不允许使用var

Question

I am writing following code to calculate Population Count as below:

package main

import (
    "fmt"
)

func main() {
    var pc [256]byte

    for i := range pc {
        pc[i] = pc[i/2] + byte(i&1)
    }

    var x uint64 = 65535
    var population uint8

    for i := 0; i < 8; i++ {
        population  = population + pc[byte(x>>(i*8))]
    }

    fmt.Printf("Population Count: %d", population)
}

On compilation I get following error:

prog.go:19:39: invalid operation: x >> (i * 8) (shift count type int, must be unsigned integer)

The problem is with the variable i which is declared while initializing the for loop. The shorthand initialization of i makes it of type int which in internally of type signed int32. This is why the following line:

population  = population + pc[byte(x>>(i*8))]

is causing the problem. Because shift operator >> is expecting only unsigned integer, but the expression i*8 results into int which is internally signed int32.

As var can not be used in for initialization.

for var i uint8 = 0; i < 8; i++ { ... }

syntax error: var declaration not allowed in for initializer

I can declare variable i outside of for loop, but I do not find it clean and readable.

What should be the clean way to achieve the same?

Answer 1

The Go Programming Language Specification

Variable declarations

... each variable is given the type of the corresponding initialization value in the assignment.

---

i := uint(0). For example,

package main

import (
    "fmt"
)

func main() {
    var pc [256]byte

    for i := range pc {
        pc[i] = pc[i/2] + byte(i&1)
    }

    var x uint64 = 65535
    var population uint8

    for i := uint(0); i < 8; i++ {
        population = population + pc[byte(x>>(i*8))]
    }

    fmt.Printf("Population Count: %d", population)
}

Playground: https://play.golang.org/p/H_4cn9w_dOq

Output:

Population Count: 16