当参数为int时为什么会出现编译错误“无法在...的参数中使用...作为uint8类型”

Question

我刚接触Go，正在解决Go编程语言中的问题。代码应使用随机的Lissajous图形创建GIF动画，并使用以下不同颜色生成图像palate：

// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/

// Run with "web" command-line argument for web server.
// See page 13.
//!+main

// Lissajous generates GIF animations of random Lissajous figures.
package main

import (
    "image"
    "image/color"
    "image/gif"
    "io"
    "math"
    "math/rand"
    "os"
)

//!-main
// Packages not needed by version in book.
import (
    "log"
    "net/http"
    "time"
)

//!+main
// #00ff55
var palette = []color.Color{color.RGBA{0x00, 0xff, 0x55, 0xFF}, color.Black, color.RGBA{0x00, 0x00, 0xff, 0xFF}, color.RGBA{0xff, 0x00, 0xff, 0xFF}}

const (
    whiteIndex = 0 // first color in palette
)

func main() {
    //!-main
    // The sequence of images is deterministic unless we seed
    // the pseudo-random number generator using the current time.
    // Thanks to Randall McPherson for pointing out the omission.
    rand.Seed(time.Now().UTC().UnixNano())

    if len(os.Args) > 1 && os.Args[1] == "web" {
        //!+http
        handler := func(w http.ResponseWriter, r *http.Request) {
            lissajous(w)
        }
        http.HandleFunc("/", handler)
        //!-http
        log.Fatal(http.ListenAndServe("localhost:8000", nil))
        return
    }
    //!+main
    lissajous(os.Stdout)
}

func lissajous(out io.Writer) {
    const (
        cycles  = 5     // number of complete x oscillator revolutions
        res     = 0.001 // angular resolution
        size    = 100   // image canvas covers [-size..+size]
        nframes = 64    // number of animation frames
        delay   = 8     // delay between frames in 10ms units
    )
    freq := rand.Float64() * 3.0 // relative frequency of y oscillator
    anim := gif.GIF{LoopCount: nframes}
    phase := 0.0 // phase difference
  colorIndex := 2
  for i := 0; i < nframes; i++ {
    rect := image.Rect(0, 0, 2*size+1, 2*size+1)
        img := image.NewPaletted(rect, palette)
        for t := 0.0; t < cycles*2*math.Pi; t += res {
            x := math.Sin(t)
            y := math.Sin(t*freq + phase)
      img.SetColorIndex(size+int(x*size+0.5), size+int(y*size+0.5), colorIndex)
      colorIndex++
        }
        phase += 0.1
        anim.Delay = append(anim.Delay, delay)
        anim.Image = append(anim.Image, img)
    }
    gif.EncodeAll(out, &anim) // NOTE: ignoring encoding errors
}

//!-main

这是我得到的错误

lissajous/main.go:76: cannot use colorIndex (type int) as type uint8 in argument to img.SetColorIndex

int和uint8类型之间有区别吗？

Answer 1

的类型colorIndex是int。参数类型是uint8. Anint不能分配给 a uint8。以下是修复程序的一些选项：

• 声明colorIndex为无类型常量。

  const colorIndex = 2
  

• 声明colorIndex为 uint8 类型：

  colorIndex := uint8(3)
  

• 在调用时转换值：

  img.SetColorIndex(size+int(x*size+0.5), size+int(y*size+0.5), uint8(colorIndex))
  

可以更换的所有用途uint8，在这个答案byte，因为byte是一个别名为uint8。