arrays - cannot use make([]Entry, 0, 100) (type []Entry) as type Map in assignment

Question

Trying to implement go map, with below code:

package gomap

type Entry struct {
    stockName  string
    stockValue float64
}

type Bucket []Entry

type Map [4]Bucket

type bucketElementPosition [4]int

var emptyPosition = bucketElementPosition{0, 0, 0, 0}

func Newmap() Map {

    return Map{} // [(nil,0,0), (nil,0,0), (nil,0,0), (nil,0,0)]
}

func Insert(m []Map, key string, value float64) { // m is sliceofslices type

    bucketNumber := (key[0] - 'A') / 7

    if cap(m[bucketNumber]) == 0 { // first insert, so create the backing array
        m[bucketNumber] = make([]Entry, 0, 100)
    }
}

---

package main

import (
    "fmt"

    "github.com/myhub/cs61a/gomap"
)

func main() {

    var stockData = gomap.Newmap()
    gomap.Insert(stockData[:], "AAPL", 94.56)

}

---

m[bucketNumber] = make([]Entry, 0, 100) gives error:

     cannot use make([]Entry, 0, 100) (type []Entry) as type Map in assignment

---

How to pass slice header of stockData to Insert() function?

标签： arraysdictionarygoslice

Answer 1

使用指向Map

m是类型[]Map，所以m[bucketNumber]将是类型Map，不是bucket。

使用时更改mto的类型*Map和取消引用：

func Insert(m *Map, key string, value float64) { // m is sliceofslices type
    bucketNumber := (key[0] - 'A') / 7

    if cap((*m)[bucketNumber]) == 0 { // first insert, so create the backing array
        (*m)[bucketNumber] = make([]Entry, 0, 100)
    }
}

并传递地址stockData：

gomap.Insert(&stockData, "AAPL", 94.56)

请注意，Go 中的数组是次要的。如果您使用切片而不是数组（和指向数组的指针），那么代码会更简单并且可能更有效。

导出bucket和使用[]Bucket

如果可以导出bucket，则可以将类型更改m为[]Bucket：

func Insert(m []Bucket, key string, value float64) { // m is sliceofslices type
    bucketNumber := (key[0] - 'A') / 7

    if cap(m[bucketNumber]) == 0 { // first insert, so create the backing array
        m[bucketNumber] = make([]Entry, 0, 100)
    }
}

并称它为：

Insert(stockData[:], "AAPL", 94.56)