简短的回答

_, exists := timeZone[tz]    // Just checks for key existence
val, exists := timeZone[tz]  // Checks for key existence and retrieves the value

例子

这是一个在Go Playground的例子。

再回答

根据Effective Go的地图部分:

An attempt to fetch a map value with a key that is not present in the map will return the zero value for the type of the entries in the map. For instance, if the map contains integers, looking up a non-existent key will return 0. Sometimes you need to distinguish a missing entry from a zero value. Is there an entry for "UTC" or is that the empty string because it's not in the map at all? You can discriminate with a form of multiple assignment. var seconds int var ok bool seconds, ok = timeZone[tz] For obvious reasons this is called the “comma ok” idiom. In this example, if tz is present, seconds will be set appropriately and ok will be true; if not, seconds will be set to zero and ok will be false. Here's a function that puts it together with a nice error report: func offset(tz string) int { if seconds, ok := timeZone[tz]; ok { return seconds } log.Println("unknown time zone:", tz) return 0 } To test for presence in the map without worrying about the actual value, you can use the blank identifier (_) in place of the usual variable for the value. _, present := timeZone[tz]

在go-nuts电子邮件列表中搜索，找到了Peter Froehlich在2009年11月15日发布的解决方案。

package main

import "fmt"

func main() {
        dict := map[string]int {"foo" : 1, "bar" : 2}
        value, ok := dict["baz"]
        if ok {
                fmt.Println("value: ", value)
        } else {
                fmt.Println("key not found")
        }
}

或者更简洁地说，

if value, ok := dict["baz"]; ok {
    fmt.Println("value: ", value)
} else {
    fmt.Println("key not found")
}

注意，使用这种形式的if语句，value和ok变量只在if条件中可见。

    var empty struct{}
    var ok bool
    var m map[string]struct{}
    m = make(map[string]struct{})
    m["somestring"] = empty


    _, ok = m["somestring"]
    fmt.Println("somestring exists?", ok) 
    _, ok = m["not"]
    fmt.Println("not exists?", ok)

然后，去查地图，去 somestring存在吗?真正的 不存在?假

示例用法:循环遍历一个片，用于pairMap检查key是否存在。 它是一种算法，找出所有对相加到一个特定的和。

func findPairs(slice1 []int, sum int) {
    pairMap := make(map[int]int)
    for i, v := range slice1 {
        if valuei, ok := pairMap[v]; ok {
            fmt.Println("Pair Found", i, valuei)
        } else {
            pairMap[sum-v] = i
        }
    }
}

简短的回答

_, exists := timeZone[tz]    // Just checks for key existence
val, exists := timeZone[tz]  // Checks for key existence and retrieves the value

例子

这是一个在Go Playground的例子。

再回答

根据Effective Go的地图部分:

An attempt to fetch a map value with a key that is not present in the map will return the zero value for the type of the entries in the map. For instance, if the map contains integers, looking up a non-existent key will return 0. Sometimes you need to distinguish a missing entry from a zero value. Is there an entry for "UTC" or is that the empty string because it's not in the map at all? You can discriminate with a form of multiple assignment. var seconds int var ok bool seconds, ok = timeZone[tz] For obvious reasons this is called the “comma ok” idiom. In this example, if tz is present, seconds will be set appropriately and ok will be true; if not, seconds will be set to zero and ok will be false. Here's a function that puts it together with a nice error report: func offset(tz string) int { if seconds, ok := timeZone[tz]; ok { return seconds } log.Println("unknown time zone:", tz) return 0 } To test for presence in the map without worrying about the actual value, you can use the blank identifier (_) in place of the usual variable for the value. _, present := timeZone[tz]

在“索引表达式”中提到。

赋值中使用的map[K]V类型映射a上的索引表达式 或者初始化特殊形式 V, ok = a[x] V, ok:= a[x] Var v, ok = a[x] 产生一个额外的无类型布尔值。ok的值为true，如果 键x存在于映射中，否则为false。