How to clear a map in Go?

Question

I'm looking for something like the c++ function .clear() for the primitive type map.

Or should I just create a new map instead?

Update: Thank you for your answers. By looking at the answers I just realized that sometimes creating a new map may lead to some inconsistency that we don't want. Consider the following example:

var a map[string]string
var b map[string]string

func main() {
    a = make(map[string]string)
    b=a
    a["hello"]="world"
    a = nil
    fmt.Println(b["hello"])
}

I mean, this is still different from the .clear() function in c++, which will clear the content in the object.

Answer 1

You should probably just create a new map. There's no real reason to bother trying to clear an existing one, unless the same map is being referred to by multiple pieces of code and one piece explicitly needs to clear out the values such that this change is visible to the other pieces of code.

So yeah, you should probably just say

mymap = make(map[keytype]valtype)

---

If you do really need to clear the existing map for whatever reason, this is simple enough:

for k := range m {
    delete(m, k)
}

Answer 2

Unlike C++, Go is a garbage collected language. You need to think things a bit differently.

When you make a new map

a := map[string]string{"hello": "world"}
a = make(map[string]string)

the original map will be garbage-collected eventually; you don't need to clear it manually. But remember that maps (and slices) are reference types; you create them with make(). The underlying map will be garbage-collected only when there are no references to it. Thus, when you do

a := map[string]string{"hello": "world"}
b := a
a = make(map[string]string)

the original array will not be garbage collected (until b is garbage-collected or b refers to something else).

Answer 3

Go 1.21

Use the builtin clear.

func main() {
    m := map[string]int{"foo": 1}
    clear(m)
}

It's a single function call, and has the minor but important advantage that it will delete irreflexive keys (see below for details about what this means). The standard library maps package doesn't have maps.Clear anymore.

Go 1.18 to 1.20

You can use maps.Clear. The function belongs to the package golang.org/x/exp/maps (experimental and not covered by the compatibility guarantee)

Clear removes all entries from m, leaving it empty.

Example usage:

func main() {
    testMap := map[string]int{"gopher": 1, "badger": 2}
    maps.Clear(testMap)
    fmt.Println(testMap)
    
    testMap["zebra"] = 2000
    fmt.Println(testMap)
}

Playground: https://go.dev/play/p/qIdnGrd0CYs?v=gotip

If you don't want to depend on experimental packages, you can copy-paste the source, which is actually extremely simple:

func Clear[M ~map[K]V, K comparable, V any](m M) {
    for k := range m {
        delete(m, k)
    }
}

IMPORTANT NOTE: just as with the builtin delete — which the implementation of maps.Clear uses —, this does not remove irreflexive keys from the map. The reason is that for irreflexive keys, by definition, x == x is false. Irreflexive keys are NaN floats and every other type that supports comparison operators but contains NaN floats somewhere.

See this code to understand what this entails:

func main() {
    m := map[float64]string{}
    m[1.0] = "foo"

    k := math.NaN()
    fmt.Println(k == k) // false
    m[k] = "bar"

    maps.Clear(m)
    fmt.Printf("len: %d, content: %v\n", len(m), m) 
    // len: 1, content: map[NaN:bar]

    a := map[[2]float64]string{}
    a[[2]float64{1.0, 2.0}] = "foo"

    h := [2]float64{1.0, math.NaN()}
    fmt.Println(h == h) // false
    a[h] = "bar"

    maps.Clear(a)
    fmt.Printf("len: %d, content: %v\n", len(a), a) 
    // len: 1, content: map[[1 NaN]:bar]
}

Playground: https://go.dev/play/p/LWfiD3iPA8Q

Answer 4

// Method - I , say book is name of map
for k := range book {
    delete(book, k)
}

// Method - II
book = make(map[string]int)

// Method - III
book = map[string]int{}

Answer 5

For the method of clearing a map in Go

for k := range m {
    delete(m, k)
}

It only works if m contains no key values containing NaN.

delete(m, k) doesn't work for any irreflexive key (such as math.NaN()), but also structs or other comparable types with any NaN float in it. Given struct{ val float64 } with NaN val is also irreflexive (Quote by blackgreen comment)

---

To resolve this issue and support clearing a map in Go, one buildin clear(x) function could be available in the new release, for more details, please refer to add clear(x) builtin, to clear map, zero content of slice, ptr-to-array

Per Go 1.21 release note

The new function clear deletes all elements from a map or zeroes all elements of a slice.

Call	Argument type	Result
clear(m)	map[K]T	deletes all entries, resulting in an empty map (len(m) == 0)
clear(s)	[]T	sets all elements up to the length of s to the zero value of T
clear(t)	type parameter	see below

If the type of the argument to clear is a type parameter, all types in its type set must be maps or slices, and clear performs the operation corresponding to the actual type argument.

If the map or slice is nil, clear is a no-op.

Answer 6

Go 1.21 Release adds three new built-ins to the language. In particular:

• The new function clear deletes all elements from a map or zeroes all elements of a slice.

It works like that:

m := map[string]string{"hello": "world"}
clear(m)
fmt.Println(m) // map[]

https://go.dev/play/p/UdQBUEeQ4ck?v=gotip

Answer 7

If you are trying to do this in a loop, you can take advantage of the initialization to clear out the map for you. For example:

for i:=0; i<2; i++ {
    animalNames := make(map[string]string)
    switch i {
        case 0:
            animalNames["cat"] = "Patches"
        case 1:
            animalNames["dog"] = "Spot";
    }

    fmt.Println("For map instance", i)
    for key, value := range animalNames {
        fmt.Println(key, value)
    }
    fmt.Println("-----------\n")
}

When you execute this, it clears out the previous map and starts with an empty map. This is verified by the output:

$ go run maptests.go 
For map instance 0
cat Patches
-----------

For map instance 1
dog Spot
-----------