CODEWITHSUNDEEP
go
AlexB
AlexB

Reputation: 3548

Go - loop through map on the same order multiple times

How can I loop through a map on a constant order multiple times ?

In my go code I am looping through a map twice but the values are not appearing on the same order on both loops :

fieldMap := map[string]int{...}

First loop :

for k, _ := range fieldMap {...}

Second loop :

for _, v := range fieldMap {...}

Upvotes: 2

Views: 2632

Answers (2)

poopoothegorilla
poopoothegorilla

Reputation: 784

Go makes sure that you cannot rely on a map's order as stated in this blog post https://blog.golang.org/go-maps-in-action#TOC_7.

When iterating over a map with a range loop, the iteration order is not specified and is not guaranteed to be the same from one iteration to the next. Since Go 1 the runtime randomizes map iteration order, as programmers relied on the stable iteration order of the previous implementation. If you require a stable iteration order you must maintain a separate data structure that specifies that order.

Ainar-G's answer is correct, but here is a variation of it while incorporating this answer... https://stackoverflow.com/a/27848197/3536948

keys := make([]string, len(m))
i := 0
for k, _ := range m {
    keys[i] = k
    i += 1
}
for _, k := range keys {
    fmt.Println(m[k])
}

Playground: http://play.golang.org/p/64onPZNODm

Upvotes: 4

Ainar-G
Ainar-G

Reputation: 36229

Save the keys in the first loop and use them in the second loop:

keys := make([]string, 0, len(m))
for k, v := range m {
    fmt.Println(k, v)
    keys = append(keys, k)
}
for _, k := range keys {
    fmt.Println(k, m[k])
}

Playground: http://play.golang.org/p/MstH20wkNN.

Upvotes: 4

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