How can I write an array of maps [golang]

Question

I have a map which has as value an array of maps.

Example:

 thisMap["coins"][0] = aMap["random":"something"]
 thisMap["notes"][1] = aMap["not-random":"something else"]
 thisMap["coins"][2] = aMap["not-random":"something else"]

I can't figure it out how to do this as go seems to allow setting data only at one level when you deal with maps [name][value] = value.

So far I have this code which fails

package main

func main() {

    something := []string{"coins", "notes", "gold?", "coins", "notes"}

    thisMap := make(map[string][]map[string]int)

    for k, v := range something {
        aMap := map[string]string{
            "random": "something",
        }

        thisMap[v] = [k]aMap
    }
}

Edit: The slice values ("coins", "notes" etc ) can repeat so this is the reason why I need use an index [] .

Just a tangential note: you are technically using a slice of maps, not an array of maps. blog.golang.org/slices — Evan
– Evan, Commented Apr 14, 2014 at 17:58

nemo · Accepted Answer · 2014-04-15 02:19:34Z

27

Working example (click to play):

something := []string{"coins", "notes", "gold?"}

thisMap := make(map[string][]map[string]int)

for _, v := range something {
    aMap := map[string]int{
        "random": 12,
    }

    thisMap[v] = append(thisMap[v], aMap)
}

When iterating over the newly created thisMap, you need to make room for the new value aMap. The builtin function append does that for you when using slices. It makes room and appends the value to the slice.

If you're using more complex data types that can't be initialized as easily as slices, you first have to check if the key is already in the map and, if it is not, initialize your data type. Checking for map elements is documented here. Example with maps (click to play):

thisMap := make(map[string]map[string]int)

for _, v := range something {
    if _, ok := thisMap[v]; !ok {
        thisMap[v] = make(map[string]int)
    }
    thisMap[v]["random"] = 12
}

edited Apr 15, 2014 at 2:19

answered Apr 14, 2014 at 17:58

nemo

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4 Comments

nemo Over a year ago

@Dustin: yep in this case it's true that you don't need to check the key. However, I'd like to keep the answer more general.

Dustin Over a year ago

In general, I'd never make a zero element slice. I might use a literal in a special case. Not having more code than is required to solve a problem correctly is best practice.

nemo Over a year ago

@Dustin: I meant keeping the answer as general as possible if somebody with a similar problem stumbles over it. For slices you can short the code, yes. For maps, for example, you can't. These must be initialized, so you'd need key checking. I'll just reformulate things...

The user with no hat Over a year ago

@nemo this doesn't seem to work if the slice has repeating values. I was thinking to use an index/[] exactly due this reason. Should I order them first to avoid loosing some data ?

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