Equivalent of setdefault in Go?

Question

I can do:

_, ok := some_go_map[a_key]

to test for existence of key.

But I've been spoiled by Python's dict's setdefault method (if a key does not have value set in a "map" [dict == associative array], set it to a given default, then get it; otherwise just get).

Wondering if there's some idiom in Go to achieve the same thing?

Answer 1

Note that Go's default behavior is to return the "zero value" for the value type (e.g., 0 or "") when a looked-up key's missing, so if the default you want happens to be that, you're all set already.

Riffing off Buddy and larsmans' answers, here's code that attaches a new method to a named Dict type, so you can either use d[key] for Go's built-in behavior or d.SetDefault(key, val)--

http://play.golang.org/p/5SIJSWNWO7

package main

import "fmt"

type Dict map[string]float64

func (d Dict) SetDefault(key string, val float64) (result float64) {
    if v, ok := d[key]; ok {
        return v
    } else {
        d[key] = val
        return val
    }
}

func main() {
    dd := Dict{}
    dd["a"] = 3
    fmt.Println(dd.SetDefault("a", 1))
    fmt.Println(dd.SetDefault("b", 2))
}

Answer 2

You can always define it yourself:

func setDefault(h map[string]int, k string, v int) (set bool, r int) {
    if r, set = h[k]; !set {
        h[k] = v
        r = v
        set = true
    }
    return
}

But no, it's not in the stdlib. Usually, you'd just do this inline.