Even though it said to be random (randomized) (spec, blog, hashmap source, another blog, SO), the distribution is far from being perfect.
Why? Because we like maps being fast, and better random distribution tends to require more computation and/or bigger delays. A compromise had to be made. And because the intention is not to provide a quality "shuffle" functionality by for range
, but only to prevent developers relying on stable iteration order (because it could change even without explicit randomization).
But "how good" may this distribution be? Easy to get a "taste". Let's create a map of 10 pairs, and start iterating over it lot of times. And let's count the distribution of the very first index (key):
m := map[int]int{}
for i := 0; i < 10; i++ {
m[i] = i
}
dist := make([]int, 10)
for i := 0; i < 100000; i++ {
for idx := range m {
dist[idx]++
break
}
}
fmt.Println("Distribution:", dist)
Output (try it on the Go Playground):
Distribution: [25194 24904 6196 6134 6313 6274 6297 6189 6189 6310]
The first 2 keys (0
and 1
) were roughly encountered 4 times more than the rest which had roughly the same probability.
You can tell it's pretty bad for being true (or even good) random, but that's not the point. It's good enough to provide varying iteration order (and importantly: it's fast).