Go: Append if unique

append go slice
Kyle Finley picture Kyle Finley · Feb 12, 2012 · Viewed 74.3k times · Source

Is there a way to check slices/maps for the presence of a value?

I would like to add a value to a slice only if it does not exist in the slice.

This works, but it seems verbose. Is there a beter way to do this?

orgSlice := []int{1, 2, 3}
newSlice := []int{}
newInt := 2

newSlice = append(newSlice, newInt)
for _, v := range orgSlice {
    if v != newInt {
        newSlice = append(newSlice, v)
    }
}

newSlice == [2 1 3]

Answer

tux21b picture tux21b · Feb 12, 2012

Your approach would take linear time for each insertion. A better way would be to use a map[int]struct{}. Alternatively, you could also use a map[int]bool or something similar, but the empty struct{} has the advantage that it doesn't occupy any additional space. Therefore map[int]struct{} is a popular choice for a set of integers.

Example:

set := make(map[int]struct{})
set[1] = struct{}{}
set[2] = struct{}{}
set[1] = struct{}{}
// ...

for key := range(set) {
  fmt.Println(key)
}
// each value will be printed only once, in no particular order


// you can use the ,ok idiom to check for existing keys
if _, ok := set[1]; ok {
  fmt.Println("element found")
} else {
  fmt.Println("element not found")
}

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