Find element in slice/array with linear or binary search

Linear search

Go doesn’t have an out-of-the-box linear search function for slices and arrays. Here are two example linear search implemen­tations, which you can use as templates.

// Find returns the smallest index i at which x == a[i],
// or len(a) if there is no such index.
func Find(a []string, x string) int {
    for i, n := range a {
        if x == n {
            return i
        }
    }
    return len(a)
}

// Contains tells whether a contains x.
func Contains(a []string, x string) bool {
    for _, n := range a {
        if x == n {
            return true
        }
    }
    return false
}

Binary search

If the array is sorted, you can use a binary search instead. This will be much more efficient, since binary search runs in worst-case logarithmic time, making O(log n) comparisons, where n is the size of the slice.

There are the three custom binary search functions: sort.SearchInts, sort.SearchStrings or sort.SearchFloat64s.

They all have the signature

func SearchType(a []Type, x Type) int

and return

• the smallest index i at which x <= a[i]
• or len(a) if there is no such index.

The slice must be sorted in ascending order.

a := []string{"A", "C", "C"}

fmt.Println(sort.SearchStrings(a, "A")) // 0
fmt.Println(sort.SearchStrings(a, "B")) // 1
fmt.Println(sort.SearchStrings(a, "C")) // 1
fmt.Println(sort.SearchStrings(a, "D")) // 3

Generic binary search

There is also a generic binary search function sort.Search.

func Search(n int, f func(int) bool) int

It returns

• the smallest index i at which f(i) is true,
• or n if there is no such index.

It requires that f is false for some (possibly empty) prefix of the input range and then true for the remainder.

This example mirrors the one above, but uses the generic sort.Search instead of sort.SearchInts.

a := []string{"A", "C", "C"}
x := "C"

i := sort.Search(len(a), func(i int) bool { return x <= a[i] })
if i < len(a) && a[i] == x {
    fmt.Printf("Found %s at index %d in %v.\n", x, i, a)
} else {
    fmt.Printf("Did not find %s in %v.\n", x, a)
}
// Output: Found C at index 1 in [A C C].

Further reading

The 3 ways to sort in Go

The map option

If you are doing repeated searches and updates, you may want to use a map instead of a slice. A map provides lookup, insert, and delete operations in O(1) expected amortized time.

Further reading

Maps explained [code example]

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