Go: Binary search

Use one of the binary search functions: sort.SearchInts, sort.SearchFloat64s or sort.SearchStrings.

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

Use the 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].

Complexity

Binary search runs in worst-case logarithmic time, making O(log n) comparisons, where n is the size of the array.

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