Question

Consider the following example:

package main

import (
    "fmt"
    "sort"
)

func main() {
    var n int
    var a sort.IntSlice
    a = append(a, 23)   
    a = append(a, 3)
    a = append(a, 10)
    sort.Sort(a)    
    fmt.Println(a)
    n = sort.SearchInts(a, 1)
    fmt.Println(n)
    n = sort.SearchInts(a, 3)
    fmt.Println(n)
}

http://play.golang.org/p/wo4r43Zghv

And the result is:

[3 10 23]
0
0

How am I supposed to know if the number is present in the slice or not, when both the first element and the nonexistent element return 0 as index?

Update
Note that the index can also be greater than the length of the slice so the proper way to find if an element is present in the slice is:

num := 1
n = sort.SearchInts(a, num) 
if n < len(a) && a[n] == num {
  // found
}

Answer 1

Check whether the number you were looking for actually lives at the returned index. The binary search routines in sort are supposed to find the index where a value can be inserted if it’s not already present.

E.g. a search for 100 in the slice in your example will return 3 because the value must be appended to the slice.

the call Search(len(data), func(i int) bool { return data[i] >= 23 }) returns the smallest index i such that data[i] >= 23. If the caller wants to find whether 23 is in the slice, it must test data[i] == 23 separately.