Index, LastIndex

A string contains an important substring. With Index, we search for that substring. And with LastIndex we search from right to left.

Common functionality

A for-loop can be used to iterate through, and test, a string. But with Index and LastIndex this is done in a simpler way, in a single func call.

First example

We import the "strings" package. Then we assign 2 strings—value1 is part of value2. Index returns the integer index of value1.

package main

import (
    "fmt"
    "strings"
)

func main() {
    value1 := "rainbow"
    value2 := "the rainbow"

    // See if value1 is found in value2.
    if strings.Index(value2, value1) != -1 {
        fmt.Println("FOUND")
    }
}
FOUND

Index result

Index returns an int index—this is the position where the string is found. Remember strings are indexed starting at zero.

package main

import (
    "fmt"
    "strings"
)

func main() {
    value := "cat dog"

    // Get index of this substring.
    result := strings.Index(value, "dog")
    fmt.Println(result)
}
4

LastIndex

This searches from right to left. It begins its search at the last index and proceeds to the first index. So it finds rightmost matches first.

package main

import (
    "fmt"
    "strings"
)

func main() {
    input := "one two one"

    // Get last index, searching from right to left.
    i := strings.LastIndex(input, "one")
    fmt.Println(i)
}
8

IndexFunc

This method receives a func argument that indicates what characters are matched. In the func, any Go code can be used. When true is returned, the rune is matched.

package main

import (
    "fmt"
    "strings"
)

func main() {
    f := func(c rune) bool {
        // Return true if colon or space char.
        return c == ':' ||
            c == ' ';
    }

    value := "one:two"
    // Pass func to method.
    result := strings.IndexFunc(value, f)
    fmt.Println(result)

    value = "one two"
    result = strings.IndexFunc(value, f)
    fmt.Println(result)
}
3
3

IndexAny

This method receives a string that represents a set of characters. The first index of any character in that set is returned.

package main

import (
    "fmt"
    "strings"
)

func main() {
    value := "bird"
    // Finds the letter "i" index.
    // ... No "a" is found in the string.
    result1 := strings.IndexAny(value, "ai")
    fmt.Println("RESULT1:", result1, value[result1:result1+1])

    // Find index of first character in the set of chars.
    result2 := strings.IndexAny(value, "x?rd_")
    fmt.Println("RESULT2:", result2, value[result2:result2+1])
}
RESULT1: 1 i
RESULT2: 2 r

Between, before and after

We can extract parts of a string based on the surrounding substrings. With strings.Index and LastIndex we locate a substring.

Character searching is a common task in programs. If more complex tests are needed, a for-loop can be used, but for simple searches, Index and LastIndex are ideal.