`Substring`

In the Go language, no substring func is available. Instead we access parts of strings (substrings) with slice syntax.

To take a substring in this language, we specify a first index and a last index. The first integer is a start index. And the second is an end index.

First example

There are 2 ways to take a slice of a string. We can convert a string to a rune slice, and then take a slice of the runes. Or we can take a slice of the string directly.

Part 1 To handle all characters correctly, we should convert the string to a rune slice and then operate on the runes.

Part 2 To handle just 1-byte characters, we can use the slice syntax directly on a string. This should be used with care.

Here We start at character index 1, and continue until index 3, which leaves us with a 2-char substring.

package main

import "fmt"

func main() {
    // A string.
    value := "bird"

    // Part 1: take substring with runes.
    // ... This handles any kind of rune in the string.
    runes := []rune(value)
    // ... Convert back into a string from rune slice.
    safeSubstring := string(runes[1:3])
    fmt.Println(" RUNE SUBSTRING:", safeSubstring)

    // Part 2: take substring with direct byte slice.
    // ... This handles only ASCII correctly.
    asciiSubstring := value[1:3]
    fmt.Println("ASCII SUBSTRING:", asciiSubstring)
} RUNE SUBSTRING: ir
ASCII SUBSTRING: ir

Unicode characters

For non-ASCII characters, more than 1 byte is required for a character. To take substrings correctly on non-ASCII strings, we must use runes.

Here We see how using a rune slice of a string correctly handles some characters. This is important for many applications.

package main

import "fmt"

func main() {
    // This string contains Unicode characters.
    value := "ü:ü"
    // Convert string to rune slice before taking substrings.
    // ... This will handle Unicode characters correctly.
    //     Not needed for ASCII strings.
    runes := []rune(value)
    fmt.Println("First 1:", string(runes[0:1]))
    fmt.Println(" Last 1:", string(runes[2:]))
}First 1: &uuml;
 Last 1: &uuml;

Direct substrings

Here we take a slice (a substring) of a string. We start at the index 4, which is the letter "d." And this proceeds until the end of the string.

Tip The slice end index is exclusive—this means a character at that position is not included.

Info To take a slice until the end of a string, we can use the len built-in. This means "to the end" of the string.

package main

import "fmt"

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

    // Take substring from index 4 to length of string.
    substring := value[4:len(value)]
    fmt.Println(substring)
}
dog

No first index

The first index of a slice can be omitted. This always means the index 0. So if you don't like typing 0, this is a good thing.

Note Reducing syntax noise (confusing characters) can make programs more readable.

Here We take the first two characters in the string as a substring. We use the ":2" slice syntax.

package main

import "fmt"

func main() {
    value := "abcd"
    // Omit start index, this is the same as zero.
    // ... Take 2-char substring.
    substring := value[:2]

    // Test the substring.
    if substring == "ab" {
        fmt.Println(true)
    }
}
true

No end index

We can omit the end index. This takes the substring from a start index to the end of the string. This is a clearer way of using the length as the end.

package main

import "fmt"

func main() {
    value := "frog;fish"

    // We can specify just the start index.
    substring := value[5:]
    fmt.Println(substring)
}
fish

Three index error

In Go, we can use "full slice expressions" with three indexes on some types. The third index is a max index (so the slice does not go off the end).

However The three-index syntax does not work with strings. Trying it will lead to an error.

package main

import "fmt"

func main() {
    value := "one two"
    // This program does not work.
    // ... String slice cannot have three indexes.
    result := value[4:8:6]
    fmt.Println(result)
}
# command-line-arguments
invalid operation value[4:8:6] (3-index slice of string)

Out of range, panic

When taking substrings, it is important to ensure we are within the bounds of the string. If our first or last index is out of range, a panic will occur.

Tip The len built-in method, along with some if-tests, can solve these runtime errors.

package main

import "fmt"

func main() {
    value := "abc"
    // Indexes must be validated first or a panic occurs.
    result := value[10:20]
    fmt.Println(result)
}panic: runtime error: slice bounds out of range

goroutine 1 [running]:
main.main()
        C:/programs/file.go:11 +0x144....
exit status 2

To review, Go does not provide a substring() method. But with slice syntax, we take substrings based on the first and last index. We must first check indexes to prevent panics.