Slices, maps and strings

In plain English

An array has a fixed length that is part of its type, while a slice is a flexible view over a sequence and is what you use most of the time. The built-in append adds elements to a slice, returning a new slice when it needs more room. A map stores key to value pairs, and looking up a key with two values gives you the value plus an ok boolean that is true only when the key exists. A string is a sequence of bytes, and ranging over it walks the string rune by rune, where a rune is a single character code point.

Step by step

1. 1Pick a slice for a sequence whose length can change.
2. 2Add an element with append and store the result back.
3. 3Look up a map key and read the ok boolean.
4. 4Range over a string to visit each rune.

package main

import "fmt"

func main() {
    var fixed [3]string
    fixed[0] = "HTML"
    fixed[1] = "CSS"
    fixed[2] = "Go"
    fmt.Println(fixed)

    titles := []string{"HTML", "CSS"}
    titles = append(titles, "Go", "Rust")
    fmt.Println(titles)

    minutes := map[string]int{
        "HTML": 8,
        "CSS":  12,
    }
    fmt.Println(minutes)

    value, ok := minutes["Go"]
    fmt.Println(value, ok)

    name := "Go"
    fmt.Println(len(name))

    for _, r := range name {
        fmt.Println(string(r))
    }
}

Common mistakes

• Confusing an array with a slice can bite you, because an array length is fixed into its type.
• Ignoring the ok boolean from a map lookup makes a missing key look the same as a stored zero value.
• Reading a string by byte index on text outside ASCII can split a rune in half.
• Calling append without assigning the result back loses the new elements, since append may return a new slice.