Accessing Array Elements

Array elements are accessed with zero-based indexes. The first position is index 0 and the last is length minus one.

For colors := [3]string{"red", "blue", "green"}, colors[0] is red and colors[2] is green. Use len(colors) to discover the number of elements rather than hard-coding the end index.

Accessing an index outside the valid range fails: a constant invalid index is usually rejected by the compiler, while an invalid index computed at runtime causes a panic. Check indexes before accessing data derived from input.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import "fmt"

func main() {
	colors := [3]string{"red", "blue", "green"}
	fmt.Println("First:", colors[0])
	fmt.Println("Last:", colors[len(colors)-1])
	index := 1
	if index >= 0 && index < len(colors) {
		fmt.Println("Selected:", colors[index])
	}
}

Output

First: red
Last: green
Selected: blue

The array has indexes 0, 1 and 2. Its length is 3, so len(colors)-1 selects index 2. The conditional check confirms index 1 is valid before reading the middle element.

• Do not assume indexes begin at 1; the first element is index 0.
• Check bounds before using an index computed from untrusted or changing data.

Change the index variable to 4 and verify the bounds check prevents an invalid access. Then test index 0.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced Accessing Array Elements in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.