Slicing Operations

A slicing expression selects a half-open portion of an array or slice. The end index is excluded, which makes the resulting length easy to calculate.

The expression items[1:4] includes original indexes 1, 2 and 3, so its length is 4-1=3. Omitting the start defaults to zero; omitting the end defaults to the current length. Slicing usually creates a new view rather than copying elements.

A full slice expression a[low:high:max] can restrict the resulting capacity, helping avoid unexpected sharing during future append operations. Slice bounds must satisfy the language’s rules or execution panics.

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() {
	values := []int{10, 20, 30, 40, 50}
	middle := values[1:4]
	first := values[:2]
	last := values[3:]
	middle[0] = 99
	fmt.Println("Middle:", middle)
	fmt.Println("First:", first)
	fmt.Println("Last:", last)
	fmt.Println("Original:", values)
}

Output

Middle: [99 30 40]
First: [10 99]
Last: [40 50]
Original: [10 99 30 40 50]

The middle view begins at index 1 and contains three values. Changing its first element writes to original index 1. The first view also includes original index 1, so it observes 99. The last view begins at original index 3 and is unaffected.

• The high bound is exclusive, not inclusive.
• Slicing typically shares elements; it is not a cloning operation.

Create the slice values[2:5], predict its length and then modify its last element.

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

You have practiced Slicing Operations 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.