Maps vs Slices

Slices and maps both store collections of values, but they solve different problems. A slice orders elements by numeric position; a map finds values by unique keys.

Choose a slice when sequence, order, duplicates or indexed access matters. Choose a map when you need to associate keys with values or quickly test membership. Neither is a universal replacement for the other.

For example, a list of products in shopping-cart order is naturally a slice, while a quantity lookup by product name can be a map. Sometimes a program needs both: a slice to preserve order and a map for quick lookup.

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() {
	order := []string{"pen", "book", "pen"}
	quantities := make(map[string]int)
	for _, item := range order {
		quantities[item]++
	}
	fmt.Println("First item:", order[0])
	fmt.Println("Items in order:", order)
	fmt.Println("Pen quantity:", quantities["pen"])
	fmt.Println("Unique products:", len(quantities))
}

Output

First item: pen
Items in order: [pen book pen]
Pen quantity: 2
Unique products: 2

The slice keeps three positions, including two occurrences of pen. Iterating over it builds a map that counts pen twice and book once. The final map has two unique keys, even though the slice contains three items.

• A map cannot represent a guaranteed item sequence by itself.
• A slice can hold duplicates; a map stores at most one value per distinct key.

Add one more book to the order slice. Predict the order length, the number of unique products and the quantity of book.

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

You have practiced Maps vs Slices 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.