Map Programs

Learn map programs through an executable example, then connect the small demonstration to production code.

Maps associate keys with values and are useful for frequency counting, dictionaries and lookup tables. Iteration order is unspecified, so sort keys when output order matters.

The example counts the occurrences of each fruit and sorts its unique names before printing.

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"
	"sort"
)

func main() {
	fruits := []string{"pear", "apple", "pear", "banana", "apple", "pear"}
	counts := make(map[string]int)
	for _, fruit := range fruits {
		counts[fruit]++
	}
	keys := make([]string, 0, len(counts))
	for key := range counts {
		keys = append(keys, key)
	}
	sort.Strings(keys)
	for _, key := range keys {
		fmt.Println(key, counts[key])
	}
}

Output

apple 2
banana 1
pear 3

Each encounter increases a zero-default count. Sorting keys gives stable alphabetical output: apple 2, banana 1, pear 3.

• Do not depend on map iteration order.
• Distinguish an absent key from a present key with zero value when the difference matters.

Change the input words and display only those appearing more than once.

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

You have practiced Map Programs 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.