Go — Maps

Creating maps

package main

import "fmt"

func main() {
    // Literal
    ages := map[string]int{
        "Alice": 30,
        "Bob":   25,
    }

    // Make
    m := make(map[string]int)

    // Add entries
    m["Charlie"] = 35
    m["Diana"] = 28

    fmt.Println(ages, m)
}

Map operations

ages := map[string]int{"Alice": 30, "Bob": 25}

// Access
fmt.Println(ages["Alice"]) // 30

// Check existence
age, ok := ages["Charlie"]
if ok {
    fmt.Println("Charlie:", age)
} else {
    fmt.Println("Charlie not found")
}

// Delete
delete(ages, "Bob")

// Length
fmt.Println("Size:", len(ages))

Iterating maps

ages := map[string]int{"Alice": 30, "Bob": 25, "Charlie": 35}

for name, age := range ages {
    fmt.Printf("%s: %d\n", name, age)
}

Map as set

set := map[string]bool{}
set["Alice"] = true
set["Bob"] = true

// Check membership
if set["Alice"] {
    fmt.Println("Alice is in the set")
}

Nested maps

users := map[string]map[string]interface{}{
    "Alice": {
        "age":  30,
        "city": "NYC",
    },
    "Bob": {
        "age":  25,
        "city": "LA",
    },
}

fmt.Println(users["Alice"]["city"])

Best practices

• Use make for large maps to pre-allocate
• Always check the comma-ok idiom when accessing
• Maps are not safe for concurrent use — use sync.Map or mutexes
• Use maps for lookup-heavy operations

Mini Practice

Write Go code that:

1. Creates a map and iterates with range
2. Uses the comma-ok idiom to check key existence
3. Implements a word frequency counter
4. Uses a map as a set

Up Next

In the next lesson, you'll learn about Structs — grouping related data.