Maps

Key-Value Storage

Maps are Go's built-in hash table. They store key-value pairs and provide constant-time lookups.

Creating Maps

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

// Make an empty map
scores := make(map[string]int)

The type map[string]int reads as "a map from strings to ints." Keys can be any comparable type (strings, ints, booleans, structs without slice/map fields). Values can be anything.

Like the star charts on the Enterprise, a Go map lets you look up any destination in constant time. No need to scan the entire quadrant.

Operations

m := make(map[string]int)

// Insert or update
m["alice"] = 95

// Lookup
score := m["alice"] // 95

// Delete
delete(m, "alice")

// Length
fmt.Println(len(m))

The Comma-Ok Idiom

When you look up a key that does not exist, Go returns the zero value for the value type. To distinguish between "key not found" and "key exists with zero value", use the two-value form:

value, ok := m["key"]
if ok {
    fmt.Println("found:", value)
} else {
    fmt.Println("not found")
}

This is called the "comma-ok" idiom. The second value is a boolean that indicates whether the key was present.

Iterating

Use for range to iterate over a map. The iteration order is not guaranteed. It is intentionally randomized by the runtime:

for key, value := range m {
    fmt.Printf("%s: %d\n", key, value)
}

Your Task

Write a function wordCount that takes a string and returns a map[string]int where each key is a word and each value is how many times that word appears.

Use strings.Fields to split the string into words (it splits on whitespace).