Maps.md

Go Maps: Hash Tables in Go

Maps are Go's built-in associative data type (sometimes called hashes or dicts in other languages). They provide a way to store data as key-value pairs.

Quick Reference: Map Operations

Operation	Syntax / Function	Description
Declaration	var m map[K]V	Declares a map (initially nil).
Initialization	make(map[K]V, hint)	Initializes a map with an optional capacity hint.
Literal	m := map[K]V{k1: v1}	Initializes a map with values.
Insertion	m[key] = value	Adds or updates an element.
Retrieval	val := m[key]	Retrieves a value (returns zero value if key missing).
Check Existence	val, ok := m[key]	Retrieves value and a boolean indicating if key exists.
Deletion	delete(m, key)	Removes a key-value pair.
Length	len(m)	Returns the number of key-value pairs.
Clear	clear(m)	Removes all keys from the map (Go 1.21+).
Iteration	for k, v := range m	Iterates over all key-value pairs (order is random).

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1. Map Basics

Initialization

// Using make
scores := make(map[string]int)

// Using a literal
colors := map[string]string{
    "red":   "#ff0000",
    "green": "#00ff00",
}

Accessing Values

val, ok := scores["Alice"]
if ok {
    fmt.Println("Alice's score:", val)
} else {
    fmt.Println("Alice not found")
}

2. Important Characteristics

1. Unordered: Iteration order over maps is not guaranteed and can change between executions.
2. Reference Type: Like slices, maps are reference types. Passing a map to a function allows the function to modify the original map.
3. Key Constraints: Map keys can be of any type for which the equality operator == is defined (e.g., integers, floats, strings, pointers, channels, interfaces, and structs that only contain comparable types). Slices, maps, and functions cannot be map keys.

3. The maps Package (Go 1.21+)

The standard library now includes a maps package for common map operations:

• maps.Keys(m): (Proposed/Planned) Returns a slice of map keys.
• maps.Values(m): (Proposed/Planned) Returns a slice of map values.
• maps.Copy(dst, src): Copies all key/value pairs from src to dst.
• maps.Equal(m1, m2): Reports whether two maps contain the same key/value pairs.
• maps.DeleteFunc(m, f): Deletes any key/value pairs from m for which f(k, v) returns true.

Example Code

See data_structures/maps/map.go for more examples.