Stage 1 · Code

Functions & Composite Types

Maps

Store and look up key-value pairs in constant time with Go's built-in map type.

7 min readProgramming Foundations (Go)Code

Creating Maps

A map is an unordered collection of key-value pairs where every key is unique. Keys must be a comparable type (strings, integers, booleans, structs with comparable fields). Values can be any type.

Gothree-ways-to-create-a-map.go

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Always use a map literal or `make` before writing to a map. A `var` declaration gives you a nil map — reading returns zero values, but writing panics.

Writing to a nil map panics

This is one of the most common runtime panics for Go beginners. If you declare a map with var m map[K]V and then assign m[key] = value, the program panics. Always initialize with m = make(map[K]V) or a map literal before writing.

Reading and Writing

Use m[key] to read a value and m[key] = value to write. Reading a key that does not exist returns the zero value for the value type — it does not panic or produce an error.

Gobasic-map-operations.go

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Reading a missing key returns `0` for `int`, `""` for `string`, `false` for `bool`, and so on. This is convenient for counting patterns like `counts[word]++` but can hide bugs when you need to know if a key actually exists.

The Comma-Ok Idiom

To distinguish between a missing key and a key mapped to the zero value, use the two-value form: value, ok := m[key]. ok is true if the key is present, false if it is not.

Gochecking-key-existence.go

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Use the comma-ok form whenever the zero value is a valid map value and you need to distinguish 'key not present' from 'key present with zero value'. This pattern appears throughout the Go standard library.

Deleting Entries

The built-in delete(m, key) removes a key from a map. Deleting a key that does not exist is safe — it is a no-op. There is no return value.

Godeleting-map-entries.go

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`delete` is always safe to call even if the key is absent. Check existence with comma-ok if you need to know whether a deletion actually removed something.

Iterating with range

Use range to iterate over all key-value pairs in a map. The iteration order is random on every run — Go deliberately randomizes it to prevent programs from accidentally depending on insertion order.

Goranging-over-a-map.go

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If you need to print or process map entries in a consistent order, collect the keys into a slice, sort it, then iterate the sorted keys. This is a common Go idiom.

Maps are reference types

Like slices, maps are passed by reference. Passing a map to a function does not copy the data — the function operates on the same underlying hash table. Modifications inside the function are visible to the caller.

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