Maps

Creating a Map

A map associates keys with values, similar to a dictionary in Python or an object in JavaScript. It's created either with make(map[KeyType]ValueType) or a map literal listing initial key-value pairs.

Example: Creating a Map

package main

import "fmt"

func main() {
	prices := map[string]float64{
		"apple":  0.5,
		"banana": 0.25,
	}
	fmt.Println(prices["apple"])
}

The nil Map Trap

A map declared with just 'var m map[string]int' is nil, and while reading from a nil map is safe (it returns the zero value), writing to it causes a runtime panic. This is a very common beginner mistake.

Note: Always initialize a map with make() or a literal before writing to it.

Example: The nil Map Trap

package main

import "fmt"

func main() {
	var scores map[string]int
	fmt.Println("read from nil map is safe:", scores["nobody"])
	scores = make(map[string]int)
	scores["alice"] = 90
	fmt.Println(scores)
}

Checking If a Key Exists

Reading a key that doesn't exist returns the zero value for the map's value type rather than an error, which can be ambiguous. The 'comma ok' form -- value, ok := m[key] -- tells you definitively whether the key was actually present.

Note: Always use the comma-ok form when the zero value is a valid, ambiguous result (like 0 for a real score).

Example: Checking If a Key Exists

package main

import "fmt"

func main() {
	ages := map[string]int{"amit": 30}
	age, ok := ages["priya"]
	fmt.Println("age:", age, "found:", ok)
}