Checking for Key Existence

Part of the Fundamentals section of Coddy's GO journey — lesson 89 of 109.

In Go, you can check if a key exists in a map using the two-value assignment form. This is important because accessing a non-existent key returns the zero value, which might be misleading.

Create a map of country codes:

countries := map[string]string{
    "US": "United States",
    "CA": "Canada",
}

// Check if "UK" exists in the map
value, exists := countries["UK"]
fmt.Println("Value:", value, "Exists:", exists)

Result:

Value:  Exists: false

The exists boolean tells you whether the key was found. Use this in conditionals:

if _, exists := countries["US"]; exists {
    fmt.Println("US is in the map")
}

Result:

US is in the map

Challenge

Beginner

In this challenge, you'll practice checking if a key exists in a map. You have a map of student grades, and you need to check if a specific student's grade exists in the map.

Complete the code to check if the student 'Emma' exists in the grades map, and print an appropriate message.

Try it yourself

package main

import "fmt"

func main() {
	// Map of student grades
	grades := map[string]int{
		"John":  85,
		"Sarah": 92,
		"Mike":  78,
		"Lisa":  95,
	}

	// Student to check
	studentToCheck := "Emma"

	// TODO: Check if studentToCheck exists in the grades map
	// and store the result in a variable called 'exists'
	// Hint: Use the comma-ok idiom


	// Print the result
	if exists {
		fmt.Printf("%s's grade exists in the map\n", studentToCheck)
	} else {
		fmt.Printf("%s's grade does not exist in the map\n", studentToCheck)
	}
}