Checking for Keys

Part of the Logic & Flow section of Coddy's C++ journey — lesson 26 of 56.

While the square bracket operator is convenient for accessing map values, there's a potential problem: what happens if you try to access a key that doesn't exist? As you learned in the previous lesson, using [] with a non-existent key automatically creates that key with a default value.

Sometimes you want to check if a key exists before accessing it, without accidentally creating new entries. This is where the .count() method becomes useful. It tells you whether a specific key is present in the map.

The .count() method returns 1 if the key exists and 0 if it doesn't:

std::map<std::string, int> scores;
scores["Alice"] = 95;

if (scores.count("Alice")) {
    std::cout << "Alice's score: " << scores["Alice"] << std::endl;
} else {
    std::cout << "Alice not found" << std::endl;
}

This approach lets you safely check for a key's existence and handle both cases appropriately, preventing unwanted entries from being created in your map.

Challenge

Easy

Create a program that manages a library book checkout system using a std::map. Your program will track which books are available and use the .count() method to safely check if books exist before attempting to access their information.

The following inputs will be provided:

An integer n representing the number of books in the library
Then n pairs of inputs:
- A string representing the book title
- An integer representing the number of copies available
An integer m representing the number of book requests to check
Then m strings representing book titles to search for

Your program should:

Create a std::map<std::string, int> named library
Read the number of books and populate the map with book titles and their available copies
Read the number of book requests
For each book request, use the .count() method to check if the book exists in the library
Print the appropriate message based on whether the book is found or not

Use the following exact output format:

For each book request:

If the book exists: Book "[book title]" is available with [number] copies
If the book doesn't exist: Book "[book title]" is not available in the library

Use the .count() method in an if-statement to check for the book's existence before accessing its value. If library.count(bookTitle) returns 1, the book exists and you can safely access library[bookTitle] to get the number of copies. If it returns 0, the book doesn't exist in the library.

Cheat sheet

The .count() method checks if a key exists in a map without creating new entries. It returns 1 if the key exists and 0 if it doesn't:

std::map<std::string, int> scores;
scores["Alice"] = 95;

if (scores.count("Alice")) {
    std::cout << "Alice's score: " << scores["Alice"] << std::endl;
} else {
    std::cout << "Alice not found" << std::endl;
}

This prevents accidentally creating unwanted entries when checking for key existence, unlike using the square bracket operator [] which automatically creates keys with default values.

Try it yourself

#include <iostream>
#include <map>
#include <string>
using namespace std;

int main() {
    // Read number of books
    int n;
    cin >> n;
    
    // Create the library map
    map<string, int> library;
    
    // Read book information and populate the map
    for (int i = 0; i < n; i++) {
        string title;
        int copies;
        cin >> title >> copies;
        library[title] = copies;
    }
    
    // Read number of book requests
    int m;
    cin >> m;
    
    // TODO: Write your code below
    // Process each book request using .count() method
    // Check if book exists and print appropriate message
    
    return 0;
}