Creating a Map

Part of the Logic & Flow section of Coddy's C++ journey — lesson 24 of 56.

Now that you understand what a map is, let's learn how to create one and add key-value pairs to it. When declaring a std::map, you need to specify both the key type and the value type using angle brackets.

Here's the basic syntax for creating a map:

std::map<KeyType, ValueType> mapName;

For example, to create a map that stores student names as keys and their test scores as values:

std::map<std::string, int> studentScores;

To add elements to your map, you can use the square bracket notation with the key, then assign a value:

studentScores["Alice"] = 95;
studentScores["Bob"] = 87;
studentScores["Carol"] = 92;

This creates three key-value pairs in the map. Each student's name serves as the key that allows you to quickly retrieve their corresponding score.

To iterate over all entries in a map, use a range-based for loop with auto. Each element in a std::map is a key-value pair, so you access the key with .first and the value with .second:

for (const auto& pair : studentScores) {
    std::cout << pair.first << ": " << pair.second << std::endl;
}

Here, auto automatically deduces the type of each element, pair.first gives the key (student name), and pair.second gives the value (score). Note that std::map always iterates in sorted order by key.

Challenge

Easy

Create a program that builds a student grade tracking system using a std::map. Your program will store student names as keys and their test scores as values.

The following inputs will be provided:

An integer n representing the number of students
Then n pairs of inputs:
- A string representing the student's name
- An integer representing the student's score

Your program should:

Create a std::map<std::string, int> named grades
Read the number of students
For each student, read their name and score, then add them to the map using the square bracket notation
After adding all students, print each student's information in the format shown below
Calculate and print the total number of students stored in the map

Use the following exact output format:

Student Grades:\n[student1]: [score1]\n[student2]: [score2]\n[student3]: [score3]\n...\nTotal students: [number of students]

The students should be printed in the order they appear when iterating through the map (which will be alphabetical order since maps automatically sort by key). Use a range-based for loop to iterate through the map, accessing each key-value pair and printing the student name followed by their score.

Cheat sheet

To create a std::map, specify the key and value types in angle brackets:

std::map<KeyType, ValueType> mapName;

Example with string keys and integer values:

std::map<std::string, int> studentScores;

Add elements using square bracket notation:

studentScores["Alice"] = 95;
studentScores["Bob"] = 87;
studentScores["Carol"] = 92;

Iterate over a map using a range-based for loop. Each element is a pair — use .first to access the key and .second to access the value:

for (const auto& pair : studentScores) {
    std::cout << pair.first << ": " << pair.second << std::endl;
}

auto automatically deduces the type of each element, and const auto& avoids unnecessary copying.

Try it yourself

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

int main() {
    // Read the number of students
    int n;
    cin >> n;
    
    // Create the map to store grades
    map<string, int> grades;
    
    // TODO: Write your code here
    // Read each student's name and score, then add to the map
    
    
    // Print the results
    cout << "Student Grades:" << endl;
    // TODO: Use a range-based for loop to print each student's grade
    
    
    // Print total number of students
    cout << "Total students: " << grades.size() << endl;
    
    return 0;
}