Arrays

Many values, one name

Suppose you need the marks of 5 students. Five separate variables (mark1, mark2, ...) would get messy. An array holds many values of the same type under one name:

#include <iostream>
using namespace std;

int main() {
    int marks[5] = {72, 85, 90, 64, 78};
    cout << "First mark: " << marks[0] << "\n";
    cout << "Third mark: " << marks[2] << "\n";
    cout << "Last mark:  " << marks[4] << "\n";
    return 0;
}

Output

First mark: 72
Third mark: 90
Last mark:  78

Think of it like this: An array is like a row of numbered lockers. All the lockers share one name (marks), and each has a number on its door (the index).

Remember: Indexes start at 0, not 1! An array of 5 items has indexes 0, 1, 2, 3 and 4.

Changing an item

#include <iostream>
using namespace std;

int main() {
    int marks[5] = {72, 85, 90, 64, 78};
    marks[3] = 70;              // change the 4th mark
    marks[0] += 5;              // add 5 to the first mark
    cout << marks[0] << " " << marks[3] << "\n";
    return 0;
}

Output

77 70

Looping over an array

A for loop from 0 up to (but not including) the size visits every item:

#include <iostream>
using namespace std;

int main() {
    const int SIZE = 5;
    int marks[SIZE] = {72, 85, 90, 64, 78};
    int total = 0;
    for (int i = 0; i < SIZE; i++) {
        cout << "Student " << i + 1 << ": " << marks[i] << "\n";
        total += marks[i];
    }
    cout << "Average: " << total / static_cast<double>(SIZE) << "\n";
    return 0;
}

Output

Student 1: 72
Student 2: 85
Student 3: 90
Student 4: 64
Student 5: 78
Average: 77.8

Common mistake: Going past the end. marks[5] in a 5-item array is out of bounds. C++ does not stop you – it reads or overwrites whatever memory is there. This bug (a buffer overflow) is behind many famous security attacks.

Tip: Arrays have a fixed size that can never change. For lists that grow and shrink, use a vector (next lesson).