Arrays — 1D and 2D

What Is an Array?

An array is a fixed-size, ordered block of memory that holds multiple values of the same type, laid out one right after another. Instead of creating five separate variables to hold five test scores, you create one array that holds all five.

In C++, a plain array has a size that is fixed the moment it's declared — you cannot grow or shrink it later. Every element sits at a specific position called an index, and indexing always starts at 0, not 1.

Declaring and Initializing Arrays

You declare a fixed-size array by writing the element type, a name, and the number of elements in square brackets. You can initialize it immediately with a comma-separated list of values inside curly braces.

// arrays_basic.cpp
#include <iostream>
using namespace std;

int main() {
    int scores[5] = {90, 85, 78, 92, 88};

    for (int i = 0; i < 5; i++) {
        cout << "Score " << i << ": " << scores[i] << endl;
    }

    return 0;
}

The array scores holds exactly 5 int values, indexed 0 through 4. You can also declare an array without listing every value up front — uninitialized elements of a local array will contain garbage values until you assign them, so always initialize your arrays.

Iterating Arrays with Loops

A plain for loop is the most common way to walk through every element of an array — reading each value, summing them, or searching for something specific.

// arrays_sum_max.cpp
#include <iostream>
using namespace std;

int main() {
    int nums[6] = {12, 45, 7, 89, 23, 56};
    int size = sizeof(nums) / sizeof(nums[0]); // 6

    int sum = 0;
    int maxVal = nums[0];

    for (int i = 0; i < size; i++) {
        sum += nums[i];
        if (nums[i] > maxVal) {
            maxVal = nums[i];
        }
    }

    cout << "Sum: " << sum << endl;
    cout << "Max: " << maxVal << endl;

    return 0;
}

Notice how size is calculated once at the top and reused in the loop condition — that's a much safer habit than hardcoding 6 in multiple places, especially once the array grows or shrinks during development.

Array Size Gotchas

Unlike Python lists or JavaScript arrays, a plain C++ array does not know its own length at runtime. There is no .length or .size() built into it. The sizeof(arr) / sizeof(arr[0]) trick you just saw works — but only in the exact same scope where the array was declared.

For now, the practical rule is simple: always know your array's size independently — either compute it with sizeof right where the array is declared, or store it in a constant or variable you pass around together with the array.

2D Arrays — A Grid of Values

A 2D array is an array of arrays — think of it as a table with rows and columns. You declare it with two sets of square brackets: type name[rows][cols]. To visit every cell, you nest two for loops — an outer loop for rows, an inner loop for columns.

// board.cpp
#include <iostream>
using namespace std;

int main() {
    char board[3][3] = {
        {'X', 'O', 'X'},
        {'O', 'X', 'O'},
        {'X', 'X', 'O'}
    };

    for (int row = 0; row < 3; row++) {
        for (int col = 0; col < 3; col++) {
            cout << board[row][col] << " ";
        }
        cout << endl;
    }

    return 0;
}

Here board[row][col] reaches the exact cell at that row and column. This same pattern — outer loop for rows, inner loop for columns — is how you'd process a small spreadsheet, a game board, a black-and-white image, or a simple matrix.

Lesson Summary

Let's recap everything you learned in this lesson: