📘 Section 4 — 2D Arrays

Expand your array skills into two dimensions — rows, columns, matrices, and tabular data.

🎯 What You’ll Learn

How to declare and initialize 2D arrays
Navigating rows and columns with nested loops
Working with matrices (comparison, row operations)
Building a real-world sales report

1. Two-Dimensional Arrays

A 2D array is like a table with rows and columns:

// int array[row][col];
// int array[][col];
int array[][3] = {
    {25, 50, 100},
    {12, 20,  32}
};

This creates a 2×3 table:

	Col 0	Col 1	Col 2
Row 0	25	50	100
Row 1	12	20	32

💡 You can omit the row count when initializing with values, but you must always specify the column count.

Accessing Elements

array[0][2]  // → 100 (row 0, column 2)
array[1][0]  // → 12  (row 1, column 0)

Traversing with Nested Loops

for (int i = 0; i < 2; i++) {        // rows
    for (int j = 0; j < 3; j++) {    // columns
        printf("%d ", array[i][j]);
    }
    printf("\n");
}

🧪 Exercises

Exercise 1: Sales Report

Task: A company has four salespeople (1 to 4) who sell five different products (1 to 5). Once a day, each salesperson passes in a slip for each different type of product sold. Each slip contains the salesperson number, the product number, and the total dollar value of that product sold that day.

Write a program that reads all this sales information and summarizes the total sales by salesperson by product. All totals should be stored in the two-dimensional array sales. After processing all the information, print the results in tabular format with each column representing a particular salesperson and each row representing a particular product. Cross-total each row to get the total sales of each product; cross-total each column to get the total sales by salesperson.

💡 Hint

Use sales[product-1][salesperson-1] to map 1-based input to 0-based indices
Use += amount to accumulate sales for the same product/salesperson combo
Create a separate array totalSalesPerPerson[4] for column totals
Use while(1) with break to handle unknown number of inputs until user enters -1

🟢 Click to Show Solution

#include <stdio.h>

int main() {
    double sales[5][4] = {0};
    int salesperson, product;
    double amount;

    printf("Enter salesperson (1-4), product (1-5), and sales amount.\n");
    printf("Enter -1 to stop:\n");

    while (1) {
        scanf("%d", &salesperson);
        if (salesperson == -1) {
            break;
        }
        scanf("%d%lf", &product, &amount);
        sales[product - 1][salesperson - 1] += amount;
    }

    printf("\nSales Report\n");

    double totalSalesPerPerson[4] = {0};
    double totalOfProduct;

    for (int i = 0; i < 5; i++) {
        totalOfProduct = 0;
        for (int j = 0; j < 4; j++) {
            printf("%7.1f", sales[i][j]);
            totalOfProduct += sales[i][j];
            totalSalesPerPerson[j] += sales[i][j];
        }
        printf("%12.1f\n", totalOfProduct);
    }

    printf("----------------------------------------------\n");

    double totalSales = 0;
    for (int j = 0; j < 4; j++) {
        printf("%7.1f", totalSalesPerPerson[j]);
        totalSales += totalSalesPerPerson[j];
    }
    printf("%12.1f\n", totalSales);

    return 0;
}

Key concepts:

while (1) with break — loop until the user enters -1
%7.1f — 7 total places, 1 after the decimal point (aligns columns nicely)
%lf is for double in scanf
Cross-totals track both row sums (per product) and column sums (per salesperson)

Exercise 2: Matrix Equality

Task: Write a C program that checks if 2 matrices 3×3 entered by the user are equal or not.

💡 Hint

Use nested loops to read both matrices
Compare element by element — if any pair differs, print “NOT EQUAL” and exit immediately

🟢 Click to Show Solution

#include <stdio.h>

int main() {
    int m1[3][3], m2[3][3];

    for (int i = 0; i < 3; i++)
        for (int j = 0; j < 3; j++)
            scanf("%d", &m1[i][j]);

    for (int i = 0; i < 3; i++)
        for (int j = 0; j < 3; j++)
            scanf("%d", &m2[i][j]);

    for (int i = 0; i < 3; i++) {
        for (int j = 0; j < 3; j++) {
            if (m1[i][j] != m2[i][j]) {
                printf("NOT EQUAL MATRIX");
                return 0;
            }
        }
    }

    printf("EQUAL MATRIX");
    return 0;
}

💡 Early exit: As soon as one pair doesn’t match, we know the result — no need to keep checking.

Exercise 3: Maximum per Row

Task: Find maximum element of each row in a matrix.

💡 Hint

For each row, assume the first element is the max
Compare all elements in that row and update if larger

🟢 Click to Show Solution

#include <stdio.h>

int main() {
    int arr[3][5] = {
        {10, 2, 3, 4, 4},
        { 1, 2, 1, 3, 3},
        { 3, 2, 5, 7, 8}
    };

    for (int i = 0; i < 3; i++) {
        int max = arr[i][0];
        for (int j = 0; j < 5; j++) {
            if (arr[i][j] > max) {
                max = arr[i][j];
            }
        }
        printf("Max element in row number %d equal %d\n", i + 1, max);
    }

    return 0;
}

Output:

Max element in row number 1 equal 10
Max element in row number 2 equal 3
Max element in row number 3 equal 8

💡 This is the same “find max” pattern from 1D arrays, applied once per row using the outer loop.

📋 Quick Reference

Task	Pattern
Declare 2D array	`int arr[rows][cols];`
Initialize with values	`int arr[][3] = { {1,2,3}, {4,5,6} };`
Access element	`arr[row][col]`
Traverse all elements	Nested `for` loops (outer = rows, inner = cols)
Row total	Sum inner loop, reset before each row
Column total	Use a separate array, accumulate across rows

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