Two-dimensional arrays in C - Mini Project: Build & Apply

Working with Two-Dimensional Arrays in C

📖 Scenario: You are helping a small shop owner keep track of daily sales for different products over a week. The sales data is stored in a table where each row represents a product and each column represents a day of the week.

🎯 Goal: Create a program that stores sales data in a two-dimensional array, calculates the total sales for each product, and then prints the totals.

📋 What You'll Learn

Create a two-dimensional array called sales with 3 rows and 7 columns to store sales numbers.

Create an integer variable called total to hold the sum of sales for each product.

Use a for loop with variable i to go through each product (row).

Use a nested for loop with variable j to go through each day (column) for the current product.

Calculate the total sales for each product by adding the daily sales.

Print the total sales for each product using printf.

💡 Why This Matters

🌍 Real World

Two-dimensional arrays are useful for storing tables of data like sales, schedules, or pixel colors in images.

💼 Career

Understanding how to work with two-dimensional arrays is important for jobs involving data analysis, software development, and any task that requires managing grid-like data.

1

Create the sales data array

Create a two-dimensional integer array called sales with 3 rows and 7 columns. Initialize it with these exact values: first row {10, 12, 11, 14, 13, 15, 16}, second row {8, 9, 7, 10, 11, 12, 13}, third row {5, 6, 5, 7, 8, 6, 7}.

C

# Create the sales array with the exact values
# Your code here

Hint

Use curly braces to group each row's values inside the array initialization.

2

Create a variable for total sales

Create an integer variable called total and set it to 0. This will hold the sum of sales for each product.

C

int sales[3][7] = {
    {10, 12, 11, 14, 13, 15, 16},
    {8, 9, 7, 10, 11, 12, 13},
    {5, 6, 5, 7, 8, 6, 7}
};

// Create the total variable and set it to 0
// Your code here

Hint

Remember to specify the type int and initialize total to zero.

3

Calculate total sales for each product

Use a for loop with variable i from 0 to 2 to go through each product (row). Inside it, set total to 0. Then use a nested for loop with variable j from 0 to 6 to go through each day (column). Add sales[i][j] to total inside the inner loop.

C

int sales[3][7] = {
    {10, 12, 11, 14, 13, 15, 16},
    {8, 9, 7, 10, 11, 12, 13},
    {5, 6, 5, 7, 8, 6, 7}
};

int total = 0;

// Use for loops to calculate total sales for each product
// Your code here

Hint

Reset total to zero for each product before adding daily sales.

4

Print total sales for each product

Inside the outer for loop, after the inner loop, use printf to print the total sales for product i. Use this exact format: Product %d total sales: %d\n, where the first %d is i + 1 and the second %d is total.

C

int sales[3][7] = {
    {10, 12, 11, 14, 13, 15, 16},
    {8, 9, 7, 10, 11, 12, 13},
    {5, 6, 5, 7, 8, 6, 7}
};

int total = 0;

for (int i = 0; i < 3; i++) {
    total = 0;
    for (int j = 0; j < 7; j++) {
        total += sales[i][j];
    }
    // Print the total sales for product i
    // Your code here
}

Hint

Use printf("Product %d total sales: %d\n", i + 1, total); to print the results.

Practice

1. What is a two-dimensional array in C?

easy

A. An array of arrays, storing data in rows and columns

B. A single list of numbers stored in a line

C. A function that returns multiple values

D. A variable that holds only one value

Solution

1. Step 1: Understand array structure

   A two-dimensional array stores data in a grid format with rows and columns.

2. Step 2: Compare options

   Only An array of arrays, storing data in rows and columns describes this structure as an array of arrays.

3. Final Answer:

   An array of arrays, storing data in rows and columns -> Option A

4. Quick Check:

   2D array = array of arrays [OK]

Hint: Think of a table with rows and columns [OK]

Common Mistakes:

• Confusing 2D arrays with single arrays
• Thinking 2D arrays are functions
• Assuming 2D arrays hold only one value

2. Which of the following is the correct way to declare a 3x3 integer two-dimensional array in C?

easy

A. int arr(3,3);

B. int arr[3][3];

C. int arr[3,3];

D. int arr{3}{3};

Solution

1. Step 1: Recall C array declaration syntax

   In C, arrays use square brackets with sizes inside, like arr[rows][columns].

2. Step 2: Check each option

   Only int arr[3][3]; uses correct syntax: int arr[3][3];

3. Final Answer:

   int arr[3][3]; -> Option B

4. Quick Check:

   Use square brackets for array sizes [OK]

Hint: Use square brackets for array sizes, not parentheses [OK]

Common Mistakes:

• Using parentheses instead of square brackets
• Using commas inside brackets
• Using curly braces for declaration

3. What will be the output of the following C code?

int arr[2][2] = {{1, 2}, {3, 4}};
printf("%d", arr[1][0]);

medium

A. 3

B. 1

C. 2

D. 4

Solution

1. Step 1: Understand array initialization

   The array arr has two rows: first row {1, 2}, second row {3, 4}.

2. Step 2: Access element arr[1][0]

   Row index 1 is second row, column index 0 is first element in that row, which is 3.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   arr[1][0] = 3 [OK]

Hint: Remember indexing starts at 0 for rows and columns [OK]

Common Mistakes:

• Confusing row and column indexes
• Starting index at 1 instead of 0
• Mixing up elements in initialization

4. Identify the error in the following C code snippet:

int matrix[2][3];
matrix[2][1] = 5;

medium

A. Wrong data type for matrix

B. No error, code is correct

C. Syntax error due to missing semicolon

D. Index out of bounds error on matrix[2][1]

Solution

1. Step 1: Check array size and valid indexes

   matrix is declared with 2 rows (0 and 1) and 3 columns (0 to 2).

2. Step 2: Analyze matrix[2][1]

   Row index 2 is invalid because valid rows are 0 and 1 only, causing out of bounds error.

3. Final Answer:

   Index out of bounds error on matrix[2][1] -> Option D

4. Quick Check:

   Row index must be less than 2 [OK]

Hint: Array indexes start at 0 and go to size-1 [OK]

Common Mistakes:

• Using index equal to array size
• Ignoring zero-based indexing
• Assuming no error for out-of-range access

5. Given the 2D array declaration int grid[3][3] = {{1,2,3},{4,5,6},{7,8,9}};, which code snippet correctly calculates the sum of all elements?

hard

A. int sum=0; for(int i=1;i<3;i++) for(int j=1;j<3;j++) sum += grid[i][j]; printf("%d", sum);

B. int sum=0; for(int i=0;i<=3;i++) for(int j=0;j<=3;j++) sum += grid[i][j]; printf("%d", sum);

C. int sum=0; for(int i=0;i<3;i++) for(int j=0;j<3;j++) sum += grid[i][j]; printf("%d", sum);

D. int sum=0; for(int i=0;i<3;i++) sum += grid[i]; printf("%d", sum);

Solution

1. Step 1: Understand array size and loops

   grid has 3 rows and 3 columns, so indexes go from 0 to 2.

2. Step 2: Check loop bounds and summation

   int sum=0; for(int i=0;i<3;i++) for(int j=0;j<3;j++) sum += grid[i][j]; printf("%d", sum); uses correct loop bounds (i<3, j<3) and sums all elements correctly.

3. Step 3: Analyze other options

   int sum=0; for(int i=0;i<=3;i++) for(int j=0;j<=3;j++) sum += grid[i][j]; printf("%d", sum); uses <=3 causing out-of-bounds access; the option starting loops at 1 misses first row and column elements; the option trying sum += grid[i] is invalid for 2D arrays.

4. Final Answer:

   int sum=0; for(int i=0;i<3;i++) for(int j=0;j<3;j++) sum += grid[i][j]; printf("%d", sum); -> Option C

5. Quick Check:

   Use correct loop bounds and access grid[i][j] [OK]

Hint: Loop from 0 to size-1 for both rows and columns [OK]

Common Mistakes:

• Using <= instead of < in loops
• Starting loops at 1 instead of 0
• Trying to add entire row instead of elements