Defensive programming practices - Mini Project: Build & Apply

Defensive programming practices

📖 Scenario: You are writing a small program that calculates the average of three test scores entered by a user. To make sure your program works well even if the user makes mistakes, you will use defensive programming practices.

🎯 Goal: Build a C program that safely reads three test scores, checks if they are valid (between 0 and 100), calculates the average only if all scores are valid, and prints the result.

📋 What You'll Learn

Create three integer variables to store test scores.

Create a helper variable to track if all inputs are valid.

Use if statements to check if each score is between 0 and 100 inclusive.

Calculate the average only if all scores are valid.

Print the average or an error message if any score is invalid.

💡 Why This Matters

🌍 Real World

Defensive programming helps prevent bugs and crashes by checking data before using it, which is important in real software that interacts with users or external data.

💼 Career

Many programming jobs require writing safe and reliable code that handles unexpected or incorrect inputs gracefully.

1

Create variables for test scores

Create three integer variables called score1, score2, and score3 and set them to 0.

C

# Create three integer variables score1, score2, and score3 and set them to 0
// Your code here

Need a hint?

Use int to declare integer variables and assign 0 to each.

2

Add a variable to track input validity

Add an integer variable called valid and set it to 1 to assume inputs are valid initially.

C

int score1 = 0;
int score2 = 0;
int score3 = 0;
// Add an integer variable valid and set it to 1
// Your code here

Need a hint?

This variable will help us check if all scores are valid before calculating the average.

3

Check if scores are valid

Use three if statements to check if score1, score2, and score3 are each between 0 and 100 inclusive. If any score is outside this range, set valid to 0.

C

int score1 = 0;
int score2 = 0;
int score3 = 0;
int valid = 1;
// Check if score1, score2, and score3 are between 0 and 100 inclusive
// If not, set valid to 0
// Your code here

Need a hint?

Use logical OR || to check if a score is less than 0 or greater than 100.

4

Calculate and print average or error

If valid is 1, calculate the average of score1, score2, and score3 as a float variable average and print it with printf. Otherwise, print "Error: Invalid score(s)".

C

int score1 = 0;
int score2 = 0;
int score3 = 0;
int valid = 1;

if (score1  100) {
    valid = 0;
}
if (score2  100) {
    valid = 0;
}
if (score3  100) {
    valid = 0;
}

// If valid is 1, calculate average and print it
// Else print "Error: Invalid score(s)"
// Your code here

Need a hint?

Use printf with %.2f to print the average with two decimal places.