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

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

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

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

Hint

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

Practice

1. What is the main goal of defensive programming in C?

easy

A. To check for errors before they cause problems

B. To write code faster without testing

C. To ignore invalid inputs

D. To make the program run without any comments

Solution

1. Step 1: Understand the purpose of defensive programming

   Defensive programming is about preventing errors by checking inputs and conditions early.

2. Step 2: Identify the correct goal from options

   To check for errors before they cause problems clearly states checking for errors before problems happen, which matches defensive programming.

3. Final Answer:

   To check for errors before they cause problems -> Option A

4. Quick Check:

   Defensive programming = check errors early [OK]

Hint: Defensive programming means catching errors early [OK]

Common Mistakes:

• Thinking it means ignoring errors
• Confusing it with faster coding
• Believing it removes need for comments

2. Which of the following is the correct way to check for a NULL pointer before using it in C?

easy

A. if (ptr = NULL) { /* use ptr */ }

B. if (ptr != NULL) { /* use ptr */ }

C. if (ptr == 0) { /* use ptr */ }

D. if (ptr = 0) { /* use ptr */ }

Solution

1. Step 1: Identify correct NULL pointer check syntax

   In C, to check if a pointer is not NULL, use 'ptr != NULL'. Assignment '=' is wrong here.

2. Step 2: Analyze each option

   'if (ptr != NULL) { /* use ptr */ }' is correct because it checks that ptr is not NULL before use. B and D use '=' for assignment, setting ptr to NULL/0 and making the condition false. 'if (ptr == 0) { /* use ptr */ }' checks if ptr IS null (equivalent to NULL) and then uses it, causing a null dereference crash.

3. Final Answer:

   if (ptr != NULL) { /* use ptr */ } -> Option B

4. Quick Check:

   Use '!=' to compare pointer with NULL [OK]

Hint: Use '!=' to compare pointer with NULL, not '=' [OK]

Common Mistakes:

• Using '=' instead of '==' or '!=' in conditions
• Confusing NULL with zero without clarity
• Not checking pointer before use

3. What will be the output of this C code snippet?

int *ptr = NULL;
if (ptr != NULL) {
    printf("Pointer is valid\n");
} else {
    printf("Pointer is NULL\n");
}

medium

A. Pointer is NULL

B. Pointer is valid

C. Compilation error

D. No output

Solution

1. Step 1: Analyze pointer initialization

   The pointer 'ptr' is set to NULL, meaning it points to nothing.

2. Step 2: Check the if condition

   The condition 'ptr != NULL' is false because ptr is NULL, so the else block runs.

3. Final Answer:

   Pointer is NULL -> Option A

4. Quick Check:

   NULL pointer triggers else block [OK]

Hint: NULL pointer means else block runs [OK]

Common Mistakes:

• Assuming NULL pointer is valid
• Expecting compilation error
• Thinking no output occurs

4. Identify the error in this defensive programming snippet:

void print_length(char *str) {
    if (str = NULL) {
        printf("String is NULL\n");
        return;
    }
    printf("Length: %d\n", strlen(str));
}

medium

A. strlen() cannot be used on strings

B. Missing semicolon after printf

C. Using assignment '=' instead of comparison '==' in if condition

D. Function return type should be int

Solution

1. Step 1: Check the if condition syntax

   The condition uses 'str = NULL' which assigns NULL to str instead of comparing it.

2. Step 2: Understand impact of assignment in condition

   Assignment returns NULL (false), so the if block never runs as intended. It should be 'str == NULL'.

3. Final Answer:

   Using assignment '=' instead of comparison '==' in if condition -> Option C

4. Quick Check:

   Use '==' to compare pointers, not '=' [OK]

Hint: Use '==' to compare, '=' is assignment [OK]

Common Mistakes:

• Confusing '=' and '==' in conditions
• Thinking strlen() is invalid here
• Ignoring function return type

5. You want to safely read an integer from user input in C. Which defensive programming practice should you apply?

hard

A. Use scanf without checking its return value

B. Use gets() to read input without validation

C. Assume input is always correct and proceed

D. Check scanf's return value and validate the input range

Solution

1. Step 1: Understand safe input reading

   Using scanf requires checking its return value to confirm input was read correctly.

2. Step 2: Validate input range

   After reading, check if the integer is within expected limits to avoid errors later.

3. Final Answer:

   Check scanf's return value and validate the input range -> Option D

4. Quick Check:

   Check input success and validity [OK]

Hint: Always check scanf return and input validity [OK]

Common Mistakes:

• Ignoring scanf return value
• Using unsafe input functions like gets()
• Assuming user input is always valid