Defensive programming practices - Step-by-Step Execution

Concept Flow - Defensive programming practices

Start Program

↓

Input Data

↓

Check Input Validity?

No→Handle Error

Yes↓

Process Data Safely

↓

Check for Errors During Processing?

Yes→Handle Error

No↓

Output Result

↓

End Program

The program starts by getting input, checks if input is valid, handles errors if needed, processes data safely, checks for errors again, then outputs the result and ends.

Execution Sample

C

#include <stdio.h>
#include <stdlib.h>

int main() {
    int num;
    if (scanf("%d", &num) != 1) {
        printf("Invalid input\n");
        return 1;
    }
    if (num <= 0) {
        printf("Number must be positive\n");
        return 1;
    }
    printf("You entered: %d\n", num);
    return 0;
}

This code reads an integer, checks if input is valid and positive, handles errors by printing messages, then prints the number.

Execution Table

Step	Action	Input/Condition	Result	Next Step
1	Read input with scanf	User enters 'abc'	scanf returns 0 (failure)	Print 'Invalid input' and exit
2	Print error message	Invalid input detected	Output: 'Invalid input'	Program ends with return 1
3	Program ends	Exit code 1	No further action	End
4	Read input with scanf	User enters '5'	scanf returns 1 (success), num=5	Check if num <= 0
5	Check if num <= 0	num=5	False	Print 'You entered: 5'
6	Print valid input	num=5	Output: 'You entered: 5'	Program ends with return 0
7	Program ends	Exit code 0	No further action	End

💡 Program stops when input is invalid or after printing the valid number.

Variable Tracker

Variable	Start	After Step 1	After Step 4	After Step 5	Final
num	undefined	undefined (scanf failed)	5	5	5

Key Moments - 3 Insights

Why do we check the return value of scanf?

Because scanf returns the number of successfully read items. If it is not 1, input was invalid. This is shown in execution_table row 1 where scanf fails and we handle the error.

Why do we check if num <= 0 after reading input?

To ensure the number meets expected conditions (positive). This prevents invalid data from causing problems later. See execution_table row 5 where the condition is checked.

What happens if input is invalid?

The program prints an error message and exits early to avoid further errors. This is shown in execution_table rows 2 and 3.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'num' after step 4 when input is '5'?

A 5

B undefined

C 0

D -1

💡 Hint

Check variable_tracker column 'After Step 4' for 'num'

At which step does the program detect invalid input and stop?

A Step 2

B Step 5

C Step 1

D Step 6

💡 Hint

Look at execution_table row 1 where scanf fails and program decides to stop

If the input was '-3', which step would handle this error?

A Step 1

B Step 5

C Step 4

D Step 6

💡 Hint

Check where the condition 'num <= 0' is evaluated in execution_table

Concept Snapshot

Defensive programming means checking inputs and conditions before using data.
Always verify input success (e.g., scanf return value).
Check data validity (e.g., positive numbers).
Handle errors early by printing messages and exiting.
This prevents crashes and unexpected behavior.

Full Transcript

This example shows defensive programming in C. The program reads an integer from the user. It first checks if the input was successfully read by checking scanf's return value. If input is invalid, it prints an error and stops. If input is valid, it checks if the number is positive. If not, it prints an error and stops. Otherwise, it prints the number and ends normally. This approach avoids problems caused by bad input or unexpected values.