4.5. Debugging for loops

In this section, we debug an example program that uses a nested loop to print a pattern. The program is shown below. We go through common mistakes and types of errors that you might encounter when writing a code with a loop.

4.5.1. Goal of debugging exercises

• Practice reviewing/reading other people’s code

• Fix mistakes in other people’s code (debugging)

• See/avoid mistakes in your own code

4.5.2. Common mistakes

The first step in debugging is to identify the common mistakes. Here are some common mistakes that you might make when writing a loop.

• Wrong initialization of the loop variable, e.g., initializing i = 0 instead of i = 1 for a loop that starts from 1.

• Did not have a statement that changes the condition appropriately, e.g., i -= 1 instead of i += 1.

• Using incorrect logical operator, e.g., used && instead of || or vice versa.

• Using uncomfortable loop type, e.g., using a while loop in case where it is easier to deal with do-while loop.

• Missing an = or adding an = in the wrong place, e.g., having while(x > 0) instead of while ( x >= 0).

4.5.3. Debugging a program with a loop

The following is a code to print a pattern of stars. The code is shown below. Download triangle-with-bugs.c to debug the program yourself.

#include <stdio.h>

int main(void) {
  int n = 0;
  printf("Enter the number of rows: ");
  scanf("%d", &n);

  for (int row = 1; row <= n; row += 1) {
    for (int col = 1; col < n; col += 1) {
      if (col <= n - row) {
        printf(" ");
      } else if (col >= n - row || col <= n - 1 + row) {
        printf("*");
      }
    }
    printf("\n");
  }
  return 0;
}

Expected output

Enter the number of rows: 5
    *
   ***
  *****
 *******
*********

Enter the number of rows: 4
   *
  ***
 *****
*******

Enter the number of rows: 3
  *
 ***
*****

Actual Output

Enter the number of rows: 5
    
   *
  **
 ***
****

You can find the corrected code below. Download triangle-fixed.c to see the corrected code.

Corrected Code

#include <stdio.h>

int main(void) {
  int n = 0;
  printf("Enter the number of rows: ");
  scanf("%d", &n);

  for (int row = 1; row <= n; row += 1) {
    for (int col = 1; col < 2 * n; col += 1) {
      if (col <= n - row) {
        printf(" ");
      } else if (col > n - row && col <= n - 1 + row) {
        printf("*");
      }
    }
    printf("\n");
  }
  return 0;
}

4.5.4. Debugging a program with a do-while loop

The following code is supposed to take input int from the user into maxRange variable, then finds a random prime number between \(1\) and maxRange. However, it has a few bugs that we discuss and fix in the video above. You can try out debugging the code yourself here before watching the video: random-primeNum-with-bugs.c.

Code with bugs

// Question 11 in Winter 2018 Midterm Exam
// Debug the following code
// The program is supposed to take in from the user
// an int variable named maxRange.
// Then, it should print a prime number randomly generated
// between 1 and maxRange.
// Prime number is a number that is not divisible by any number
// except for itself and 1.

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

int main(void) {
  // Enter input
  int maxRange = 0;
  printf("Enter the maximum range: ");
  scanf("%d", &maxRange);

  // Generate a random number, check if it is prime, if yes, great, if not
  // re-generate
  bool isPrime = true;
  int primeRandNum = 0;
  do {
    primeRandNum = rand() % (maxRange - 1 + 1) + 1;

    isPrime = true;
    for (int denom = 2; denom < primeRandNum; denom++) {
      if (primeRandNum % denom == 0) {
        break;
      }
    }

  } while (!isPrime);

  printf("Random prime number generated is %d", primeRandNum);

  return 0;
}

Download the fixed code: random-primeNum.c.

Fixed code

// Question 11 in Winter 2018 Midterm Exam
// Fixed code!
// The program is supposed to take in from the user
// an int variable named maxRange.
// Then, it should print a prime number randomly generated
// between 1 and maxRange.
// Prime number is a number that is not divisible by any number
// except for itself and 1, e.g. 2, 3, 5, 11, 13 ...

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

int main(void) {
  // Enter input
  int maxRange = 0;
  printf("Enter the maximum range: ");
  fflush(stdout);
  scanf(" %d", &maxRange);

  // Generate a random number, check if it is prime, if yes, great, if not
  // re-generate
  bool isPrime = true;
  int primeRandNum = 0;
  do {
    primeRandNum = rand() % (maxRange - 1 + 1) + 1;

    isPrime = true;
    for (int denom = 2; denom < primeRandNum && isPrime; denom++) {
      if (primeRandNum % denom == 0) {
        isPrime = false;
      }
    }
    if (primeRandNum == 1) {
      isPrime = false;
    }
  } while (!isPrime);

  printf("Random prime number generated is %d\n", primeRandNum);

  return 0;
}