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;
}

Quiz

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