Lesson 15: Debugging Techniques

What is Debugging — The Art of Finding and Fixing Bugs

Basic debugging steps

Reading Compile Errors

Common compile errors

int x = 10   // ← no ; !
printf("%d", x);

printf Debugging — The Easiest Technique

Techniques

// Technique 1: print variable values
printf("DEBUG: x=%d, y=%d\n", x, y);

// Technique 2: check if execution reached this point
printf("DEBUG: reached before loop\n");

// Technique 3: print loop variables every iteration
for (int i = 0; i < n; i++) {
  printf("DEBUG: i=%d, sum=%d\n", i, sum);
  sum += a[i];
}

// Technique 4: see which branch was taken
if (x > 0) {
  printf("DEBUG: x is positive\n");
} else {
  printf("DEBUG: x is 0 or negative\n");
}

Try it yourself

Common Bug Patterns

// NG: accesses a[5] (out of bounds)
for (int i = 0; i <= 5; i++) sum += a[i];
// OK:
for (int i = 0; i < 5; i++) sum += a[i];

int sum;       // ← initial value is garbage!
for (...) sum += a[i]; // garbage + a[i] = ?
// Correct: int sum = 0;

if (x = 5)  // NG: assigns 5 to x (always true)
if (x == 5) // OK: checks whether x equals 5

int i = 0;
while (i < 10) {
  printf("%d\n", i);
  // forgot i++! -> i stays 0 forever
}

scanf("%d", x);   // NG: crash!
scanf("%d", &x);  // OK

Bug Fix Challenge

#include<stdio.h>
int main(void) {
  int data[] = {3, 8, 2, 7, 5};
  int n = 5;
  int sum = 0;              // ← fix 1: initialize
  for (int i = 0; i < n; i++) { // ← fix 2: from 0, < n
    sum += data[i];
  }
  double avg = (double)sum / n; // ← fix 3: double & cast
  printf("average = %.2f\n", avg);
}