Debugging CS Thinking Example 4

Follow the full solution, then compare it with the other examples linked below.

Example 4

medium
A program to sum the numbers 1 through 5 uses `SET total = 1; FOR i = 1 TO 5: total = total + i`. It outputs 16 instead of 15. Identify the bug and fix it.

Solution

  1. 1
    Step 1: The bug is the initial value. total starts at 1, so the algorithm adds an extra 1 before the loop even begins.
  2. 2
    Step 2: Fix it by starting with total = 0. Then the loop adds 1 + 2 + 3 + 4 + 5 = 15.

Answer

Initialize total to 0, not 1. Then the program outputs 15.
Debugging often comes down to checking assumptions about initial values and state. A small setup error can make the whole program wrong even when the loop logic is correct.

About Debugging

The systematic process of finding, diagnosing, and correcting errors (bugs) in a program. Debugging involves reproducing the problem, isolating its cause through testing and inspection, applying a targeted fix, and verifying the fix resolves the issue without introducing new problems.

Learn more about Debugging →

More Debugging Examples

Example 1 medium

This code should print numbers 1 to 5 but has a bug: SET i = 1. WHILE i < 5: OUTPUT i. i = i + 1. Fi

Example 2 medium

This code calculates an average but gives the wrong answer: SET total = 0. FOR each num in [10, 20,

Example 3 medium

This code should output the largest number in [3, 7, 2, 9, 4] but outputs 3: SET largest = nums[0].

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