# Lecture 13: Mid-Semester Exam Review CS 103: Computer Programming September 22, 2025 Examination Analysis

## Exam Structure Overview **Total Marks:** 40 | **Duration:** 2 hours - **Group A** (8 marks): Short answer questions - **Group B** (4 marks): True/False with justification - **Group C** (16 marks): Code output or error identification - **Group D** (12 marks): Implementation problems --- ### Key Focus Areas - Fundamental C++ concepts - Error detection and debugging - Practical programming skills

# Group A ## Succinct Answers Conceptual Understanding

## Group A: Question 1 [2 marks] **What is the difference between a compile-time error and a run-time error?** --- <div class="answer-box"> **Answer:** A compile-time error occurs during compilation (e.g., syntax, type errors), preventing execution. A run-time error occurs while the program runs (e.g., divide by zero, null pointer). Compile-time errors are detected before execution; run-time errors are detected during execution. </div> --- **Key Points:** - **Compile-time:** Caught by compiler (syntax, types, declarations) - **Run-time:** Occurs during execution (logic errors, invalid operations) - **Detection timing** is the critical distinction

## Group A: Question 2 [2 marks] **What is pass-by-reference and why is it used?** --- <div class="answer-box"> **Answer:** Pass-by-reference passes the address of a variable instead of a copy. It allows the function to directly modify the caller's variable. It avoids copying large data and improves efficiency. </div> --- **Key Benefits:** - Modify original variables - Avoid expensive copying - Efficient for large data structures **Syntax:** `void func(int& x)` or `void func(const string& s)`

## Group A: Question 3 [2 marks] **When is it essential to declare a function and when is it optional?** --- <div class="answer-box"> **Answer:** A function must be declared before use if its definition appears later in the code. It is optional if the definition comes before its first call. </div> --- **Examples:** ```cpp // Declaration needed int add(int, int); // Forward declaration int main() { return add(1,2); } int add(int a, int b) { return a+b; } // Declaration optional int add(int a, int b) { return a+b; } int main() { return add(1,2); } ```

## Group A: Question 4 [2 marks] **Why is it that a pointer of type `int` cannot store the address of a variable of type `float`?** --- <div class="answer-box"> **Answer:** Each pointer type is tied to its data type for correct dereferencing. An `int*` expects memory formatted as an integer, but a `float` has a different representation. Mismatching causes undefined behavior during pointer arithmetic. </div> --- **Key Concepts:** - Type safety in C++ - Memory representation differs (int vs float) - Pointer arithmetic depends on type size - Dereferencing interprets bytes according to type

# Group B ## True/False with Justification Testing Precise Understanding

## Group B: Question 1 [1 mark] **Statement:** The scope of a global variable is the entire program. --- <div class="answer-box"> **Answer: False** Global variables can be accessed anywhere only in the same file after declaration and across files with `extern`. </div> --- **Important Details:** - Scope limited to translation unit (file) by default - Use `extern` for cross-file access - Declaration point matters within the file - Not automatically visible in all files

## Group B: Question 2 [1 mark] **Statement:** The name of a static array is a pointer variable. --- <div class="answer-box"> **Answer: False** The array name decays to a pointer in most contexts, but it is not itself a pointer variable. It is a constant pointer. </div> --- **Key Distinction:** ```cpp int arr[5] = {1,2,3,4,5}; int* ptr = arr; // Decay: arr converts to pointer arr++; // ERROR: array name is not modifiable ptr++; // OK: ptr is a pointer variable ``` Array name is a **constant address**, not a variable

## Group B: Question 3 [1 mark] **Statement:** An assignment inside an `if` condition (e.g., `if (x = 0)`) is always a compile-time error in C++. --- <div class="answer-box"> **Answer: False** It compiles but assigns 0 to x. There is only compile time warning. The assignment operation to 0 returns false. </div> --- **Common Pitfall:** ```cpp int x = 5; if (x = 0) { // Assignment, not comparison! cout << "True"; // Never executes } // x is now 0, condition evaluates to false ``` Use `if (x == 0)` for comparison. Modern compilers warn about this.

## Group B: Question 4 [1 mark] **Statement:** The execution of a program always starts at the main function. --- <div class="answer-box"> **Answer: True** In C++, program execution always begins with `main()`. </div> --- **Important Notes:** - `main()` is the entry point for all C++ programs - Global variable initialization happens before `main()` - Static initializers run before `main()` - But logical execution flow starts at `main()`

# Group C ## Output / Error Type Code Analysis Skills

## Group C: Question 1 [2 marks] ```cpp #include <iostream> int main() { int a = 5; std::cout << a++ << " " << ++a << '\n'; return 0; } ``` --- <div class="answer-box"> **Answer:** Output: `5 7` or `6 7` </div> --- **Explanation:** - `a++` is post-increment (use then increment) - `++a` is pre-increment (increment then use) - **Undefined behavior** due to sequence point violation - Different compilers may produce different results - Avoid modifying same variable multiple times in one statement

## Group C: Question 2 [2 marks] ```cpp #include <iostream> int main() { int arr[3] = {1,2,3}; std::cout << arr[3] << '\n'; return 0; } ``` --- <div class="answer-box"> **Answer:** Runtime error (out-of-bounds access, undefined behavior) </div> --- **Why This Happens:** - Array indices: 0, 1, 2 for size 3 - `arr[3]` accesses memory beyond array - May crash, print garbage, or appear to work - C++ does not check array bounds automatically **Prevention:** Use `std::vector` with `.at()` for bounds checking

## Group C: Question 3 [2 marks] ```cpp #include <iostream> int main() { char s[] = {'h','e','l','l','o','\0'}; std::cout << s << '\n'; return 0; } ``` --- <div class="answer-box"> **Answer:** Output: `hello` </div> --- **Key Points:** - Character array initialized with null terminator `'\0'` - `cout` prints C-style strings until null terminator - Without `'\0'`, behavior would be undefined - Equivalent to: `char s[] = "hello";`

## Group C: Question 4 [2 marks] ```cpp #include <iostream> int f() { return 3.7; } int main() { std::cout << f() << '\n'; return 0; } ``` --- <div class="answer-box"> **Answer:** Output: `3` (fractional part truncated) </div> --- **Explanation:** - Function `f()` has return type `int` - Returning `3.7` causes implicit conversion - Truncation (not rounding): `3.7` → `3` - Compiler may warn about narrowing conversion - Loss of precision occurs

## Group C: Question 5 [2 marks] ```cpp #include <iostream> int main() { int *p = nullptr; std::cout << *p << '\n'; return 0; } ``` --- <div class="answer-box"> **Answer:** Runtime error (dereferencing null pointer) </div> --- **Details:** - `nullptr` means pointer doesn't point to valid memory - Dereferencing (`*p`) attempts to access that memory - Results in segmentation fault or crash - **Always check pointers before dereferencing:** ```cpp if (p != nullptr) { std::cout << *p << '\n'; } ```

## Group C: Question 6 [2 marks] ```cpp #include <iostream> int main() { if (1 = 2) std::cout << "yes\n"; return 0; } ``` --- <div class="answer-box"> **Answer:** Compile-time error (`1 = 2` is invalid; left operand must be a modifiable lvalue) </div> --- **Why This Fails:** - Assignment requires a modifiable lvalue on left - `1` is a literal constant (rvalue) - Cannot assign to literals - Error: `lvalue required as left operand of assignment` **Correct comparison:** `if (1 == 2)`

## Group C: Question 7 [2 marks] ```cpp #include <iostream> int main() { bool x = (5 > 3); std::cout << ((int)x - 1) << '\n'; return 0; } ``` --- <div class="answer-box"> **Answer:** Output: `0` (`true` is 1, so 1-1=0) </div> --- **Step-by-Step:** 1. `5 > 3` evaluates to `true` 2. `x` stores `true` (boolean value) 3. `(int)x` converts `true` to integer `1` 4. `1 - 1` = `0` 5. Output: `0` **Note:** `true` = 1, `false` = 0 when converted to int

## Group C: Question 8 [2 marks] ```cpp #include <iostream> int a[] = {1,2,3}; int main() { a++; std::cout << *a << '\n'; return 0; } ``` --- <div class="answer-box"> **Answer:** Compile-time error (array name is not assignable / constant pointer cannot be incremented) </div> --- **Why This Fails:** - Array name is a constant pointer to first element - Cannot modify array name itself - Error: `lvalue required as increment operand` **Alternative:** ```cpp int* ptr = a; ptr++; // This works! std::cout << *ptr << '\n'; // Prints 2 ```

# Group D ## Implementation Questions Practical Programming Skills

## Group D: Question 1 [3 marks] **Task:** Reverse an array in-place Implement the `reverse` function that reverses the elements of array `a` of size `n` in-place. --- **Expected Output:** `{3, 5, 7, 2, 9}` → `{9, 2, 7, 5, 3}`

## Group D: Question 1 - Solution ```cpp void reverse(int* a, int n) { for(int i=0; i < n; i++) { int temp = a[i]; a[i] = a[n-i-1]; a[n-i-1] = temp; } } ``` --- Code does not work!! **Reason:** It swaps twice and so the elements return to their original positions.

## Group D: Question 1 - Solution ```cpp void reverse(int* a, int n) { int i = 0, j = n - 1; while (i < j) { int temp = a[i]; a[i] = a[j]; a[j] = temp; i++; j--; } } ``` --- **Algorithm:** - Use two pointers: start and end - Swap elements and move pointers toward center - Stop when pointers meet - **Time Complexity:** O(n) - **Space Complexity:** O(1) - in-place

## Group D: Question 2 [3 marks] **Task:** Find the middle of an array Implement `find_middle` to return the middle element's value. For even length, return the first of the two middle elements. --- **Example:** `{3,5,2,1,6}` → middle is `2` (index 2)

## Group D: Question 2 - Solution ```cpp int find_middle(int* arr, int size) { int mid = (size - 1) / 2; return arr[mid]; } ``` --- **Key Points:** - For odd size (5): `(5-1)/2 = 2` ✓ - For even size (4): `(4-1)/2 = 1` (first of two middle) - Integer division automatically floors the result - **Time Complexity:** O(1) **Alternative for exact middle of even:** ```cpp int mid = size / 2; // Returns second of two middle ```

## Group D: Question 3 [3 marks] **Task:** Multiply two complex numbers Implement `compMult` to multiply `c1 = a+bi` and `c2 = c+di`, storing result in `real` and `imag`. --- **Formula:** (a+bi)(c+di) = (ac - bd) + (ad + bc)i **Example:** (3+2i)(1+4i) = -5+14i

## Group D: Question 3 - Solution ```cpp void compMult(double a, double b, double c, double d, double &real, double &imag) { real = a * c - b * d; imag = a * d + b * c; } ``` --- **Mathematical Derivation:** (a + bi)(c + di) = ac + adi + bci + bdi² Since i² = -1: = ac + adi + bci - bd = (ac - bd) + (ad + bc)i **Note:** Using pass-by-reference for output parameters

## Group D: Question 4 [3 marks] **Task:** Arrange characters alphabetically Implement `alphabeticOrder` that sorts a character array in-place alphabetically using any sorting algorithm (e.g., bubble sort). --- **Example:** `{'q', 'w', 'e', 'r', 't', 'y'}` → `{'e', 'q', 'r', 't', 'w', 'y'}`

## Group D: Question 4 - Solution ```cpp void alphabeticOrder(char* arr, int n) { for (int i = 0; i < n - 1; i++) { for (int j = 0; j < n - i - 1; j++) { if (arr[j] > arr[j + 1]) { char temp = arr[j]; arr[j] = arr[j + 1]; arr[j + 1] = temp; } } } } ``` --- **Bubble Sort Algorithm:** - Compare adjacent elements - Swap if out of order - Largest element "bubbles" to end each pass - **Time Complexity:** O(n²) - **Space Complexity:** O(1)

# Common Mistakes ## What to Avoid

## Common Mistakes - Memory & Pointers ### 1. Null Pointer Dereferencing ```cpp int* p = nullptr; cout << *p; // ❌ CRASH! ``` ### 2. Array Out of Bounds ```cpp int arr[5]; cout << arr[5]; // ❌ Undefined behavior ``` ### 3. Using Uninitialized Variables ```cpp int x; cout << x; // ❌ Garbage value ```

## Common Mistakes - Syntax & Logic ### 1. Assignment vs Comparison ```cpp if (x = 0) // ❌ Assignment if (x == 0) // ✓ Comparison ``` ### 2. Missing Semicolons ```cpp cout << "Hello" << endl // ❌ cout << "Hello" << endl; // ✓ ``` ### 3. Off-by-One Errors ```cpp for(int i = 0; i <= n; i++) // ❌ One too many for(int i = 0; i < n; i++) // ✓ Correct ```

## Key Takeaways from This Exam ### Conceptual Understanding - Know the difference between compile-time and run-time errors - Understand pointer types and memory safety - Master pass-by-value vs pass-by-reference ### Practical Skills - Array manipulation (reversal, searching) - Pointer arithmetic and dereferencing - Algorithm implementation (sorting, mathematical operations) --- ## Key Takeaways ### Error Prevention - Always initialize variables - Check array bounds - Validate pointers before dereferencing - Use comparison (`==`) not assignment (`=`) in conditions

## Study Tips for Future Success ### 1. Practice Code Reading - Trace through code line by line - Predict output before running - Identify potential errors ### 2. Understand, Don't Memorize - Know *why* things work, not just *how* - Understand error messages - Learn from mistakes --- ## More Tips ### 3. Write Clean Code - Use meaningful variable names - Add comments for complex logic - Test edge cases ### 4. Debug Systematically - Identify error type (compile/runtime/logic) - Use print statements to trace values - Check assumptions about data

# Questions? Review these solutions carefully and practice similar problems. --- **Remember:** Programming is a skill that improves with practice!