Lab 24 · C++ Basics Lab: Student Marks Calculator with Decisions and Loops

🎯 Objective

Turn the C++ Basics lesson into a finished, tested program: input, validation, a loop, a function and a grade decision, all working together.

🧩 Skills You Will Practice

• C++ variables and types
• cin/cout and input validation
• for loops
• Functions and return values
• if-else grade logic

🧰 What You Need

• A C++ compiler: g++ on your computer, or a free online C++ compiler for practice
• VS Code or any text editor
• A file marks.cpp
• The C++ Basics lesson open for reference

📋 Step-by-Step Tasks

1. Set up and prove the compiler works: create marks.cpp with the Hello program from the C++ Basics lesson, compile it (g++ marks.cpp -o marks) and run it. If the compiler is not found, install a compiler or switch to a free online C++ compiler before writing anything else.
2. Add the structure: include iostream and string, write int main(), and declare int total = 0; — initialise it to zero now, because an uninitialised total is a classic C++ bug that gives garbage results.
3. Build the input loop: for 5 subjects, print a clear prompt (Subject 1 marks out of 100:), read an integer with cin, and add it to total. Compile and run with simple values first — for example five 80s — and check the total is 400 before adding anything clever.
4. Add validation inside the loop: if a mark is below 0 or above 100, print Invalid marks — enter a value between 0 and 100 and ask again for the same subject (step the loop back by one). Test this deliberately with -5 and 150.
5. Handle bad types: if the user types a word instead of a number, cin fails and the loop can spin forever. Add the standard recovery after reading: if cin fails, clear the error with cin.clear(), discard the bad input with cin.ignore(), print a message, and ask again. Test it by typing a word.
6. Add the average function above main: double average(int total, int count) { return (double)total / count; } — the (double) conversion is essential, otherwise integer division rounds the average down. Call it from main and print the average with one decimal place idea in words (for example 82.4).
7. Add the grade decision with an if-else chain: 80+ Grade A, 65+ B, 50+ C, otherwise Needs improvement. Print the total, the average and the grade together in a clean result block, so the output reads like a report, not a jumble of numbers.
8. Run the full test set and record every output: (a) 90, 85, 78, 92, 88; (b) five 40s; (c) one invalid mark then a valid one; (d) a word typed as a mark; (e) all 100s. Every case must finish with a correct report and no crash or infinite loop.
9. Polish and document: add a comment at the top with your name, what the program does and how to compile and run it. Save the final code and the five test outputs as your deliverable.

✅ Expected Result / Deliverable

A working marks.cpp plus a test record: five test cases (normal, low, invalid number, word input, perfect scores) all producing a correct total, average and grade, with no crash or infinite loop — and a top comment explaining how to compile and run it.

⚠️ Common Mistakes

• Forgetting to initialise total to 0 (garbage totals)
• Integer division making the average wrong (missing the (double) conversion)
• A validation loop that never re-asks, or never ends, on bad input
• Using = instead of == in a condition
• Skipping the cin-failure test and discovering it only in a demo