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Prompt lab: prompt engineering for hiring tests

Campus and graduate hiring tests (on platforms like HackerRank) now include a prompt engineering question: write one prompt that gets an AI to give the exact right answer on hidden test inputs. Learn the six things that make prompts pass, then practise on 25 real-style tasks with 108 test cases. Run your prompt in your own AI (ChatGPT, Claude, Gemini…), paste the reply back, and see “3 / 5 test cases passed” just like the real test.

🧪 Start a task 📖 Read the guide (5 min) 🎯 Quick-fire questions

The guide: how to write prompts that pass hidden tests

1How these questions work

In tests like HackerRank, you do not chat with the AI. You write one prompt with a {input} placeholder. The system puts in hidden test inputs, runs an AI, and checks that each reply matches the expected answer exactly, like a coding test. “Sure! The answer is Dune,7” fails. So does “Dune, 7” with a space.

2The six parts of a strong prompt

Most passing prompts have the same parts. Not every prompt needs a role, but the other five are almost always needed.

WhoOptional: “You are a careful data checker.”
TaskWhat to do, in one clear sentence.
InputWhat the data looks like, and where it is: {input}.
RulesRepeats, ties, capitals, empty input, rounding: the things hidden tests check.
OutputThe EXACT format, and “reply with ONLY the answer”.
ExampleOne small worked example.

3Say exactly what the output looks like

Copy the required format into your prompt word for word, then add: “Reply with ONLY the answer, nothing else.” Add one tiny example. Mention spaces, capitals, decimals and separators if they matter, and say “no code block” when the answer is JSON or code-like.

4Think like the hidden tests

The sample cases are easy; the hidden ones are not. Before you submit, ask: what if an item appears twice? What if two items tie? What if the input is empty? Different capitals, extra spaces, punctuation, a $ sign, a single item, a boundary value (exactly 75, exactly midnight)? Give a rule for each one that could happen.

5Describe the input, then put it last

Tell the AI what each line looks like (“each line is <Book> - Borrowed <n> times”), then put the data at the very end after a label such as Records: and {input}. Clear markers stop the AI mixing your instructions with the data.

6Let the AI think, but show only the answer

For counting or calculating, you can say “work it out carefully step by step, but reply with only the final answer”. Short, plain sentences beat long clever ones. Test with tricky inputs of your own before you submit.

Practice tasks

Start with the Easy ones. Each task has 2 sample cases you can see and more cases like the hidden ones. Your prompts and results are saved on this device.

Count and total

Most borrowed bookWrite a prompt that finds the book borrowed the most times in total, and its total.Easy6 testsTop sales regionWrite a prompt that finds the region with the highest total sales.Medium5 testsMost common wordWrite a prompt that finds the word used most often, and how many times.Medium5 testsClass averageWrite a prompt that works out the class average.Easy5 testsTop three productsWrite a prompt that lists the three best-selling products by total units.Medium4 testsThe longest wordWrite a prompt that finds the longest word and its length.Easy4 testsBusiest hourWrite a prompt that finds the hour with the most visits.Medium4 testsAdd up a billWrite a prompt that gives the total.Medium4 tests

Extract data

Pull out email addressesWrite a prompt that lists every email address in the text.Easy4 testsCollect the hashtagsWrite a prompt that lists the hashtags.Easy4 tests

Reformat

Dates to YYYY-MM-DDWrite a prompt that rewrites every date as YYYY-MM-DD.Medium4 testsClean up phone numbersWrite a prompt that writes each number as + followed by digits only.Medium4 testsCSV to JSONWrite a prompt that turns it into a JSON array of objects.Hard4 testsFix the capitals in namesWrite a prompt that writes each name with a capital first letter on every part.Easy4 tests24-hour to 12-hour timesWrite a prompt that writes them in 12-hour form with AM or PM.Easy4 testsSort by surnameWrite a prompt that sorts the names by surname (the last word).Medium4 testsRemove duplicatesWrite a prompt that keeps only the first time each item appears.Easy4 testsMarks to gradesWrite a prompt that gives each student their grade.Easy4 tests

Classify

Positive, negative or neutral?Write a prompt that labels a review POSITIVE, NEGATIVE or NEUTRAL.Easy5 testsRoute a support ticketWrite a prompt that returns the team for a message.Medium5 tests

Validate

Check a passwordWrite a prompt that checks one password.Medium5 testsIs it a palindrome?Write a prompt that says whether the text is a palindrome.Easy4 testsLow stock alertWrite a prompt that lists items with stock below their minimum.Easy4 tests

Reason

The missing numberWrite a prompt that finds the missing number.Medium4 testsFirst letter that does not repeatWrite a prompt that finds the first letter that appears only once.Hard4 tests

Also practise

Prompt engineering quick-fire questions (spot the problem, pick the better prompt, which test fails?) · A full campus-test style mock · Aptitude, reasoning and data