DSA interview question

Switch Case: Mini Calculator

Easy · Algorithms · Topics: language basics

The problem

You're building the tiny calculator that sits inside a chat bot. A user sends two whole numbers and an operator character, one of +, -, *, / or %, and the bot replies with the result. Division and remainder by zero must return a friendly error instead of crashing the bot, and any other character must be reported as an unknown operator. Since the operator is a single character with a handful of possible values, this is the textbook use case for a switch statement.

Example: calculate(7, 3, '+') → "10", calculate(7, 0, '/') → "Error: division by zero".

Function signature

string calculate(long long a, long long b, char op);  // result as text, or an error message

Test cases

Input `(a, b, op)`	Output	Explanation
`(7, 3, '+')`	`10`	Addition
`(7, 10, '-')`	`-3`	Negative result
`(6, 7, '*')`	`42`	Multiplication
`(7, 2, '/')`	`3`	Integer division truncates
`(-7, 2, '/')`	`-3`	Truncates toward zero in C++
`(7, 3, '%')`	`1`	Remainder
`(7, 0, '/')`	`Error: division by zero`	Guarded instead of crashing
`(7, 2, '^')`	`Error: unknown operator`	Falls to `default`

(Confirm: integer arithmetic only; results fit in a long long; unknown operators return an error string.)

How interviewers phrase it

"Write a simple calculator that takes two numbers and an operator."
"Given a day number from 1 to 7, print the day's name" or "Given a month number, print the number of days in it."
Frequently used to probe language knowledge: "What happens if you forget break in a switch?"

Clarifying questions to ask

Are the numbers integers or decimals? (Changes how / and % behave.)
What should happen on division by zero: return an error, throw, or print a message?
Which operators must be supported, and what should an unsupported one do?
Can the inputs be large enough to overflow on multiplication?

Follow-ups to expect

"What happens if you leave out break?"
"Can you switch on a string in C++?"
"Why can't I declare a variable inside a case without braces?"
"When is switch better than if-else?"
"Where does default go?"
"How does C++ integer division treat negative numbers?"

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