DSA interview question

If / Else-If Ladder: Grade Calculator

Easy · Algorithms · Topics: language basics

The problem

Your school's result portal needs a function that turns a student's marks (0 to 100) into a grade. The rules from the exam office are: 90–100 → A, 70–89 → B, 50–69 → C, 35–49 → D, and below 35 → Fail. Marks outside 0–100 come from typos in data entry and must be reported as Invalid rather than silently graded. Last year's version checked the conditions in the wrong order and gave a student with 95 marks a "B". Write a function that gets every boundary right.

Example: gradeFor(90) → "A", gradeFor(89) → "B", gradeFor(101) → "Invalid".

Function signature

string gradeFor(int marks);   // returns "A", "B", "C", "D", "Fail" or "Invalid"

Test cases

Input `marks`	Output	Explanation
95	`A`	Inside the A band
90	`A`	Lower boundary of A is inclusive
89	`B`	Just below A
70	`B`	Lower boundary of B
50	`C`	Lower boundary of C
35	`D`	Lower boundary of D
34	`Fail`	Just below the pass mark
0	`Fail`	Minimum valid marks
101	`Invalid`	Above the allowed range
-5	`Invalid`	Below the allowed range

(Confirm: every lower boundary is inclusive, marks are whole numbers, and out-of-range input returns "Invalid".)

How interviewers phrase it

A classic first-round or campus warm-up: "Write a function that returns a grade for given marks."
Variants with the same structure: income tax slabs, BMI categories, shipping cost by weight, "Fizz/Buzz"-style classification.
Often a lead-in to discussing code quality: ordering of conditions, boundary handling, and input validation.

Clarifying questions to ask

Are the lower boundaries inclusive? Is exactly 90 an A?
What should happen for marks outside 0–100: return an error value, throw an exception, or clamp?
Can marks be decimals like 89.5? (Then the boundaries need care with floating-point comparison.)
Is the grading scheme fixed, or should it be configurable?

Follow-ups to expect

"Why does the order of the conditions matter?"
"What's the difference between an else-if ladder and several separate ifs?"
"Could you use switch here?"
"How would you make the grading scheme configurable?"
"What if marks can be 89.999?"

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