DSA interview question

Functions: Pass by Value vs Pass by Reference

Easy · Algorithms · Topics: language basics

The problem

A teammate wrote a helper swapValues(a, b) to swap two scores in a sorting routine, but after calling it, nothing changes, and the sort never finishes correctly. Separately, a function that "adds a bonus point to every score" works on its own but the caller's list never updates. Both bugs have the same root cause: the functions received copies of the data. Your task: write the swap three ways (by value, by reference, and by pointer), show which ones actually change the caller's variables, and write a helper that returns two results (minimum and maximum) through reference parameters without copying the input.

Example: with a = 3, b = 7, swapByValue(a, b) leaves them as 3, 7, while swapByReference(a, b) makes them 7, 3.

Function signature

void swapByValue(int a, int b);                            // receives copies
void swapByReference(int& a, int& b);                      // receives aliases
void swapByPointer(int* a, int* b);                        // receives addresses
void minMax(const vector<int>& nums, int& lo, int& hi);   // read-only input, two outputs

Test cases

Input	Output	Explanation
`a = 3, b = 7`, call `swapByValue(a, b)`	`a = 3, b = 7`	Only the local copies were swapped
`a = 3, b = 7`, call `swapByReference(a, b)`	`a = 7, b = 3`	References refer to the caller's variables
`a = 3, b = 7`, call `swapByPointer(&a, &b)`	`a = 7, b = 3`	Writes through the addresses
`minMax({4, -2, 9}, lo, hi)`	`lo = -2, hi = 9`	Two results returned through references
`minMax({5}, lo, hi)`	`lo = 5, hi = 5`	Single element
`v = {1, 2}`, `addBonusByValue(v)` then `addBonusByReference(v)`	`v = {2, 3}`	Only the by-reference call changes the caller's vector

(Confirm: nums is non-empty; the caller's variables are inspected after each call.)

How interviewers phrase it

"Write a function to swap two numbers. Why doesn't this version work?"
"What's the difference between pass by value, pass by reference and pass by pointer?"
"How would you return more than one value from a function in C++?"
In Java or Python interviews: "Is Java pass-by-value or pass-by-reference?"

Clarifying questions to ask

Which language? (C++ supports all three; Java is always pass-by-value; Python passes object references.)
Should the function modify the caller's data, or must the input stay unchanged?
How large is the data being passed? (Large objects shouldn't be copied needlessly.)
Is a null or missing argument possible? (Pointers can be null; references can't.)

Follow-ups to expect

"Why does swapByValue not work?"
"Reference vs pointer?"
"Why pass a vector as const vector<int>&?"
"Is Java pass-by-reference for objects?"
"Can you return a reference to a local variable?"
"How else can you return multiple values?"

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