C Output Prediction and Pointer Debugging round·Engineering·Medium·20 min

TCS Ninja Interview — C Output Prediction and Pointer Debugging

20 min · 1 credit · scorecard at the end
Z
ZeroPitch Research Desk
Updated 2026-06-11 · Newly added · be among the first to take it
Field
Engineering
Company
Tata Consultancy Services
Role
Assistant System Engineer Trainee (Ninja)
Duration
20 min
Difficulty
Medium
Completions
New
Updated
2026-06-11

How to prepare

What this round tests, what strong and weak answers sound like, and the traps to sidestep.

What this round is about

  • Topic focus. You trace short C snippets line by line and state the exact printed output, covering increment operators, pointer arithmetic, storage classes, sizeof, and undefined behaviour.
  • Conversation dynamic. The interviewer shows one snippet at a time, makes you finish the trace, then adds a twist like fix the bug or why is this undefined behaviour.
  • What gets tested. Whether you read C the way the compiler does, reasoning about precedence and memory rather than guessing a single number.
  • Round format. A voice plus whiteboard round where you draw the stack and the memory boxes as you trace each value.

What strong answers look like

  • Running trace on the board. You write the variables as boxes and update them each line, for example showing num1 taking num2 before num2 is incremented.
  • Precedence stated out loud. You say postfix increment binds tighter than prefix and that star ptr plus plus is star of ptr plus plus evaluated right to left, then compute.
  • Honest undefined behaviour. You name a freed or uninitialised pointer as a dangling or wild pointer and call the result undefined rather than inventing a clean output.
  • Memory model named. You distinguish sizeof of an array from sizeof of a pointer and say what changes on a 64-bit machine.

What weak answers look like (and how to avoid them)

  • Single guessed number. Blurting an output with no trace reads as hand-waving, so narrate each line instead.
  • Left-to-right pointer read. Reading star ptr plus plus as a pre-increment of the pointed value is wrong, so apply right-to-left associativity.
  • Clean output for a bad pointer. Predicting a tidy value for a freed or uninitialised pointer fails, so call it undefined behaviour.
  • sizeof confusion. Giving the pointer size for a whole array is a classic miss, so state what sizeof measures on each.

Pre-interview checklist (2 minutes before you start)

  • Recall increment precedence. Postfix binds tighter than prefix, and prefix and dereference share precedence with right-to-left associativity.
  • Have pointer fundamentals ready. A pointer holds an address, a null pointer is value zero, and calloc zero-initialises the block.
  • Identify dangling-pointer causes. Free, a returned non-static local address, or a variable going out of scope all create one.
  • Pull up your sizeof facts. Array size is element size times count; pointer size depends on the machine, typically eight bytes on 64-bit.
  • Think of the storage classes. Be ready to say what a static variable prints across repeated calls versus an auto variable.

How the AI behaves

  • Probes every trace. It asks why an operator binds first or why a value is what you said, not just for the final number.
  • No mid-interview praise. It will not say great answer; it acknowledges the specific line you traced and pushes on.
  • Interrupts on a guess. If you state output with no trace, it slows you down and makes you trace one line at a time.
  • One snippet at a time. It finishes each trace and its twist before moving to the next card.

Common traps in this type of round

  • Output without a trace. Stating a number and stopping, with no line-by-line reasoning to back it.
  • Precedence inversion. Evaluating star ptr plus plus or a mixed pre and post increment expression in the wrong order.
  • Undefined behaviour as defined. Treating modification of a variable more than once between sequence points as having one guaranteed value.
  • Dangling pointer denied. Saying a returned local-variable address is safe instead of naming the dangling pointer.
  • Resume bluff. Listing C as a strength but not knowing what calloc returns or what a null pointer is.
  • sizeof on arrays versus pointers. Reporting the pointer size for an array passed into a function as if it were the full array.

Sample problems you'll face

The 2 problems below are the same ones you'll work through in the live session — no surprises. Read the constraints carefully; the AI persona will refer you to the on-canvas card by problem number.

  1. 1Predict the Output: Pointer Walk Over a String Literal

    char *ptr = "Linux"; printf("[%c]", *ptr++); printf("[%c]", *ptr); Trace this snippet. State which character each printf sends to the screen and where ptr points after the first line. The interviewer will then ask why the evaluation goes in the direction it does, and what would change if the second line were plus plus star ptr instead.

    Example inputchar *ptr = "Linux";
    Example output[L][i]
    • star ptr plus plus is parsed as star of (ptr plus plus): since dereference and postfix increment are involved, the postfix increment binds to ptr and yields the old pointer, which is then dereferenced.
    • The first printf therefore reads the original first byte (capital L) and the post-increment advances ptr to point at the second byte; the second printf reads lowercase i.
    • A string literal is read-only storage, so writing through ptr would be undefined behaviour even though reading is fine.
    • ON THE CANVAS: draw the bytes of Linux as boxes with their addresses, place ptr as an arrow on the first box, then move the arrow after the first printf and write the character each printf prints before you say done.
  2. 2Find the Bug: Function Returning a Local Array

    int *makeArray(void) { int arr[3] = {10, 20, 30}; return arr; } /* main calls int *p = makeArray(); then prints p[0], p[1], p[2] */ Trace what main attempts to print, then debug it. State whether the storage at that address is still valid after makeArray returns, name the defect precisely, and propose a fix that makes the behaviour defined without changing what main expects.

    Example inputint *p = makeArray(); printf("%d %d %d", p[0], p[1], p[2]);
    Example outputUndefined behaviour (dangling pointer; may print garbage or crash)
    • arr is a non-static local array; its storage lives in makeArray's stack frame and is released the instant the function returns, so the returned pointer is a dangling pointer.
    • Reading through that pointer in main is undefined behaviour: it may appear to print 10 20 30, may print garbage, or may cause a segmentation fault, and the result is not guaranteed by the language.
    • Two correct fixes: declare the array static int arr[3] so it has program lifetime, or allocate with malloc inside makeArray and have the caller free it; do not just change the printed values.
    • ON THE CANVAS: draw makeArray's stack frame with arr inside it, draw the returned pointer, then cross out the frame at the return and state in words why the address in main is no longer backed by valid storage before you give the fix.
Reference

The full breakdown

How you're scored, the questions candidates ask most, and the research this interview is built on. Skim it — or just start the interview.

Interview framework

You will be scored on these 5 dimensions. The full rubric with definitions is below.

Line-by-line Trace Discipline
How consistently you externalise each line's effect on the whiteboard before stating any output, instead of jumping to a single number.
25%
Precedence And Associativity Reasoning
How accurately you state which operator binds first and in which direction, especially for mixed increment and dereference expressions.
20%
Pointer And Memory Model
How soundly you reason about addresses, the array-pointer duality, and where variables physically live across function calls.
20%
Undefined Behaviour Honesty
Whether you correctly name dangling or wild pointers and call out undefined behaviour rather than inventing a clean output.
20%
Debug And Fix Fluency
How well you locate the defect and propose a concrete, correct fix when the interviewer adds the find-the-bug twist.
15%

What we evaluate

Your final scorecard breaks down across these dimensions. The full rubric and tier criteria are revealed inside the interview itself.

  • Line-By-Line Trace Discipline20%
  • Precedence and Associativity Reasoning18%
  • Pointer and Memory Model18%
  • Undefined Behaviour Honesty18%
  • Debug and Fix Fluency14%
  • Core C Fundamentals Recall12%

Common questions

What does the TCS Ninja C output-prediction round actually test?
It tests whether you can read C the way the compiler does. The interviewer shows short five-to-eight-line snippets and asks you to trace memory and values line by line and state the exact printed output, then debug. It covers pre-increment and post-increment in one expression, operator precedence and associativity, pointer arithmetic, the array-pointer duality, pass by value versus pass by pointer, storage classes, sizeof on arrays versus pointers, string literals versus char arrays, dangling or uninitialised pointers, integer overflow, and format-specifier mismatches. The skill being checked is methodical tracing and honesty about undefined behaviour, not memorised answers.
How should I structure my answer when I get a snippet?
Do not blurt a single number. Read the snippet aloud, write the variables as boxes on the whiteboard, and trace each line, updating the boxes as the program runs. Say which operator binds first and in which direction before you compute. State the printed output explicitly. When the interviewer adds a twist such as fix the bug or why is this undefined behaviour, answer the twist directly. A clear running trace beats a fast guess every time in this round.
What are the most common mistakes candidates make?
The biggest one is guessing a single output with no trace, which the interviewer treats as hand-waving. Others include reading star ptr plus plus left to right instead of right to left, claiming a clean output for a freed or uninitialised pointer instead of calling it undefined behaviour, saying a function that returns the address of a non-static local variable is fine instead of naming the dangling pointer, and giving the pointer size for sizeof of a whole array. Listing C as a strength and then not knowing pointers or storage classes also fails fast.
How is this AI interviewer different from a real TCS interviewer?
It behaves like a real Ninja-tier technical interviewer named Aditya. It presents one snippet at a time, makes you finish the trace before the twist, and probes every answer at least once. It will not coach you or hand you the answer, and it will not praise you mid-round. The difference is consistency and patience: it gives the same depth of probing every time, never reacts to accent or delivery, and produces a written scorecard at the end naming exactly where your trace went wrong.
How is scoring done in this round?
You are scored on the quality of your tracing, not on getting lucky. The signals are correct operator-precedence and associativity reasoning, a sound pointer and memory mental model, accurate output, honest identification of undefined behaviour, and whether you use the whiteboard to externalise your trace. The scorecard maps each snippet to these dimensions and quotes the specific moment a trace broke or a guess replaced reasoning. Applied tracing skill is weighted above recall of definitions.
What should I do in the first two minutes before I start?
Recall the operator-precedence and associativity facts for increment and dereference: postfix binds tighter than prefix, and star ptr plus plus is star of ptr plus plus evaluated right to left. Have your pointer fundamentals ready: a pointer holds an address, a null pointer is value zero, calloc zero-initialises. Be ready to draw a stack and memory boxes on the whiteboard. Decide that you will trace every snippet line by line rather than guess. Keep the difference between defined and undefined behaviour fresh.
How do I handle a snippet I think is undefined behaviour?
Say so plainly and name why. If a pointer is dereferenced after free or before initialisation, call it a dangling or wild pointer and undefined behaviour that may crash with a segmentation fault. If a variable is modified more than once between sequence points, say the result is not defined by the language rather than predicting one value. Do not invent a single confident output for undefined behaviour: in this round, naming it correctly scores higher than guessing a number that happens to print on one compiler.
What does a strong answer sound like in this round?
A strong answer narrates the trace: this line declares the variables, here are the boxes, this operator binds first because its precedence is higher, evaluation goes right to left here, so after this line num1 holds this value. It states the printed output explicitly and then addresses the twist, for example by pointing out that returning the address of a local variable creates a dangling pointer and by fixing it with a static variable or heap allocation. It separates what the language guarantees from what merely prints on one machine.
Why does the interviewer keep asking about precedence and associativity?
Because most wrong outputs in this round come from mis-evaluating expressions that mix operators. Postfix increment has higher precedence than prefix and the dereference operator, and prefix and dereference share precedence with right-to-left associativity. If you cannot say which operator binds first and in which direction, your trace of star ptr plus plus, or of an expression mixing pre and post increment, will be wrong. The interviewer probes this to check that your output came from reasoning, not from memory of a similar question.
What happens on a 64-bit machine versus a 32-bit machine in these snippets?
The twist usually targets sizeof and pointers. A pointer is typically eight bytes on a 64-bit machine and four bytes on a 32-bit machine, so sizeof of a pointer changes, while sizeof of an array stays the element size times the count regardless of machine. Integer width and overflow behaviour can also differ by platform and type. A strong candidate states what is implementation-defined, like pointer size, versus what is fixed by the array's declared type, and does not assume one universal number.

Sources this interview is built on

Real candidate-report URLs (Glassdoor / AmbitionBox / PrepInsta / GeeksforGeeks / Medium) reviewed when authoring the questions, persona, and rubric. Verify the realism yourself.