Stack Practice questions
Duration: 5 min
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The video presents a C programming problem asking students to predict the output of a recursive function f(n) called with the argument 11. The instructor systematically traces the execution of the code, analyzing the base cases and recursive steps. He writes down the call tree on the whiteboard, breaking down f(11) into smaller sub-problems like f(5) and f(6). By evaluating each recursive call step-by-step, determining return values based on integer division and modulo operations, he calculates the final result. The process involves checking conditions like n <= 1 and n % 2 == 0 to decide the return path. The final output is determined to be 5, corresponding to option (D).
Chapters
0:00 – 2:00 00:00-02:00
The instructor introduces the problem "Predict the output of following program" and displays the C code on screen. He starts tracing the function f(11) written on the board. He evaluates the first condition if(n <= 1) as false for 11. Then he checks if(n%2 == 0), noting that 11 is odd, so this is also false. Consequently, he moves to the final return statement return f(n/2) + f(n/2+1), substituting 11 to get return f(5) + f(6). He begins expanding f(5) by writing Ret f(2) + f(3) underneath it, indicating the next level of recursion. He then analyzes f(2), checking 2 <= 1 (false) and 2%2 == 0 (true), leading to return f(1). He establishes that f(1) returns 1 because 1 <= 1 is true, writing f(2) = 1 on the side.
2:00 – 5:00 02:00-05:00
Continuing the trace, the instructor evaluates f(3). Since 3 is odd, it hits the final return statement: return f(1) + f(2). Using the previously calculated values where f(1) = 1 and f(2) = 1, he determines f(3) = 2. He writes f(3) = 2 on the board. Returning to f(5), he sums the results of its children: f(2) + f(3) becomes 1 + 2, so f(5) = 3. Next, he analyzes f(6). Since 6 is even, the condition n%2 == 0 is true, so it returns f(6/2) which is f(3). Since f(3) is already known to be 2, f(6) = 2. Finally, he combines the results for the initial call: f(11) = f(5) + f(6) becomes 3 + 2, resulting in 5. He writes f(11) = 5 clearly on the board.
5:00 – 5:08 05:00-05:08
The instructor concludes the problem by confirming the calculated result is 5. He points to option (D) 5 and places a checkmark next to it, indicating it is the correct answer. He briefly dismisses the other options, crossing out (A) Stack Overflow, (B) 3, and (C) 4 to show they are incorrect. The video ends with the final answer selected.
The lecture demonstrates a methodical approach to solving recursive function problems by manually tracing the call stack. The instructor breaks down complex calls into simpler sub-problems, utilizing a top-down expansion strategy. By calculating base cases first and working backwards up the recursion tree, he avoids stack overflow errors and correctly computes the final integer value. This step-by-step evaluation highlights the importance of understanding integer division and modulo operators in C recursion.