Call stack behavior - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this recursive function?

Consider the following C code that uses recursion. What will it print when run?

C

#include <stdio.h>

void printNumbers(int n) {
    if (n == 0) return;
    printNumbers(n - 1);
    printf("%d ", n);
}

int main() {
    printNumbers(3);
    return 0;
}

A 3 2 1

B 1 2 3

C 0 1 2 3

D 3 2 1 0

💡 Hint

Think about when the printf statement runs during the recursion.

The function calls itself with n-1 before printing. So it prints numbers in ascending order after reaching the base case.

❓ Predict Output

intermediate

What is the output of nested function calls?

Analyze the output of this C program with nested function calls.

C

#include <stdio.h>

void f1() {
    printf("A");
}

void f2() {
    printf("B");
    f1();
    printf("C");
}

int main() {
    f2();
    return 0;
}

A CAB

B ABC

C BCA

D BAC

💡 Hint

Follow the order of function calls and prints carefully.

f2 prints 'B', calls f1 which prints 'A', then f2 prints 'C'. So output is 'BAC'.

🔧 Debug

advanced

What error occurs in this recursive function?

Examine this recursive function. What error will it cause when run?

C

#include <stdio.h>

void recurse(int n) {
    printf("%d ", n);
    recurse(n + 1);
}

int main() {
    recurse(1);
    return 0;
}

A Stack overflow (infinite recursion)

B Segmentation fault due to null pointer

C Compilation error: missing return type

D No error, prints numbers from 1 to 10

💡 Hint

Consider what happens when the function never stops calling itself.

The function calls itself without a base case, causing infinite recursion and stack overflow.

📝 Syntax

advanced

Which option causes a stack corruption due to wrong function pointer usage?

Identify which code snippet will cause stack corruption because of incorrect function pointer usage.

A void (*fp)() = (void (*)())1234; fp();

B void (*fp)() = &main; fp();

C void (*fp)() = NULL; if(fp) fp();

D void (*fp)() = &printf; fp();

💡 Hint

Think about what happens when you call a function pointer set to an invalid address.

Option A sets a function pointer to an invalid address (1234) and calls it, causing stack corruption or crash.

🚀 Application

expert

How many stack frames are active at the deepest point?

Given this recursive function, how many stack frames exist at the deepest recursion call?

C

#include <stdio.h>

void countDown(int n) {
    if (n == 0) return;
    countDown(n - 1);
    printf("%d ", n);
}

int main() {
    countDown(5);
    return 0;
}

A 5

B 4

C 6

D 1

💡 Hint

Remember the base case call also creates a stack frame.

The function calls itself with n=5 down to n=0. At n=0, the deepest call, there are 6 active stack frames (for n=5,4,3,2,1,0).

Practice

1. What does the call stack do in a C program?

easy

A. Handles input and output operations

B. Stores global variables permanently

C. Manages memory allocation for dynamic data

D. Keeps track of function calls and local variables

Solution

1. Step 1: Understand the role of the call stack

   The call stack stores information about active function calls and their local variables during program execution.

2. Step 2: Differentiate from other memory areas

   Global variables and dynamic memory are stored elsewhere, not on the call stack.

3. Final Answer:

   Keeps track of function calls and local variables -> Option D

4. Quick Check:

   Call stack = function calls + locals [OK]

Hint: Call stack tracks active functions and locals [OK]

Common Mistakes:

• Confusing call stack with heap memory
• Thinking global variables are on the stack
• Assuming call stack manages I/O

2. Which of the following is the correct way to declare a function in C that will be called and added to the call stack?

easy

A. void myFunction() { /* code */ }

B. function myFunction() { /* code */ }

C. def myFunction() { /* code */ }

D. func myFunction() { /* code */ }

Solution

1. Step 1: Identify correct C function syntax

   In C, functions are declared with a return type, name, and parentheses, e.g., void myFunction().

2. Step 2: Recognize incorrect syntax

   Keywords like function, def, or func are not valid in C.

3. Final Answer:

   void myFunction() { /* code */ } -> Option A

4. Quick Check:

   C function declaration uses return type + name [OK]

Hint: C functions start with return type, no keywords like def [OK]

Common Mistakes:

• Using Python or JavaScript keywords in C
• Omitting return type in function declaration
• Using incorrect braces or parentheses

3. What will be the output of this C program?

void func(int n) {
    if (n == 0) return;
    printf("%d ", n);
    func(n - 1);
}

int main() {
    func(3);
    return 0;
}

medium

A. 3 2 1

B. 3 2 1 0

C. 0 1 2 3

D. 1 2 3

Solution

1. Step 1: Trace the recursive calls

   The function prints the current value of n, then calls itself with n-1 until n is 0.

2. Step 2: Determine printed values

   It prints 3, then 2, then 1, then stops before printing 0.

3. Final Answer:

   3 2 1 -> Option A

4. Quick Check:

   Print before recursion = descending order [OK]

Hint: Print before recursive call means descending output [OK]

Common Mistakes:

• Assuming print happens after recursion
• Including zero in output incorrectly
• Confusing order of prints

4. Consider this code snippet:

void foo() {
    int x = 5;
    bar();
}

void bar() {
    printf("%d", x);
}

int main() {
    foo();
    return 0;
}

What is the error here?

medium

A. Missing semicolon after bar() call

B. Function foo() is missing a return type

C. Variable x is not declared in bar()'s scope

D. main() should return void

Solution

1. Step 1: Check variable scope

   Variable x is declared inside foo(), so it is local to foo() and not visible in bar().

2. Step 2: Identify the error cause

   bar() tries to print x, but x is undefined in bar(), causing a compile error.

3. Final Answer:

   Variable x is not declared in bar()'s scope -> Option C

4. Quick Check:

   Local variables are not shared across functions [OK]

Hint: Local variables can't be accessed outside their function [OK]

Common Mistakes:

• Thinking local variables are global
• Ignoring variable scope rules
• Assuming implicit variable sharing

5. Given this recursive function:

int factorial(int n) {
    if (n <= 1) return 1;
    return n * factorial(n - 1);
}

int main() {
    int result = factorial(4);
    printf("%d", result);
    return 0;
}

How many call stack frames are active at the deepest point during execution?

hard

A. 3

B. 4

C. 5

D. 1

Solution

1. Step 1: Count recursive calls for factorial(4)

   Calls happen for n=4, 3, 2, and 1 (base case triggers at n=1).

2. Step 2: Determine deepest call stack frames

   At n=1, the deepest call, there are 4 active frames: factorial(4), factorial(3), factorial(2), and factorial(1).

3. Final Answer:

   4 -> Option B

4. Quick Check:

   Number of calls = 4 frames [OK]

Hint: Count calls including base case, exclude main [OK]

Common Mistakes:

• Counting main() as a call stack frame for this question
• Off-by-one error in counting frames
• Assuming base case does not add a frame