Call stack behavior - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a function named 'foo'.

C

void [1]() {
    // function body
}

Drag options to blanks, or click blank then click option'

Afoo

Bbar

Cmain

Dstart

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different function name than 'foo'.

The function name should be 'foo' as requested.

2fill in blank

medium

Complete the code to call the function 'foo' inside 'main'.

C

int main() {
    [1]();
    return 0;
}

Drag options to blanks, or click blank then click option'

Amain

Bfoo

Cstart

Dbar

Attempts:

3 left

💡 Hint

Common Mistakes

Calling a function that is not defined or named differently.

To call the function 'foo', write 'foo();' inside main.

3fill in blank

hard

Fix the error in the recursive function call to 'countdown'.

C

void countdown(int n) {
    if (n == 0) return;
    printf("%d\n", n);
    [1](n - 1);
}

Drag options to blanks, or click blank then click option'

Acountdownn

Bcount_down

Ccount

Dcountdown

Attempts:

3 left

💡 Hint

Common Mistakes

Misspelling the function name in the recursive call.

The function calls itself by its exact name 'countdown' for recursion.

4fill in blank

hard

Fill both blanks to create a function that prints numbers from 1 to n using recursion.

C

void print_numbers(int [1]) {
    if ([2] == 0) return;
    print_numbers([2] - 1);
    printf("%d\n", [2]);
}

Drag options to blanks, or click blank then click option'

Anum

Bn

Ccount

Di

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names for parameter and condition.

The parameter name is 'n' and the base case checks if 'n' is zero.

5fill in blank

hard

Fill all three blanks to create a recursive factorial function.

C

int factorial(int [1]) {
    if ([2] <= 1) return 1;
    return [2] * factorial([3] - 1);
}

Drag options to blanks, or click blank then click option'

Anum

Bn

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing different variable names for the parameter and recursive call.

The parameter is named 'n'. The base case checks 'n'. The recursive call uses 'n - 1'.

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