Stack Implementation Using Array in DSA C - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this stack push and pop sequence?

Consider a stack implemented using an array with a maximum size of 3. The following operations are performed in order:

push(10), push(20), pop(), push(30), push(40), pop(), pop()

What is the printed output of the popped elements?

DSA C

int stack[3];
int top = -1;

void push(int x) {
    if (top == 2) return; // stack full
    stack[++top] = x;
}

int pop() {
    if (top == -1) return -1; // stack empty
    return stack[top--];
}

int main() {
    push(10);
    push(20);
    printf("%d ", pop());
    push(30);
    push(40);
    printf("%d ", pop());
    printf("%d", pop());
    return 0;
}

A 10 30 40

B 10 40 30

C 20 40 30

D 20 30 40

💡 Hint

Remember that pop removes the top element and returns it.

The first pop removes 20 (last pushed), then push 30 and 40 fills the stack. Next pop removes 40, then pop removes 30.

❓ Predict Output

intermediate

What is the value of 'top' after these operations?

Given a stack implemented with an array of size 5, initially empty (top = -1). The operations are:

push(5), push(10), push(15), pop(), push(20), pop(), pop()

What is the value of 'top' after these operations?

DSA C

int stack[5];
int top = -1;

void push(int x) {
    if (top == 4) return;
    stack[++top] = x;
}

void pop() {
    if (top == -1) return;
    top--;
}

int main() {
    push(5);
    push(10);
    push(15);
    pop();
    push(20);
    pop();
    pop();
    printf("%d", top);
    return 0;
}

A 0

B 1

C -1

D 2

💡 Hint

Track each push and pop carefully to update 'top'.

After push(5), top=0; push(10), top=1; push(15), top=2; pop(), top=1; push(20), top=2; pop(), top=1; pop(), top=0.

🔧 Debug

advanced

What error does this stack push function cause?

Identify the error in this push function for a stack implemented using an array of size 3:

void push(int x) { if (top == 3) { printf("Stack Overflow\n"); return; } stack[++top] = x; }

Assume 'top' is initialized to -1.

DSA C

int stack[3];
int top = -1;

void push(int x) {
    if (top == 3) {
        printf("Stack Overflow\n");
        return;
    }
    stack[++top] = x;
}

A Syntax error due to missing semicolon

B No error, works correctly

C Stack underflow error

D Stack overflow check misses last valid index, causing out-of-bounds write

💡 Hint

Array indices go from 0 to size-1.

The condition should be 'top == 2' for size 3, because valid indices are 0,1,2. Checking 'top == 3' allows writing beyond array bounds.

❓ Predict Output

advanced

What is the output after these stack operations?

Given a stack of size 4, initially empty, perform:

push(1), push(2), push(3), pop(), push(4), push(5), pop(), pop(), pop()

Print the popped values in order separated by spaces.

DSA C

int stack[4];
int top = -1;

void push(int x) {
    if (top == 3) return;
    stack[++top] = x;
}

int pop() {
    if (top == -1) return -1;
    return stack[top--];
}

int main() {
    push(1);
    push(2);
    push(3);
    printf("%d ", pop());
    push(4);
    push(5);
    printf("%d ", pop());
    printf("%d ", pop());
    printf("%d", pop());
    return 0;
}

A 3 5 4 2

B 3 4 5 2

C 3 4 5 1

D 3 5 4 1

💡 Hint

Remember push fails silently if stack is full.

Pop 1 removes 3, push 4 succeeds, push 5 succeeds (stack full now). Pop removes 5, pop removes 4, pop removes 2.

🧠 Conceptual

expert

Which option correctly describes the time complexity of stack operations using an array?

Consider a stack implemented using an array with fixed size. What is the time complexity of push and pop operations?

A Push is O(n), pop is O(1)

B Both push and pop have O(1) time complexity

C Push is O(1), pop is O(n)

D Both push and pop have O(n) time complexity

💡 Hint

Think about how many elements need to be moved during push or pop.

Push and pop only change the 'top' index and assign or read one element, so both are constant time O(1).

Practice

1. What is the main purpose of using an array in stack implementation?

easy

A. To store elements in a fixed-size linear structure

B. To allow random access to elements anywhere in the stack

C. To automatically resize the stack when full

D. To sort elements in ascending order

Solution

1. Step 1: Understand stack storage

   A stack stores elements in a linear order where the last added element is accessed first.

2. Step 2: Role of array in stack

   An array provides a fixed-size linear structure to hold stack elements in order.

3. Final Answer:

   To store elements in a fixed-size linear structure -> Option A

4. Quick Check:

   Array = fixed-size linear storage [OK]

Hint: Stack uses array for fixed-size linear storage [OK]

Common Mistakes:

• Thinking array allows automatic resizing
• Confusing stack with random access structures
• Assuming array sorts elements automatically

2. Which of the following is the correct way to declare a stack array of size 5 in C?

easy

A. int[5] stack;

B. int stack[5];

C. stack int[5];

D. array stack = int[5];

Solution

1. Step 1: Recall C array declaration syntax

   In C, arrays are declared as: type name[size];

2. Step 2: Match syntax with options

   int stack[5]; matches correct syntax: int stack[5];

3. Final Answer:

   int stack[5]; -> Option B

4. Quick Check:

   C array declaration = int stack[5]; [OK]

Hint: C arrays declared as type name[size]; [OK]

Common Mistakes:

• Placing type after variable name
• Using incorrect keywords like 'array'
• Misplacing brackets in declaration

3. What will be the output after executing the following code?

int stack[3];
int top = -1;
// Push 10
stack[++top] = 10;
// Push 20
stack[++top] = 20;
// Pop
int x = stack[top--];
printf("%d", stack[top]);

medium

A. Runtime error

B. 20

C. 10

D. Garbage value

Solution

1. Step 1: Trace push operations

   Initially top = -1.
   Push 10: top becomes 0, stack[0] = 10.
   Push 20: top becomes 1, stack[1] = 20.

2. Step 2: Trace pop and print

   Pop: x = stack[1] = 20, then top becomes 0.
   Print stack[top] = stack[0] = 10.

3. Final Answer:

   10 -> Option C

4. Quick Check:

   After pop, top points to 10 [OK]

Hint: Pop decreases top, print uses updated top [OK]

Common Mistakes:

• Printing popped element instead of current top
• Confusing pre-increment and post-decrement
• Assuming top points beyond last element

4. Identify the error in this stack push function and select the fix:

void push(int stack[], int *top, int size, int value) {
    if (*top == size) {
        printf("Stack Overflow\n");
        return;
    }
    stack[++(*top)] = value;
}

medium

A. No error, code is correct

B. Change condition to *top == size + 1

C. Change stack[++(*top)] to stack[(*top)++]

D. Change condition to *top == size - 1

Solution

1. Step 1: Understand top indexing in stack

   Top starts at -1, max index is size - 1.
   When top == size - 1, stack is full.

2. Step 2: Analyze overflow condition

   Condition *top == size is wrong; it should be *top == size - 1 to detect full stack.

3. Final Answer:

   Change condition to *top == size - 1 -> Option D

4. Quick Check:

   Overflow when top reaches size-1 [OK]

Hint: Top max index is size-1, not size [OK]

Common Mistakes:

• Checking top == size instead of size-1
• Misusing increment operators in array indexing
• Ignoring zero-based indexing

5. Given a stack implemented using an array of size 4, initially empty, what will be the stack content after these operations?

push(5);
push(10);
pop();
push(15);
push(20);
pop();
push(25);

Assume top starts at -1 and push/pop update top accordingly. Choose the correct final stack content from bottom to top.

hard

A. 5 -> 15 -> 25 -> null

B. 5 -> 10 -> 15 -> 25 -> null

C. 10 -> 15 -> 25 -> null

D. 5 -> 15 -> 20 -> 25 -> null

Solution

1. Step 1: Trace each operation step-by-step

   Start: top = -1, stack empty.
   push(5): top=0, stack[0]=5
   push(10): top=1, stack[1]=10
   pop(): removes 10, top=0
   push(15): top=1, stack[1]=15
   push(20): top=2, stack[2]=20
   pop(): removes 20, top=1
   push(25): top=2, stack[2]=25

2. Step 2: Final stack content

   Stack from bottom (index 0) to top (index 2): 5, 15, 25

3. Final Answer:

   5 -> 15 -> 25 -> null -> Option A

4. Quick Check:

   Push/pop update top correctly [OK]

Hint: Track top index after each push/pop [OK]

Common Mistakes:

• Forgetting to update top after pop
• Assuming pop removes bottom element
• Mixing order of elements after operations