Check if Stack is Empty or Full in DSA C - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Stack Empty Check

What is the output of this C code that checks if the stack is empty after some push and pop operations?

DSA C

#include <stdio.h>
#define MAX 3

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

int isEmpty() {
    return top == -1;
}

void push(int val) {
    if (top < MAX - 1) {
        stack[++top] = val;
    }
}

void pop() {
    if (!isEmpty()) {
        top--;
    }
}

int main() {
    push(10);
    push(20);
    pop();
    pop();
    if (isEmpty()) {
        printf("Stack is empty\n");
    } else {
        printf("Stack is not empty\n");
    }
    return 0;
}

A Compilation error

B Stack is not empty

C Runtime error

D Stack is empty

💡 Hint

Think about how many elements are pushed and popped before the check.

After pushing 10 and 20, the stack has two elements. Then two pops remove both elements, making the stack empty. So isEmpty() returns true and prints 'Stack is empty'.

❓ Predict Output

intermediate

Output of Stack Full Check

What is the output of this C code that checks if the stack is full after pushing elements?

DSA C

#include <stdio.h>
#define MAX 2

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

int isFull() {
    return top == MAX - 1;
}

void push(int val) {
    if (!isFull()) {
        stack[++top] = val;
    }
}

int main() {
    push(5);
    push(15);
    if (isFull()) {
        printf("Stack is full\n");
    } else {
        printf("Stack is not full\n");
    }
    return 0;
}

A Stack is full

B Stack is not full

C Runtime error

D Compilation error

💡 Hint

Check how many elements the stack can hold and how many are pushed.

The stack size is 2. After pushing two elements, top becomes 1 which equals MAX-1, so isFull() returns true and prints 'Stack is full'.

❓ Predict Output

advanced

Output after Mixed Stack Operations

What is the output of this C code after performing push and pop operations and checking if the stack is empty or full?

DSA C

#include <stdio.h>
#define MAX 4

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

int isEmpty() {
    return top == -1;
}

int isFull() {
    return top == MAX - 1;
}

void push(int val) {
    if (!isFull()) {
        stack[++top] = val;
    }
}

void pop() {
    if (!isEmpty()) {
        top--;
    }
}

int main() {
    push(1);
    push(2);
    push(3);
    pop();
    push(4);
    push(5);
    if (isFull()) {
        printf("Stack is full\n");
    } else if (isEmpty()) {
        printf("Stack is empty\n");
    } else {
        printf("Stack is neither full nor empty\n");
    }
    return 0;
}

A Runtime error

B Stack is full

C Stack is empty

D Stack is neither full nor empty

💡 Hint

Count the number of elements after all operations and compare with MAX.

After pushing 1,2,3, popping once removes 3, then pushing 4 and 5 fills the stack to 4 elements. So isFull() returns true and prints 'Stack is full'.

❓ Predict Output

advanced

Stack State After Invalid Push

What is the output of this C code after trying to push an element when the stack is already full, then checking if the stack is full?

DSA C

#include <stdio.h>
#define MAX 2

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

int isFull() {
    return top == MAX - 1;
}

void push(int val) {
    if (!isFull()) {
        stack[++top] = val;
    }
}

int main() {
    push(100);
    push(200);
    push(300); // This push should be ignored
    if (isFull()) {
        printf("Stack is full\n");
    } else {
        printf("Stack is not full\n");
    }
    return 0;
}

A Stack is full

B Stack is not full

C Runtime error

D Compilation error

💡 Hint

The third push should not add an element because the stack is full.

The stack size is 2. After pushing 100 and 200, the stack is full. The third push is ignored. So isFull() returns true and prints 'Stack is full'.

❓ Predict Output

expert

Output of Stack Empty and Full Checks with Underflow and Overflow Attempts

What is the output of this C code that attempts to pop from an empty stack and push beyond full capacity, then checks if the stack is empty or full?

DSA C

#include <stdio.h>
#define MAX 3

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

int isEmpty() {
    return top == -1;
}

int isFull() {
    return top == MAX - 1;
}

void push(int val) {
    if (!isFull()) {
        stack[++top] = val;
    }
}

void pop() {
    if (!isEmpty()) {
        top--;
    }
}

int main() {
    pop(); // pop on empty stack, should do nothing
    push(10);
    push(20);
    push(30);
    push(40); // push beyond full, ignored
    pop();
    pop();
    if (isEmpty()) {
        printf("Stack is empty\n");
    } else if (isFull()) {
        printf("Stack is full\n");
    } else {
        printf("Stack is neither full nor empty\n");
    }
    return 0;
}

A Stack is empty

B Stack is full

C Stack is neither full nor empty

D Runtime error

💡 Hint

Track the stack size after each operation carefully.

Pop on empty stack does nothing. Then push 3 elements fills stack. The 4th push is ignored. Two pops remove two elements, leaving one element. So stack is neither full nor empty.