stack.c

#include <stdio.h>
#include <stdlib.h>

// Self-referential structure
struct stackNode {
    int data;  // A integer data in the stack
    struct stackNode *nextPtr;   // Pointer to next element of stack
}; // end struct stackNode

typedef struct stackNode StackNode; // alias for the struct
typedef StackNode *StackNodePtr;  // alias for StackNode*

// Declarations
// Pointer to the top plays the main role
void push(StackNodePtr *topPtr, int info);
int pop(StackNodePtr *topPtr);
int isEmpty(StackNodePtr topPtr);
void showStack(StackNodePtr currentPtr);
void showMenu(void);

int main() {
    // Points to stack top
    StackNodePtr stackPtr = NULL;
    // User's choice
    unsigned int ch;
    // User's entered character
    int item;
    
    // Display the menu
    showMenu();
    printf("? ");
    scanf("%u", &ch);
    
    // Loop unless user chooses 3
    while(ch != 3) {
        switch(ch) {
            case 1:
                printf("Enter an integer: ");
                scanf("%d", &item);
                
                // insert the item in the top of stack
                push(&stackPtr, item);
                showStack(stackPtr);
                break;
            case 2:
                // if stack is not empty
                if(!isEmpty(stackPtr)) {
                    printf("The popped integer is %d.\n", pop(&stackPtr));
                } // end if 
                showStack(stackPtr);
                break;
            default:
                printf("Invalid choice.\n");
                showMenu();
                break;
        }
        showMenu();
        printf("? ");
        scanf("%u", &ch);
    }
    printf("Thank You!\n");
    return 0;
}

// Display Menu Function
void showMenu(void) {
    printf("Enter your choice:\n"
        "   1 to push integer into the stack.\n"
        "   2 to pop integer from the stack.\n"
        "   3 to end" );
}

// insert a node to the top of stack
void push(StackNodePtr *topPtr, int value) {
    StackNodePtr newPtr; // pointer to new node
    
    newPtr = malloc(sizeof(StackNode)); // Allocate memory for StackNode
    
    // if allocated
    if (newPtr != NULL) {
        newPtr->data = value;   // put value in the node
        newPtr->nextPtr = *topPtr;
        *topPtr = newPtr; // new node is top
    } // end if
    else {
        printf("%d is not inserted. Memory error!\n", value);
    }
}

// pop from the stack
int pop(StackNodePtr *topPtr) {
    StackNodePtr tmpPtr; // temporary node pointer
    int popValue; // node value
    
    tmpPtr = *topPtr;
    popValue = tmpPtr->data;
    
    *topPtr = tmpPtr->nextPtr; 
    free(tmpPtr);
    
    return popValue;
}

// return 1 if the stack is empty, otherwise 0
int isEmpty(StackNodePtr topPtr) {
    return topPtr == NULL;
}

// show the list
void showStack(StackNodePtr currentPtr) {
    // if stack is empty
    if (isEmpty(currentPtr)) {
        printf("The stack is empty!\n");
    }
    else {
        printf("The stack is: \n");
        
        // while not the end of stack
        while(currentPtr != NULL) {
            printf("%d ==> ", currentPtr->data);
            currentPtr = currentPtr->nextPtr;
        }
        printf("NULL\n");
    }
}