There is a Empty character in my Character Array at the end and idkwhy

Question

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

char * stack = NULL;
int top = -1;

void push (char letter);
void pop();

int main()
{
    char stringy [100];
    stack = (char *)malloc(100*sizeof(char));
    printf("Enter a String : ");
    scanf("%s",stringy);

    char letter = '1';
    for (int i = 0 ; letter != '\0'; i++){
        letter = stringy[i];
        push(letter);
        printf("Top = %d, Character = %c",top,letter);
    }

    top--;
    for (top; top != -1; top){
        pop();
    }
    return 0;
}

void push(char letter){
    top++;
    stack[top] = letter;
}

void pop(){
    
    printf("%c",stack[top]);
    top--;
}

THERE IS A EMPTY CHARACTER THAT IS BEING PUSHED AND I DONT KNOW WHY PLEASE HELP There is a fourth element being pushed into my stack as well which has no display on the stdout.

Answer 1

Look at this loop in your code:

    for (int i = 0; letter != '\0'; i++){
        letter = stringy[i];
        push(letter);

When the value of i is equal to index of stringy array where null character exists (i.e. stringy[i] is '\0', it will first assign that '\0' character to letter and then that null character is pushed to stack.

Instead of checking letter != '\0', you should directly check the stringy[i] != '\0' in for loop condition.

Few other problems in your code:

In main() function, you are doing

    top--;
    for (top; top != -1; top){
        pop();
    }

It can be implemented in a better way, like this:

    while (top != -1) {
        pop();
    }

A suggestion - Instead of printing character in pop() function, better to return the character, popped from stack, from pop() function.

Since, the size of stringy array is 100 characters, put a restriction in scanf() to not to read more than 99 characters (the remain one character space is for null terminating character):

    scanf("%99s",stringy);

Follow good programming practice, always check malloc function return and free the dynamically allocated memory once your program done with it.

Putting these altogether, you can do:

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

char * stack = NULL;
int top = -1;

void push (char letter);
void pop();

int main (void) {
    char stringy [100];

    stack = (char *)malloc(100*sizeof(char));
    if (stack == NULL) {
        printf ("Failed to allocate memory\n");
        exit (EXIT_FAILURE);
    }

    printf("Enter a String : ");
    scanf("%99s",stringy);

    for (int i = 0; stringy[i] != '\0'; i++) {
        push(stringy[i]);
        printf("Top = %d, Character = <%c>\n", top, stringy[i]);
    }

    while (top != -1) {
        pop();
    }

    free (stack);
    return 0;
}

void push(char letter) {
    top++;
    stack[top] = letter;
}

void pop() {
    printf("%c",stack[top]);
    top--;
}

A stack data structure can be implemented in much better way. Leaving it up to you to learn and modify the implementation.

Answer 2

Your program had few bugs. I fixed those. Now, it is working correctly.

There were two changes:

1. The condition in the for loop was changed.
2. top-- was removed from the main() function.

The updated code is below:

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

char *stack = NULL;
int top = -1;

void push (char letter);
void pop();

int main()
{
    char stringy [100];
    stack = (char *)malloc(100*sizeof(char));

    printf("Enter a String : ");
    scanf("%s",stringy);

    char letter = '1';

    for (int i = 0 ; ((letter = stringy[i]) != '\0'); i++){
        push(letter);
        printf("Top = %d, Character = %c\n",top,letter);
    }

    for (top; top != -1; top) {
        pop();
    }

    return 0;
}

void push(char letter)
{
    top++;
    stack[top] = letter;
}

void pop()
{
    printf("%c",stack[top]);
    top--;
}