Thread: properly nested

Hey all, Ive been working on this program for a while and keep changing it around. Im trying to have it verify that the user inputted braces are properly nested. I keep coming up with the outcome that they are not, even if they are. any input on where im going wrong is appreciated.

Code:

#include<stdbool.h>
#include<stdio.h>


#define STACK_SIZE 100


char contents[STACK_SIZE];
int top = 0;


void stack_overflow(void)
{
    printf("invalid under");
}


void stack_underflow(void)
{
    printf("invalid over");
}


void make_empty(void)
{
    top = 0;
}


bool is_empty(void)
{
    return top==0;
}


bool is_full(void)
{
    return top==STACK_SIZE;
}


char push(char ch)
{
    if(is_full())
        stack_overflow();
    else
        contents[top++]=ch;
}


char pop(char contents[])
{
   
    if(is_empty())
        stack_underflow();
    else
        return contents[--top];
}


int main(void)
{
    char  ch,fst,lst;           
             
    int i = 0;         




    printf( "Enter parenthesesor braces:\n" );




    while ( ( i < STACK_SIZE ) && ( ( ch = getchar() ) != '\n' ) ) {
       contents[i++] = ch;


    if(('('==ch)||('{'==ch))
             {push(ch);
            fst = ch;
        }
        else if((')'==ch)||('}'==ch))
              {
                pop(contents);
                lst = ch;
                }
    }


        if ( fst == lst )
        printf("\nParenthesis/brackets nested properly....");
        else printf("\nParenthesis/brackets not nested properly...");


    return 0;
}