Greetings!

The program works fine as long as I dont put any '(' and ')' in the infix, and I can't find the reason why is it doing that. Also, I can't seem to make the program calculate numbers bigger than 9. In terms of let's say 23+17. And after placing 23+17 in the infix, I can't seem to make postfix look like 23 17 +. Could you please help me?

Side note: My fault i started to work on that today, and i need to finish it before today 23:59 CET. So I'd be thankful if you could help me asap.

Code starts here:
----------------------------------------------------------------------
Code:
#include<iostream.h>
#include<stdio.h>
#include<conio.h>
#include<string.h>
#include<stdlib.h>
#include <ctype.h>

using namespace std;


const int size =255;
char infix[size],postfix[size],stack[size];
int top=-1;

int precedence(char ch);   // function to get the precedence of the operator
char pop();  //function to pop an element from the stack
char topelement();  // returns the top element of the stack
void push(char ch);  // pushes an element into the stack
int eval(char *postf); // evaluation of postfix

int precedence(char ch)
{
       switch(ch)
          {
               case '/' : return 4;
               case '*' : return 4;
               case '+' : return 3;
               case '-' : return 3;
               default  : return 0;
          }
}

char pop()                  //function to pop the element from the stack
{
     char ret;
     if(top!=-1)
       {  ret =stack[top];
          top--;
          return ret;
       }
     else
        return '#';
}

char topelement()          // function to return top element from the stack without popping
{
      char ch;
      if(top!=-1)
        ch=stack[top];
      else
         ch='#';
       return ch;
}

void push(char ch)          // function to push an element in the stack
{
     if(top!=size-1)
         {
            top++;
            stack[top]= ch;
         }
}


int eval(char *postf){      //Evaluation of postfix
    char *p;
    int op1,op2,result;

    p=&postf[0];
    while(*p!='\0'){

        if(isdigit(*p)){
                        push(*p-48);}
                        else{
                             op1=pop();
                             op2=pop();

            switch(*p)
            {
                case '+':
                    result = op2 + op1;
                    break;

                case '-':
                    result = op2 - op1;
                    break;

                case '/':
                    result = op2 / op1;
                    break;

                case '*':
                    result = op2 * op1;
                    break;

                default:
                    printf("\nInvalid Operator");
                    return 0;
            }
            push(result);
        }

        p++;
     }
     result=pop();
     return result;
}



//-----------------MAIN----------------------

int main()
{
     char ele,elem,st[2];
     int prep,pre,popped,j=0,chk=0,p;
     strcpy(postfix," ");

     cout<<"infix: ";
     gets(infix);

     for(int i=0;infix[i]!=0;i++){
           while(infix[i] == ' ' || infix[i] == '\t'){
               i++;
           }
           if(infix[i]!='('&&infix[i]!=')'&&infix[i]!='*'&&infix[i]!='/'&&infix[i]!='+'&&infix[i]!='-')
                       postfix[j++]=infix[i];
                  else if(infix[i]=='(')
                      {
                         elem=infix[i];
                         push(elem);
                      }
                  else if(infix[i]==')')
                      {
                         while(popped=pop() != '(')
                             postfix[j++]=popped;
                      }
                  else
                      {
                         elem=infix[i];
                         pre=precedence(elem);//stores the precedence of operator coming frm infix
                         ele=topelement();
                         prep=precedence(ele);//stores the precedence of operator at the top of the stack

                         if(pre > prep)
                           push(elem);

                         else
                           {
                                while(prep >= pre)
                                  {
                                     if(ele=='#')
                                       break;
                                     popped=pop();
                                     ele=topelement();
                                     postfix[j++]=popped;
                                     prep=precedence(ele);
                                   }
                                   push(elem);
                            }
                         }
             }

          while((popped=pop())!='#')
              postfix[j++]=popped;
          postfix[j]='\0';

          cout<<"\npost fix :"<<postfix<<endl;
          cout<<"equals: "<<eval(&postfix[0])<<endl;
           system("pause");
           return 0;
}