//Convert an infix expression to postfix expression...

#include<iostream>
#include<string>
#include<cstdlib>
using namespace std;
char ifx[50],pfx[50],stk[50];
int top=-1,n;
char push(char ch)
{
    if(top!=n)
    {
        ch=stk[top];
        top++;
        return ch;
    }
    else
        cout<<"\nThe stack is full.";
}
void pop()
{
    char rmv;
    if(top!=-1)
    {
        rmv=stk[top];
        top--;
        cout<<"\nElement removed was "<<rmv;
    }
    else
        cout<<"\nThe stack is empty.";
}
char topele()
{
    char t;
    if(top==-1)
        t='#';
    else
        t=stk[top];
    return t;
}
int chkpres()
{
    char ch;
    switch(ch)
    {
        case '^': return 5;
            break;
        case '*': return 4;
            break;
        case '/': return 3;
            break;
        case '+': return 2;
            break;
        case '-': return 1;
            break;
        default: exit(1);
            break;
    }
}
int main()
{
    int i;

    char pre,pres,ele,elem,chk,popp,topp;
    cout<<"\nEnter how many elements you want to enter in the infix expression: ";
    cin>>n;
    topp=stk[top];
    cout<<"\nEnter the characters of the infix expression one by one: ";

    // Make this its own loop to simplify the rest
    for(i=0;i<n;++i)
    {
        cin>>ifx[i];
    }
    // You can't assume the array will end in '\0', you have to put it there
    ifx[i]='\0';

    // uncommented this.  It was a good idea, but didn't 
    // work before because you forgot the !
    for(i=0;ifx[i]!='\0';++i)
    {
            // Is this really what you want it to do?  Nothing except letters
            // and brackets will make to the second if-statement.
            if(ifx[i]!='^' && ifx[i]!='*' && ifx[i]!='/' && ifx[i]!='+' && ifx[i]!='-')
                pfx[i]=ifx[i];
            // 'ele' may not even have been set yet, in which case its value will
            // be undefined.  Which could be (.  Or could be \xfe.  What do you
            // want 'ele' to be by default?  set it when you create the var.
            else if(ele=='(')
            {
                ele=ifx[i];
                push(ele);
                top++;
                // Shouldn't you set pfx[i] to something here?  Otherwise it
                // will stay at an undefined value at index i -- possibly \0,
                // possibly garbage
            }
            else if(ifx[i]=='^'||ifx[i]=='*'||ifx[i]=='/'||ifx[i]=='+'||ifx[i]=='-')
            {
                if(topp=='^'||topp=='*'||topp=='/'||topp=='+'||topp=='-')
                {
                    pre=ele;
                    pres=topp;
                    chk=chkpres();
                    if(pre>pres)
                    {
                        topp=pre;
                        top++;
                        // Shouldn't you set pfx[i] to something here?  Otherwise it
                        // will stay at an undefined value at index i -- possibly \0,
                        // possibly garbage
                    }
                    else if(pres>=pre)
                        pfx[i]=pres;
                }
            }
            // There is a huge difference between = and ==.
            // you were changing ele to ')' here.
            // 'ele' may not even have been set yet, either, see above.
            else if(ele==')')
            {
                while(topp!='#')
                {
                    popp=topp;
                    pfx[i]=popp;
                }
            }
       // }
    }

    // You can't assume the array will end in '\0', you have to put it there
    pfx[i]='\0';

    cout<<"\nThe postfix expression is: ";
    for(i=0; pfx[i]!='\0'; ++i)
    {
        cout<<pfx[i];
    }
return 0;
}