Stack infix to postfix - c

Question

My code runs well to my surprise!!
The only problem is that whenever I use a bracketed infix input it comes out a 'J' at the end of postfix expression !! Any suggestions??
Here the algorithm is the basic one all expressions are getting converted and all is right but the tailing 'J' is i just cann't understand !! Suggestions??
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<ctype.h>
#define MAX 50
char stack[MAX];
int top = -1;
void push(char element)
{
stack[++top] = element;
}
char pop()
{
return(stack[top--]);
}
char tope()
{
return(stack[top]);
}
int prec(char c)
{
switch(c){
case '+':
case '-' : return 1;
break;
case '*' :
case '/' : return 2;
break;
default:
return 0;
break;
}
}
int main()
{
char post[MAX],in[MAX],ch,element;
printf("Infix expression : ");
scanf("%s",in);
int i=0,k=0;
in[strlen(in)] = ')';
push('(');
while((ch = in[i++]) != '\0')
{
if(isalnum(ch))
post[k++] = ch;
if(ch == '(')
push(ch);
if(ch == ')')
{
while(tope() != '(')
{
post[k++] = pop();
}
pop();
}
if(ch == '+' || ch =='-' || ch == '*' || ch == '/')
{
while(prec(ch) <= prec(tope()))
{
post[k++] = pop();
}
push(ch);
}
}
post[k] = '\0';
printf("%s",post);
return 0;
}

Answer 1

in[strlen(in)] = ')';
overwrites the nul-terminating character, which explains strange chars when printing (printing stops only when meeting another nul char by luck: undefined behaviour which may even lead to a crash if no nul char is found in the in 50-byte buffer)
You have to shift it, for instance like this:
int l = strlen(in);
in[l] = ')';
in[l+1] = '\0';
Note: you have to store the length of your string in l instead of calling strlen(in) twice not only because of the performance loss but, because putting the parenthesis means that strlen doesn't work properly until you null-terminate.
(also you probably want to protect your scanf like this: scanf("%48s",in); so you're sure to have enough room for the extra parenthesis for your 50-size buffer, even compatible with your macro definition, see scanf: template with macro (#define constant) inside)