Arduino: Infix to Postfix - data-structures

Question

I want to make calculator on Arduino so, this calculator should be carries about "operation sequence ".
I belive my "convertering infix to postfix" codes are true, but executable program stops working and i got results which are not effective. I try this code on other arduinos but i get no result. Writing on lcd sometimes works but sometimes code resets arduino. I fail to see why this error was occured.
#include <Keypad.h>
#include <LiquidCrystal.h>
const byte SATIR = 4;
const byte SUTUN= 4;
char keys[SATIR][SUTUN] = {
{'1','2','3','+'},
{'4','5','6','-'},
{'7','8','9','*'},
{'C','0','=','/'}
};
byte rowPins[SATIR] = { 13, 10, 9, 8 };
byte colPins[SUTUN] = { 7, 6, 1, 0 };
Keypad kpd = Keypad( makeKeymap(keys), rowPins, colPins, SATIR, SUTUN );
LiquidCrystal lcd(12, 11, 5, 4, 3, 2);
int SIZE = 20;
char stack[20];
int top = -1;
String num = "";
int number;
void setup() {
lcd.begin(16, 2); //16x2 kullaniyoruz
lcd.print("fatay");
delay(3000);
lcd.clear();
lcd.setCursor(0, 0);
}
int p = 0;
void loop(){
char infix[20];
char postfix[20];
char key = kpd.getKey();
if (key != NO_KEY){
if(key == '=') {
for(int z = 0; z<num.length(); z++){
infix[z] = num.charAt(z);
}
number = num.length();
delay (100);
InfixToPostfix(infix,postfix);
} else {
lcd.print(key);
num+=key;
delay(400);
}
}
}
////////////////////////////////////////////////////////////////////////////////////////
void push(char item) {
if(top >= SIZE-1)
{
printf("\nStack Overflow.");
}
else
{
top = top+1;
stack[top] = item;
}
}
/* define pop operation */
char pop() {
char item ;
if(top <0)
{
printf("stack under flow: invalid infix expression");
getchar();
/* underflow may occur for invalid expression */
/* where ( and ) are not matched */
exit(1);
}
else
{
item = stack[top];
top = top-1;
return(item);
}
}
////////////////////////////////////////////////////////////////////////////////////////
int is_operator(char symbol) {
if(symbol == '^' || symbol == '*' || symbol == '/' || symbol == '+' || symbol =='-')
{
return 1;
}
else
{
return 0;
}
}
int precedence(char symbol) {
if(symbol == '^')/* exponent operator, highest precedence*/
{
return(3);
}
else if(symbol == '*' || symbol == '/')
{
return(2);
}
else if(symbol == '+' || symbol == '-') /* lowest precedence */
{
return(1);
}
else
{
return(0);
}
}
void InfixToPostfix(char infix_exp[], char postfix_exp[])
{
int i, j;
char item;
char x;
push('('); /* push '(' onto stack */
strcat(infix_exp,")"); /* add ')' to infix expression */
i=0;
j=0;
item=infix_exp[i]; /* initialize before loop*/
while(item != '\0') /* run loop till end of infix expression */
{
if(item == '(')
{
push(item);
}
else if( isdigit(item) || isalpha(item))
{
postfix_exp[j] = item; /* add operand symbol to postfix expr */
j++;
}
else if(is_operator(item) == 1) /* means symbol is operator */
{
x=pop();
while(is_operator(x) == 1 && precedence(x)>= precedence(item))
{
postfix_exp[j] = x; /* so pop all higher precendence operator and */
j++;
x = pop(); /* add them to postfix expresion */
}
push(x);
/* because just above while loop will terminate we have
oppped one extra item
for which condition fails and loop terminates, so that one*/
push(item); /* push current oprerator symbol onto stack */
}
else if(item == ')') /* if current symbol is ')' then */
{
x = pop(); /* pop and keep popping until */
while(x != '(') /* '(' encounterd */
{
postfix_exp[j] = x;
j++;
x = pop();
}
}
else
{ /* if current symbol is neither operand not '(' nor ')' and nor
operator */
printf("\nInvalid infix Expression.\n"); /* the it is illegeal symbol */
getchar();
exit(1);
}
i++;
item = infix_exp[i]; /* go to next symbol of infix expression */
} /* while loop ends here */
if(top>0)
{
printf("\nInvalid infix Expression.\n"); /* the it is illegeal symbol */
getchar();
exit(1);
}
if(top>0)
{
printf("\nInvalid infix Expression.\n"); /* the it is illegeal symbol */
getchar();
exit(1);
}
delay(100);
lcd.setCursor(0,1);
delay (100);
for(int k = 0; k<number; k++) {
lcd.print(postfix_exp[k]);
}
}
Thanks for your answer in advance.