Infix program in C program data structure

Infix notation is the common arithmetic and logical formula notation, in which operators are written infix-style between the operands they act on (e.g. 2 + 2). It is not as simple to parse by computers as prefix notation  but many programming languages use it due to its familiarity.

#include<stdio.h>
#include<conio.h>
#include<string.h>
#define MAX 20
char stack[MAX];
int top=-1;
char pop();
void push(char item);
int prcd(char symbol)
{




switch(symbol)
{
case '+':
case '-':return 2;
//break;
case '*':
case '/':return 3;
break;
case '^':
case '$':return 4;
break;
case '(':
case ')':
case '#':return 1;
break;
}}
 int isoperator(char symbol)
{
switch(symbol)
{
case '+':
case '-':
case '*':
case '/':
case '^':
case '$':
case '(':
case ')':
return 1;
break;
default:return 0;
}}
void convertip(char infix[],char postfix[])
{
int i,symbol,j=0;
stack[++top]='#';
for(i=0;i<strlen(infix);i++)
{
symbol=infix[i];
if(isoperator(symbol)==0)
{
postfix[j]=symbol;
j++;
}
else
{
if(symbol=='(')
push(symbol);
else if(symbol==')')
{
while(stack[top]!='(')
{
postfix[j]=pop();
j++;
}
pop();//pop out (.
} else{
if(prcd(symbol)>prcd(stack[top]))
push(symbol);
else{
while(prcd(symbol)<=prcd(stack[top]))
{
postfix[j]=pop();
j++;
}
push(symbol);
}//end of else.
}//end of else.
}//end of else.
}//end of for.
while(stack[top]!='#')
{
postfix[j]=pop();
j++;
}
postfix[j]='\0';//null terminate string.
}
void main()
{
char infix[20],postfix[20];
clrscr();
printf("Enter the valid infix string:\n");
gets(infix);
convertip(infix,postfix);
printf("The corresponding postfix string is:\n");
puts(postfix);
getch();
}
void push(char item)
{ top++;
stack[top]=item;
} char pop()
{ char a;
a=stack[top];
top--;
return a;
}