Program to convert Infix expression to Postfix expression in c

infix_to_postfix.c

#include<stdio.h>
#define max 100
int top=-1, a[max];
void push(char x)
{
    a[++top]=x;
}
char pop()
{ if(top==-1)
return -1;
else
return a[top--];
}

int prcd(char c)
{   if(c=='(')
    return 0;
    else if(c=='+'||c=='-')
    return 1;
    else if(c=='*'||c=='/')
    return 2;
}
int infixtopostfix(char infix[max],char postfix[max])
{
    char temp,x;
    int i=0,j=0;
    while(infix[i]!='\0')
    {
        temp=infix[i];
       if(isalnum(temp))
        {
            postfix[j++]=temp;
        }
        else if(temp=='(')
         push(temp);
        else if(temp==')')
            {
         while((x=pop())!='(')
         {
          postfix[j++]=x;
           }
          }
        else
        {  while(prcd(a[top])>=prcd(temp))
            {postfix[j++]=pop();}
                push(temp);
        }
        i++;
    }
    while(top!= -1)
    postfix[j++]=pop();
    postfix[j]='\0';
}
main()
{
    char infix[max],postfix[max];
    printf("Enter the infix expression\n");
    gets(infix);
    printf("The infix expression is %s\n",infix);
    infixtopostfix(infix, postfix);
    printf("The postfix expression is %s\n",postfix);

}