infixto_postfix

#include<stdio.h>
#include<process.h>
#include<string.h>
char stack[20]; //character type
int top=-1;
char pop();
void push(char item);
int isoperand(char symbol);
int isoperator(char symbol);
void converttopost(char infix[20],char postfix[20]);
int prcd(char symbol);
char pop()
{
	char ch;
	ch=stack[top];
	top--;
	return ch;
}
void push(char item)
{
	top++;
	stack[top]=item;
}
int isoperand(char symbol)
{
	if((symbol>='a'&& symbol<='z') ||
	  (symbol>='A' && symbol<='Z')||
	  (symbol>='0' && symbol<='9'))
	    return 1;
    else
	  	return 0;
}

int isoperator(char symbol)
{
	switch(symbol)
	{
		case '+':
		case '-':
		case '*':
		case '/':
		case '^':
		case '$':
		case '(':
		case ')': return 1;
		default: return 0;	
	}
}
int prcd(char symbol)
{
	switch(symbol)
	{

	    case '+':
		case '-': return 2;
		case '*':
		case '/': return 4;
	    case '^':
		case '$': return 6;
		case '(':
		case ')': return 1;	

    }
}
void converttopost(char infix[20],char postfix[20])
{
	int i,j=0;
	char symbol;
	for(i=0;i<strlen(infix);i++)
	{
		symbol=infix[i];
		if(isoperand(symbol)==1)
		{
			postfix[j]=symbol;
			j++;
		}
		else if(symbol=='(')
			push(symbol);

		else if(symbol==')')
		{
			while(stack[top]!='(')
			{
				postfix[j]=pop();
				j++;
			}
			pop();
		}
		else if(isoperator(symbol)==1)
		{
			if(prcd(symbol)>prcd(stack[top]))
				push(symbol);

			else 
			{
				while (prcd(symbol)<=prcd(stack[top]))
				{
					postfix[j]=pop();
					j++;
				}
				push(symbol);
			}
		}
		else
		{
			printf("\n Invalid Symbol: %c ",symbol);
			exit(0);
		}
	}
	while(top!=-1)
	{
		postfix[j]=pop();
		j++;
	}
	postfix[j]='\0';
}
int main()
{
		char infix[20],postfix[20];
		printf("enter the valid infix string \n");
		gets(infix);
		converttopost(infix,postfix);
		printf("the corresponding postfix string: %c \n");
		puts(postfix);
}