Conversion of Infix to Postfix and Evaluation of Postfix

/*
Write a program for conversion of infix to postfix and evaluation of postfix. It will evaluate only single digit numbers.
*/ 
#include<stdio.h>
#include<string.h>
#include<math.h>
#include<stdlib.h> 

#define BLANK ' '
#define TAB '\t'
#define MAX 50

void push(long int symbol);

long int pop();

void infix_to_postfix();

long int eval_post();

int priority(char symbol);

int isEmpty();

int white_space();

char infix[MAX], postfix[MAX];

long int stack[MAX];

int top;

main()

{

long int value;

top=-1;

printf("Enter infix : ");

gets(infix);

infix_to_postfix();

printf("Postfix : %s\n",postfix);

value=eval_post();

printf("Value of expression : %ld\n",value);

}

void infix_to_postfix()
{

unsigned int i,p=0;

char next;

char symbol;

for(i=0;i<strlen(infix);i++)

{

symbol=infix[i];

if(!white_space(symbol))

{

switch(symbol)

{

case '(':

push(symbol);

break;

case ')':

while((next=pop())!='(')

postfix[p++] = next;

break;

case '+':

case '-':

case '*':

case '/':

case '%':

case '^':

while( !isEmpty( ) && priority(stack[top])>= priority(symbol) )

postfix[p++]=pop();

push(symbol);

break;

default: /*if an operand comes*/

postfix[p++]=symbol;

}

}

}

while(!isEmpty( ))

postfix[p++]=pop();

postfix[p]='\0'; /*End postfix with'\0' to make it a string*/

}
 
/*This function returns the priority of the operator*/
int priority(char symbol)

{

switch(symbol)

{

case '(':

return 0;

case '+':

case '-':

return 1;

case '*':

case '/':

case '%':

return 2;

case '^':

return 3;

default :

return 0;

}

}

void push(long int symbol)

{

if(top>MAX)

{

printf("Stack overflow\n");

exit(1);

}

stack[++top]=symbol;

}

long int pop()

{

if( isEmpty() )

{

printf("Stack underflow\n");

exit(1);

}

return (stack[top--]);

}

int isEmpty()

{

if(top==-1)

return 1;

else

return 0;

}

int white_space(char symbol)

{

if( symbol == BLANK || symbol == TAB )

return 1;

else

return 0;

}

long int eval_post()

{

long int a,b,temp,result;

unsigned int i;

for(i=0;i<strlen(postfix);i++)

{

if(postfix[i]<='9' && postfix[i]>='0')

push(postfix[i]-'0');

else

{

a=pop();

b=pop();

switch(postfix[i])

{

case '+':

temp=b+a; break;

case '-':

temp=b-a;break;

case '*':

temp=b*a;break;

case '/':

temp=b/a;break;

case '%':

temp=b%a;break;

case '^':

temp=pow(b,a);

}

push(temp);

}

}

result=pop();

return result;

}