Notes

#include <stdio.h>
#include <stdlib.h>
#include<string.h>
#define MaxSize 50
char stack[MaxSize];
int TOP=-1;

//function to insert an element into a stack
void push(char symbol)
{
TOP=TOP+1;
stack[TOP]=symbol;
}

//function to delete an element from a stack
char pop()
{
char k;
k=stack[TOP];
TOP=TOP-1;
return k;

}

//function to check whether the symbol is operand or not
int isoperand(char symbol)
{
if(symbol>='a'&& symbol<='z')
return 1;
else
return 0;
}

//function to check whether the symbol is operator or not
int isoperator(char symbol)
{
if(symbol=='+'|| symbol=='-'|| symbol=='*'|| symbol=='/'|| symbol=='$')
return 1;
else
return 0;
}

//function to assign precedence to all operators
int precedence(char symbol)
{
int result;
switch(symbol)
{
case '(':
result=0; break;
case '+':
case '-':result=1; break;
case '*':
case '/':result=2; break;
}
return result;
}

//to find the reverse of an expression
void rev(char str[])
{
int l, i,j;
char t;
l=strlen(str);
for(i=0,j=l-1;i<l/2;i++,j--)
{
t=str[i];
str[i]=str[j];
str[j]=t;
}
for(i=0;str[i]!='';i++)
{
if (str[i]=='(')
str[i]=')';
else
{
if(str[i]==')')
str[i]='(';
}
}
}
// main function
int main()
{
char infix[MaxSize],postfix[MaxSize];
char symbol,temp;
int i,j;
i=j=0;
printf("Enter an infix expression: ");
gets(infix);

rev(infix);
push('(');

while (infix[i]!='')
{
symbol=infix[i];
if(isoperand(symbol))
{
postfix[j]=symbol;
//printf("Postfix expression P is: %c n",postfix[i]);
j=j+1;
}
if(symbol=='(')
{
push(symbol);
}

if(isoperator(symbol))
{
while(precedence(stack[TOP])>=precedence(symbol))
{
temp=pop();
postfix[j]=temp;
j=j+1;
}
push(symbol);
}
if(symbol==')')
{
while(stack[TOP]!='(')
{
temp=pop();
postfix[j]=temp;

j=j+1;
}
temp=pop();
}
i=i+1;
}//while loop ends here
while(stack[TOP]!='(')
{
temp=pop();
postfix[j]=temp;
j=j+1;
}
postfix[j]='';
printf("Postfix expression is: %s n",postfix);
rev(postfix);
printf("Prefix expression is: %s n",postfix);

return 0;
}