C Program to Convert Infix Expression to Postfix Using Stack

Program : 

  • This Program Accepts Operators : +,-,/,*,(,)
  • Sample Infix Expression : (a+b)*c/(a+b*c)
#include<stdio.h>
#include<conio.h>
#include<ctype.h>
#define MAX 100

typedef struct stack
{
int data[MAX];
int top;
}stack;

int priority(char);
void init(stack *);
int empty(stack *);
int full(stack *);
char pop(stack *);
void push(stack *,char);
char top(stack *);

void main()
{
stack s;
char x;
int token;
init(&s);
clrscr();
printf("nEnter infix expression:");
while((token=getchar())!='n')
{
if(isalnum(token))
printf("%c",token);
else
if(token == '(')
push(&s,'(');
else
{
if(token == ')')
while((x=pop(&s))!='(')
printf("%c",x);
else
{
while(priority(token)< =priority(top(&s)) && !empty(&s))
{
x=pop(&s);
printf("%c",x);
}
push(&s,token);
}
}
}
while(!empty(&s))
{
x=pop(&s);
printf("%c",x);
}
getch();
}
//---------------------------------------------
int priority(char x)
{
if(x == '(')
return(0);
if(x == '+' || x == '-')
return(1);
if(x == '*' || x == '/' || x == '%')
return(2);
return(3);
}
//---------------------------------------------
void init(stack *s)
{
s->top=-1;
}
//---------------------------------------------
int empty(stack *s)
{
if(s->top==-1)
return(1);
else
return(0);
}
//---------------------------------------------
int full(stack *s)
{
if(s->top==MAX-1)
return(1);
else
return(0);
}
//---------------------------------------------
void push(stack *s,char x)
{
s->top=s->top+1;
s->data[s->top]=x;
}
//---------------------------------------------
char pop(stack *s)
{
int x;
x=s->data[s->top];
s->top=s->top-1;
return(x);
}
//---------------------------------------------
char top(stack * s)
{
return(s->data[s->top]);
}
//---------------------------------------------

Explanation :
while((token=getchar())!='n')
  • Accepts Expression Character by Character Till Entered Character is ‘n’
  • After Accepting Single Character do all actions inside while loop.
if(isalnum(token))
printf("%c",token);
  • isalnum(token)   checks whether entered Character is Alphabetic or Numeric .
  • If Entered Character is alpha-numeric then Directly Display it – for more help [Algo] + [Example]
  • If Entered Character is Opening Bracket then Push Element Onto Stack

if(token == '(')
push(&s,'(');
  • If Entered Character is ‘Closing Bracket’ then Pop All Elements till Equivalent Opening Bracket is poped.
if(token == ')')
while((x=pop(&s))!='(')
printf("%c",x);
  • If Entered Character is Operator then – do
else
{
while(priority(token)< =priority(top(&s)) && !empty(&s))
{
x=pop(&s);
printf("%c",x);
}
push(&s,token);
}