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Infix to postfix

the following propgram can convert an infix expression to a postfix expression using stacks

Submitted By: frog

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Rating:

Views: 32,033

Language: C++

Last Modified: July 30, 2006

Instructions: the program assumes that the infix expression contains only single lettered variables

Snippet

// the program is used to convert a infix expression to a postfix expression
#include<iostream.h>
#include<stdio.h>
#include<conio.h>
#include<string.h>
#include<stdlib.h>
const int size =50;
char infix[size],postfix[size],stack[size];
int top=-1;
int precedence(char ch); // function to get the precedence of the operator
char pop(); //function to pop an element from the stack
char topelement(); // returns the top element of the stack
void push(char ch); // pushes an element into the stack
int main()
{
char ele,elem,st[2];
int prep,pre,popped,j=0,chk=0;
strcpy(postfix," ");
gets(infix);
for(int i=0;infix[i]!=0;i++)
{
if(infix[i]!='('&&infix[i]!=')'&&infix[i]!='^'&&infix[i]!='*'&&infix[i]!='/'&&infix[i]!='+'&&infix[i]!='-')
postfix[j++]=infix[i];
else if(infix[i]=='(')
{
elem=infix[i];
push(elem);
}
else if(infix[i]==')')
{
while(popped=pop() != '(')
postfix[j++]=popped;
}
else
{
elem=infix[i];
pre=precedence(elem);//stores the precedence of operator coming frm infix
ele=topelement();
prep=precedence(ele);//stores the precedence of operator at the top of the stack
if(pre > prep)
push(elem);
else
{
while(prep >= pre)
{
if(ele=='#')
break;
popped=pop();
ele=topelement();
postfix[j++]=popped;
prep=precedence(ele);
}
push(elem);
}
}
}
while((popped=pop())!='#')
postfix[j++]=popped;
postfix[j]='\0';
cout<<"\n post fix :"<<postfix<<endl;
system("pause");
return 0;
}
int precedence(char ch)
{
switch(ch)
{
case '^' : return 5;
case '/' : return 4;
case '*' : return 4;
case '+' : return 3;
case '-' : return 3;
default : return 0;
}
}
char pop() //function to pop the element from the stack
{
char ret;
if(top!=-1)
{ ret =stack[top];
top--;
return ret;
}
else
return '#';
}
char topelement() // function to return top element from the stack without popping
{
char ch;
if(top!=-1)
ch=stack[top];
else
ch='#';
return ch;
}
void push(char ch) // function to push an element in the stack
{
if(top!=size-1)
{
top++;
stack[top]= ch;
}
}

Copy & Paste

// the program is used to convert a infix expression to a postfix expression

#include<iostream.h>
#include<stdio.h>
#include<conio.h>
#include<string.h>
#include<stdlib.h>

const int size =50;
char infix[size],postfix[size],stack[size];
int top=-1;

int precedence(char ch);   // function to get the precedence of the operator
char pop();  //function to pop an element from the stack
char topelement();  // returns the top element of the stack
void push(char ch);  // pushes an element into the stack

int main()
{
     char ele,elem,st[2];
     int prep,pre,popped,j=0,chk=0;
     strcpy(postfix," ");
         
     gets(infix);
     
     for(int i=0;infix[i]!=0;i++)
          {
                  if(infix[i]!='('&&infix[i]!=')'&&infix[i]!='^'&&infix[i]!='*'&&infix[i]!='/'&&infix[i]!='+'&&infix[i]!='-')     
                       postfix[j++]=infix[i];
                  else if(infix[i]=='(')
                      {
                         elem=infix[i];
                         push(elem);
                      }
                  else if(infix[i]==')')
                      {
                         while(popped=pop() != '(')
                             postfix[j++]=popped;
                      }
                  else
                      {
                         elem=infix[i];
                         pre=precedence(elem);//stores the precedence of operator coming frm infix
                         ele=topelement();
                         prep=precedence(ele);//stores the precedence of operator at the top of the stack
                        
                         if(pre > prep)
                           push(elem);                                         
                           
                         else
                           {
                                while(prep >= pre)
                                  {
                                     if(ele=='#')
                                       break;
                                     popped=pop();
                                     ele=topelement();
                                     postfix[j++]=popped;
                                     prep=precedence(ele);
                                   }
                                   push(elem);
                            }
                         }
             } 
             
          while((popped=pop())!='#')
              postfix[j++]=popped;
          postfix[j]='\0';
          
          cout<<"\n post fix :"<<postfix<<endl;
           
           system("pause");
           return 0;
}

int precedence(char ch)
{
       switch(ch)
          {
               case '^' : return 5;
               case '/' : return 4;
               case '*' : return 4;                                            
               case '+' : return 3;
               case '-' : return 3;
               default  : return 0;
          }
}

char pop()                  //function to pop the element from the stack
{
     char ret;
     if(top!=-1)
       {  ret =stack[top];
          top--;
          return ret;
       }
     else
        return '#';
}
                         
char topelement()          // function to return top element from the stack without popping
{     
      char ch;
      if(top!=-1)
        ch=stack[top];
      else
         ch='#';
       return ch;
}

void push(char ch)          // function to push an element in the stack
{
     if(top!=size-1)
         {
            top++;
            stack[top]= ch;
         }
}

Comments
fairy 2008-03-16 06:07:30
this is a horrible code it will drive the reader mad can't you just reduce the code?????
javadmotahari 2008-08-08 08:49:56

omniaelsafty 2009-01-17 09:50:58
THANKS ALOOOT
sam442000 2009-04-08 00:03:27
brilliant
pavankvvd 2009-08-08 10:27:39
nfix.c:6: error: variably modified �infix� at file scope infix.c:6: error: variably modified �postfix� at file scope infix.c:6: error: variably modified �stack� at file scope infix.c: In function �main�: infix.c:18: error: �for� loop initial declaration used outside C99 mode infix.c:100: error: expected declaration or statement at end of input CAN ANYONE SOLVE THIS ERROR FOR THE ABOVE CODE
jl880703 2009-11-17 19:45:21
i made some modification over this code where can i post it