Stack and queue problem

Question

hi i am trying to change infix expression to postfix and also the opposite, like
((a+b)*(a-c*d)/(b+d*e)) to ab+acb*-bde+/
using stack and queue
I am so confuse

Answer 1

Hello,
See this URL :
http://www24.brinkster.com/premshree/perl
You will find the algorithms here.
Here is a Perl version :
#     Script:          infix-postfix.pl
#     Author:          Premshree Pillai
#     Description:     Using this script you can :
#               - Convert an Infix expression to Postfix
#               - Convert a Postfix expression to Infix
#               - Evaluate a Postfix expression
#               - Evaluate an Infix expression
#     Web:          http://www.qiksearch.com
#     Created:     23/09/02 (dd/mm/yy)
#     � 2002 Premshree Pillai. All rights reserved.
sub isOperand
     ($who)=@_;
     if((!isOperator($who)) && ($who ne "(") && ($who ne ")"))
          return 1;
     else
          return;
sub isOperator
     ($who)=@_;
     if(($who eq "+") || ($who eq "-") || ($who eq "*") || ($who eq "/") || ($who eq "^"))
          return 1;
     else
          return;
sub topStack
     (@arr)=@_;
     my $arr_len=@arr;
     return($arr[$arr_len-1]);
sub isEmpty
     (@arr)=@_;
     my $arr_len=@arr;
     if(($arr_len)==0)
          return 1;
     else
          return;
sub prcd
     ($who)=@_;
     my $retVal;
     if($who eq "^")
          $retVal="5";
     elsif(($who eq "*") || ($who eq "/"))
          $retVal="4";
     elsif(($who eq "+") || ($who eq "-"))
          $retVal="3";
     elsif($who eq "(")
          $retVal="2";
     else
          $retVal="1";
     return($retVal);
sub genArr
     ($who)=@_;
     my @whoArr;
     for($i=0; $i<length($who); $i++)
          $whoArr[$i]=substr($who,$i,1);
     return(@whoArr);
sub InfixToPostfix
     ($infixStr)=@_;
     my @infixArr=genArr($infixStr);
     my @postfixArr;
     my @stackArr;
     my $postfixPtr=0;
     for($i=0; $i<length($infixStr); $i++)
          if(isOperand($infixArr[$i]))
               $postfixArr[$postfixPtr]=$infixArr[$i];
               $postfixPtr++;
          if(isOperator($infixArr[$i]))
               if($infixArr[$i] ne "^")
                    while((!isEmpty(@stackArr)) && (prcd($infixArr[$i])<=prcd(topStack(@stackArr))))
                         $postfixArr[$postfixPtr]=topStack(@stackArr);
                         pop(@stackArr);
                         $postfixPtr++;
               else
                    while((!isEmpty(@stackArr)) && (prcd($infixArr[$i])<prcd(topStack(@stackArr))))
                         $postfixArr[$postfixPtr]=topStack(@stackArr);
                         pop(@stackArr);
                         $postfixPtr++;
               push(@stackArr,$infixArr[$i]);
          if($infixArr[$i] eq "(")
               push(@stackArr,$infixArr[$i])
          if($infixArr[$i] eq ")")
               while(topStack(@stackArr) ne "(")
                    $postfixArr[$postfixPtr]=pop(@stackArr);
                    $postfixPtr++;
               pop(@stackArr);
     while(!isEmpty(@stackArr))
          if(topStack(@stackArr) eq "(")
               pop(@stackArr)
          else
               $temp=@postfixArr;
               $postfixArr[$temp]=pop(@stackArr);
     return(@postfixArr);
sub PostfixToInfix
     ($postfixStr)=@_;
     my @stackArr;
     my @postfixArr=genArr($postfixStr);
     for($i=0; $i<length($postfixStr); $i++)
          if(isOperand($postfixArr[$i]))
               push(@stackArr,$postfixArr[$i]);
          else
               $temp=topStack(@stackArr);
               pop(@stackArr);
               $pushVal=topStack(@stackArr).$postfixArr[$i].$temp;
               pop(@stackArr);
               push(@stackArr,$pushVal);
     return((@stackArr));
sub PostfixEval
     ($postfixStr)=@_;
     my @stackArr;
     my @postfixArr=genArr($postfixStr);
     for($i=0; $i<length($postfixStr); $i++)
          if(isOperand($postfixArr[$i]))
               push(@stackArr,$postfixArr[$i]);
          else
               $temp=topStack(@stackArr);
               pop(@stackArr);
               $pushVal=PostfixSubEval(topStack(@stackArr),$temp,$postfixArr[$i]);
               pop(@stackArr);
               push(@stackArr,$pushVal);
     return(topStack(@stackArr));
sub PostfixSubEval
     ($num1,$num2,$sym)=@_;
     my $returnVal;
     if($sym eq "+")
          $returnVal=$num1+$num2;
     if($sym eq "-")
          $returnVal=$num1-$num2;
     if($sym eq "*")
          $returnVal=$num1*$num2;
     if($sym eq "/")
          $returnVal=$num1/$num2;
     if($sym eq "^")
          $returnVal=$num1**$num2;
     return($returnVal);
sub joinArr
     (@who)=@_;
     my $who_len=@who;
     my $retVal;
     for($i=0; $i<$who_len; $i++)
          $retVal.=$who[$i];
     return $retVal;
sub evalInfix
     ($exp)=@_;
     return PostfixEval(joinArr(InfixToPostfix($exp)));
sub init
     my $def_exit="\n\tThank you for using this Program!\n\tFor more scripts visit http://www.qiksearch.com\n";
     printf "\n\tInfix - Postfix\n";
     printf "\n\tMenu";
     printf "\n\t(0) Convert an Infix Expression to Postfix Expression";
     printf "\n\t(1) Convert a Postfix Expression to Infix Expression";
     printf "\n\t(2) Evaluate a Postifx Expression";
     printf "\n\t(3) Evaluate an Infix Expression";
     printf "\n\t(4) Exit this Program";
     printf "\n\t(5) About this Program\n";
     printf "\n\tWhat do you want to do? (0/1/2/3/4/5) ";
     $get=<STDIN>;
     chomp $get;
     if(!(($get eq "0") || ($get eq "1") || ($get eq "2") || ($get eq "3") || ($get eq "4") || ($get eq "5")))
          printf "\n\t'$get' is an illegal character.\n\tYou must enter 0/1/2/3/4/5.";
     if(($get ne "4") && ($get ne "5") && (($get eq "0") || ($get eq "1") || ($get eq "2") || ($get eq "3")))
          printf "\n\tEnter String : ";
          $getStr=<STDIN>;
          chomp $getStr;
     if($get eq "0")
          printf "\tPostfix String : ";
          print InfixToPostfix($getStr);
     if($get eq "1")
          printf "\tInfix String : ";
          print PostfixToInfix($getStr);
     if($get eq "2")
          printf "\tPostfix Eval : ";
          print PostfixEval($getStr);
     if($get eq "3")
          printf "\tExpression Eval : ";
          print evalInfix($getStr);
     if($get eq "4")
          printf $def_exit;
          exit 0;
     if($get eq "5")
          printf "\n\t======================================================";
          printf "\n\t\tInfix-Postfix Script (written in Perl)";
          printf "\n\t\t(C) 2002 Premshree Pillai";
          printf "\n\t\tWeb : http://www.qiksearch.com";
          printf "\n\t======================================================\n";
          printf "\n\tUsing this program, you can : ";
          printf "\n\t- Convert an Infix Expression to Postfix Expression.";
          printf "\n\t Eg : 1+(2*3)^2 converts to 123*2^+";
          printf "\n\t- Convert a Postfix Expression to Infix Expression.";
          printf "\n\t Eg : 123*+ converts to 1+2*3";
          printf "\n\t- Evaluate a Postfix Expression";
          printf "\n\t Eg : 37+53-2^/ evaluates to 2.5";
          printf "\n\t- Evaluate an Infix Expression";
          printf "\n\t Eg : (5+(4*3-1))/4 evaluates to 4";
          printf "\n\n\tYou can find the algorithms used in this Program at : ";
          printf "\n\t-http://www.qiksearch.com/articles/cs/infix-postfix/index.htm";
          printf "\n\t-http://www.qiksearch.com/articles/cs/postfix-evaluation/index.htm";
          printf "\n\n\tYou can find a JavaScript implementation of 'Infix-Postfix' at : ";
          printf "\n\t-http://www.qiksearch.com/javascripts/infix-postfix.htm";
     printf "\n\n\tDo you want to continue? (y/n) ";
     my $cont=<STDIN>;
     chomp $cont;
     if($cont eq "y")
          init();
     else
          printf $def_exit;
          exit 0;
init;