#!/usr/bin/perl -w
use strict;

my %ohmsLaw;  #i,v,r

print "This is an Ohm's law program (V=I*R).\n",
      "enter statements like: \n",
      "  Enter I,V,R equation> i=2.3 or 3.2 mA or 3.2 ma\n",
      "  Enter I,V,R equation> R=4\n",
      "  Enter I,V,R equation> v=?   **asks for v to be printed**\n",
      "  prints: v=9.2   ** 9.2= (2.3 x 4) **\n",
      "basically I calculate the value that you ask for\n",
      "  based upon the previous entries for the other two\n",
      "  variables\n",
      "enter q|Q|quit to quit program\n";
      
my $line;

while( (print "Enter I,V,R equation>"), 
       ($line=<STDIN>), $line !~ /^\s*q(uit)?\s*$/i)
{    
    next if $line =~ /^\s*$/;  # simple re-prompt on blank line
    
    #
    # set an I,V,R variable...
    # 
    if (my ($vir_letter, $number, $flag_ma) = 
         $line =~ /^\s*([ivrIVR])\s*\=\s*([-+]?\d*\.?\d+)\s*(ma|mA)?\s*$/)
    {
       if ($vir_letter =~ /I/i)
       {
          $ohmsLaw{i}  = $number;
          $ohmsLaw{i} /= 1000 if defined $flag_ma;
       }
       $ohmsLaw{v}=$number if $vir_letter =~ /V/i;
       $ohmsLaw{r}=$number if $vir_letter =~ /R/i;  
    } 
    #
    # output a calculated I,V,R value
    #
    elsif (my ($printValue) = $line =~ /\s*(i|v|r)\s*\=\s*\?\s*$/i)
    {
       foreach my $ltr (grep{$_ ne $printValue} keys %ohmsLaw)
       {
          print "  $ltr=$ohmsLaw{$ltr}\n"; #list other values!
       }
       
       print "v=",$ohmsLaw{v}=$ohmsLaw{i}*$ohmsLaw{r},"\n" 
                  if $printValue =~ /V/i;
       print "i=",$ohmsLaw{i}=$ohmsLaw{v}/$ohmsLaw{r},"\n" 
                  if $printValue =~ /I/i;
       print "r=",$ohmsLaw{r}=$ohmsLaw{v}/$ohmsLaw{i},"\n" 
                  if $printValue =~ /R/i;
    }
    else
    {
       print "illegal syntax!\n";
    }    
}