print "I will calculate whatever\n",
      "you specify according to\n",
      "Ohm's law. What shall I\n",
      "calculate? Type either\n",
      "voltage, current, or\n",
      "resistance.\n";

my $i = 0;
....

##</code><code>##

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

my $line;

while( (print "parameter to calculate?:"), 
       ($line=<STDIN>), $line !~ /^\s*q(uit)?\s*$/i)
{    
    next if $line =~ /^\s*$/; #blank line => simple re-prompt
    
    if ($line =~ /^\s*(resistance|R)\s*$/i)
    {
       print "sub to calculate resistance\n";    
    }
    elsif ($line =~ /^\s*(voltage|V)\s*$/i)
    {
       print "sub to calculate voltage\n";
    }
    elsif ($line =~ /^\s*(current|I)\s*$/i)
    {
       print "sub to calculate current\n";
    }
    else
    {
       print "illegal entry!\n";
    }   
}

##</code><code>##

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

my @tests = ('50 ma', '10ma', '2.3', '-.2 ma', '3.', 
             '4.0', '10ba');

foreach my $input (@tests)
{
   my ($amps, $flag_ma)= $input=~ /^\s*([-+]?\d*\.?\d+)\s*(ma|mA)?\s*$/;
  
   $amps /=1000    if defined $flag_ma;
   
   if (defined $amps)  
   {
       print "$amps amps\n";
   }
   else
   {
       print "$input is an illegal entry\n";
   }
}

__END__

0.05 amps
0.01 amps
2.3 amps
-0.0002 amps
3. is an illegal entry
4.0 amps
10ba is an illegal entry

##</code><code>##

prompt>    i  =  2.3
prompt> t=3
illegal syntax try again!
prompt> r =      3
prompt> v?
v=i*r
 i=2.3
 r=3
v=6.9
prompt>v=9
prompt>i?
i=v/r
  r=3
  v=9
i=3
prompt>q
good bye!

##</code><code>##

#!/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";
    }    
}