#!/usr/bin/perl -w

use strict;

#
# Declaring subroutine prototypes here allows us to define the
# subroutines at the end of the file.
#
sub parse_line( $ );
sub calculate( $$$ );

while(<>)
{
    chomp;

    #
    # Stop if the user typed 'exit'.
    #
    last if /^exit$/;

    #
    # Call subroutines parse_line() and calculate() to do the actual work
    #
    my($op, $n1, $n2) = parse_line($_);
    my $result = calculate($op, $n1, $n2);

    #
    # Print the results.
    #
    print "$n1 $op $n2 = $result\n";
}

#
# Extract numbers and operator from input string
#
sub parse_line( $ )
{
    local($_) = @_;             # so we don't have to use =~

    #
    # use m%% so we can use / inside the expression
    #
    if (m%^\s*(\d+)\s*([-+*/])\s*(\d+)\s*$%)
    {
        return ($2, $1, $3);    # operator, then the two numbers
    }
    else
    {
        die "Unable to parse: \"$_\"\n";
    }
}

#
# Compute the value
#
sub calculate( $$$ )
{
    my($op, $n1, $n2) = @_;
    if ($op eq '+')
    {
        return $n1 + $n2;
    }
    elsif ($op eq '-')
    {
        return $n1 - $n2;
    }
    elsif ($op eq '*')
    {
        return $n1 * $n2;
    }
    elsif ($op eq '/')
    {
        return $n1 / $n2;
    }
    else
    {
        die "Unrecognized operator: \"$op\"\n";
    }
}