Some Super Search digging in our code vaults gives:

• Simple calculator
• perl -ple'$_=eval' by Juerd (Re: •Re: Re: Simple command line calculator)

HTH

The problem with Juerd's is that it offers no error checking. This one does:

perl -ple'$_=eval||$@||0'
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For example:

$ perl -ple'$_=eval'
1/0
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vs.

$ perl -ple'$_=eval||$@||0'
1/0
Illegal division by zero at (eval 1) line 2, <> line 1.
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-sauoq
"My two cents aren't worth a dime.";

while (<STDIN>) {
    chomp;
    ($_ eq "quit") ? last : print eval "$_", "\n";
}
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This is what I have now what is wrong

 
#!/usr/bin/perl
print "What is your name?\n";
$name = <>;
chomp($name);
$greeting = "Hello, " . $name . "\nWhat do type of math do you want th
+is program to do?\n";
print $greeting;
chomp($operator = <STDIN>);
if ($operator eq 'sub') {
  $c = $a - $b;
} elsif ($operator eq '*') {
  # * code here
} else {
 $c = 'invalid operator';
}
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Edit, BazB: closed code tag.

This is what I have now what is wrong

For one thing I've never known a calculator to need to know my name. That's kind of "2001: A Space Odyssey"-esque. Especially since I'm Dave. I can just see it now if I attempt to divide by zero: "I'm sorry Dave, I can't do that." It also reminds me of the fascenating little computer exhibits at the Exploratorium in San Fransisco that I used to love to play with back in 1978 when I was 10.

Beyond that, the point that people are making with regards to using eval versus a logic tree, is that perl already knows how to do math. And Perl is an interpreted language, so dynamic code is perfectly alright. eval can be used to execute dynamic code. So by using eval, the Perl interpreter (perl) becomes the math engine for you. You don't have to go to all the trouble of teaching your Perl script how to add and subtract, and parse mathematical expressions, if perl already knows how to do that for you. The Perlish solution therefore, is to let perl parse the expression. (And this reminds me again of the common phrase, "Only perl can parse Perl." ...but in this case it can be rewritten as "Let perl parse it for you as dynamic Perl."

eval does have its pitfalls. Because it would be perfectly happy to do whatever you ask of it as long as it is legal Perl. A mathematical expression is legal Perl. But so is something like " `rm -r /`". So you don't want to just give any old user access to a free eval from a CGI script or something. :)



Dave

"If I had my life to do over again, I'd be a plumber." -- Albert Einstein

You just need to read in the operator (- *) just like you read in the Name. Then you can use if.

if ($operator eq '-') {
  $c = $a - $b;
} elsif ($operator eq '*') {
  # * code here
} else {
 $c = 'invalid operator';
}
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If those few operations are all you need, there's nothing wrong with the if/else dispatching above. However, if you ever want to extend it to other mathmatical operations, I'd use a dispatch table:

my %OPERATIONS = (
    '*' => sub { $_[0] * $_[1] }, 
    '-' => sub { $_[0] - $_[1] }, 
    '+' => sub { $_[0] + $_[1] }, 
);
# $operator, $first, and $second defined 
# elsewhere, from user input
print "Result: ", 
    $OPERATIONS{$operator}->($first, $second), 
    "\n"
    if exists $OPERATIONS{$operator};
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Also, avoid using $a and $b as variable names, as they have special meaning in sort subroutines.

add this:

print $greeting;
chomp($operator = <STDIN>);
#your other prints and stuff here
$c = $a - $b if $operator eq "-";
$c = $a * $b if $operator eq "*";
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The if statement could also be written as such:

if ($operator eq "-")
{
  $c = $a - $b;
} else {
  $c = $a * $b;
}
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use strict;
print "What operation?";
chomp(my $op=<>);
if($op eq '-')
{
  print $a-b;
}
elsif($op eq '*')
{
  print $a*b;
}
#etc
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