# Meaning of '!(...' EG: "myObject.parse!(argv)" ..?

**URL:** <https://www.ruby-forum.com/t/meaning-of-eg-myobject-parse-argv/179073>\
**Category:** Ruby\
**Created:** [November 17, 2009, 7:15am UTC](https://www.ruby-forum.com/t/meaning-of-eg-myobject-parse-argv/179073 "2009-11-17T07:15:25Z")\
**Posts on this page:** 9\
**Page:** 1

<div class="post-metadata">

**Author:** ![michelle1](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a87d85/32.png) [@michelle1](https://www.ruby-forum.com/u/michelle1)\
**Post date:** [November 17, 2009, 7:15am UTC](https://www.ruby-forum.com/t/meaning-of-eg-myobject-parse-argv/179073/1 "2009-11-17T07:15:25Z")

</div>

Hi there,

QUESTION:  
Whilst “parse(argv)” is obvious, what does “opts.parse!(argv)” do in the  
below code? And what is the word use to describe doing this?

## CONTEXT: I am looking at a class called ‘Options’ whose job is to parse command line options which get passed to a program called ‘PartInspector’. An extract of this class follows:

class Options

def initialize(argv)  
@directory = DEFAULT\_DIR  
parse(argv)  
end

private  
def parse(argv)  
OptionParser.new do |opts|  
opts.banner = “Usage: PartInspector [part\_number]”

```
  opts.on("-h", "Shows this message") do
    puts opts
    exit
  end

  begin
    argv = ["-h"] if argv.empty?
    opts.parse!(argv) ## <------- HERE?!!?!
    rescue OptionParser::ParseError => e
      STDERR.puts e.message, "\n", opts
  end
end#def parse()

```

end#private class

## end#option class

I know its probably some basic syntax for ruby, not knowing what it is  
called however makes it difficult to look it up.

Thank-you in advance, Michelle

---

<div class="post-metadata">

**Author:** ![michelle1](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a87d85/32.png) [@michelle1](https://www.ruby-forum.com/u/michelle1)\
**Post date:** [November 17, 2009, 7:59am UTC](https://www.ruby-forum.com/t/meaning-of-eg-myobject-parse-argv/179073/2 "2009-11-17T07:59:55Z")

</div>

On Nov 16, 2009, at 22:15 , Michelle P. wrote:

> QUESTION:  
> Whilst “parse(argv)” is obvious, what does “opts.parse!(argv)” do in the  
> below code? And what is the word use to describe doing this?

it is just an argument list. nothing fancy. “!” is valid as part of the  
method name, that’s all.

---

<div class="post-metadata">

**Author:** ![michelle1](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a87d85/32.png) [@michelle1](https://www.ruby-forum.com/u/michelle1)\
**Post date:** [November 17, 2009, 8:04am UTC](https://www.ruby-forum.com/t/meaning-of-eg-myobject-parse-argv/179073/3 "2009-11-17T08:04:47Z")

</div>

Ryan D. wrote:

> it is just an argument list. nothing fancy. “!” is valid as part of the  
> method name, that’s all.

Thanks for your reply Ryan… I’m surprised! So… are you saying that  
the method calls

```
     opts.parse!(argv) and opts.parse(argv)

```

effectively do the same thing?

---

<div class="post-metadata">

**Author:** ![michelle1](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a87d85/32.png) [@michelle1](https://www.ruby-forum.com/u/michelle1)\
**Post date:** [November 17, 2009, 8:17am UTC](https://www.ruby-forum.com/t/meaning-of-eg-myobject-parse-argv/179073/4 "2009-11-17T08:17:25Z")

</div>

Michelle P. wrote:

> Thanks for your reply Ryan… I’m surprised! So… are you saying that  
> the method calls
> 
> ```
> opts.parse!(argv) and opts.parse(argv)
> 
> ```
> 
> effectively do the same thing?

Section 6.2 of “The Ruby P.ming Language” by Flanagan & Matsumoto,  
explains that a method that ends with an exclamation mark is a naming  
convention indicating to the programmer that the method “should be used  
with caution.”

It goes on to explain that most of the time this is due to the fact that  
the method is a mutator; a method that alters the internal state of the  
object.

---

<div class="post-metadata">

**Author:** ![michelle1](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a87d85/32.png) [@michelle1](https://www.ruby-forum.com/u/michelle1)\
**Post date:** [November 17, 2009, 9:14am UTC](https://www.ruby-forum.com/t/meaning-of-eg-myobject-parse-argv/179073/5 "2009-11-17T09:14:50Z")

</div>

On Nov 16, 2009, at 23:04 , Michelle P. wrote:

> Ryan D. wrote:
> 
> > it is just an argument list. nothing fancy. “!” is valid as part of the  
> > method name, that’s all.
> 
> Thanks for your reply Ryan… I’m surprised! So… are you saying that  
> the method calls
> 
> ```
> opts.parse!(argv) and opts.parse(argv)
> 
> ```
> 
> effectively do the same thing?

Are they both plain happy method calls? Yes. If by “do the same thing”  
you mean call the same method, then no.

They are both equivalent to:

opts.send(:parse!, argv)

and

opts.send(:parse, argv)

respectively.

---

<div class="post-metadata">

**Author:** ![michelle1](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a87d85/32.png) [@michelle1](https://www.ruby-forum.com/u/michelle1)\
**Post date:** [November 17, 2009, 10:39am UTC](https://www.ruby-forum.com/t/meaning-of-eg-myobject-parse-argv/179073/6 "2009-11-17T10:39:43Z")

</div>

> ‘parsew’ and ‘parse’. But if thats true… then in my orginal code  
> above, there wasn’t a ‘parse!’ method defined.
> 
> –  
> Posted via [http://www.ruby-forum.com/](http://www.ruby-forum.com/).

I think you’ve got it. Basically parse!, and I mean all of it including  
the exclamation mark, is a method defined separately from parse. In your  
options class, they might look like this:

class Options  
def parse!(argv)

# parsing code

end  
def parse(argv)  
opts = self.dup  
opts.parse!(argv)  
end  
end

The example implementation is just a common idiom in ruby where a method  
ending in a “bang”, or exclamation point is a destructive method,  
altering the state of the object, whereas the non-bang version of the  
method acts on a duplicate object, so as to keep the actual object  
unaltered. There is nothing special about the bang in how it is  
processed in ruby, it’s just taken as part of the method name. So yes,  
you can just replace it with “w” or something. The reason for the bang  
is just ruby convention. If you wanted, you could reverse the  
definitions, and it would work fine, but just be confusing (don’t do  
that, go by convention).

Hope that’s clear.

- Ehsan

---

<div class="post-metadata">

**Author:** ![michelle1](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a87d85/32.png) [@michelle1](https://www.ruby-forum.com/u/michelle1)\
**Post date:** [November 17, 2009, 12:11pm UTC](https://www.ruby-forum.com/t/meaning-of-eg-myobject-parse-argv/179073/7 "2009-11-17T12:11:39Z")

</div>

Ja, great! Thanks Ehsan - I’ve got it now 🙂

Michelle

---

<div class="post-metadata">

**Author:** ![michelle1](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a87d85/32.png) [@michelle1](https://www.ruby-forum.com/u/michelle1)\
**Post date:** [November 17, 2009, 9:47am UTC](https://www.ruby-forum.com/t/meaning-of-eg-myobject-parse-argv/179073/8 "2009-11-17T09:47:59Z")

</div>

Sorry Ryan… I’m still not getting it ☹

What is that exclamation mark doing? Could I substitute lets say a ‘w’  
for the exclamation mark?

opts.send(:parse!, argv) =\> opts.send(:parsew, argv)  
opts.send(:parse, argv) =\> opts.send(:parse, argv)

If I can, then I understand it. There are in fact two different methods  
‘parsew’ and ‘parse’. But if thats true… then in my orginal code  
above, there wasn’t a ‘parse!’ method defined.

---

<div class="post-metadata">

**Author:** ![michelle1](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/a87d85/32.png) [@michelle1](https://www.ruby-forum.com/u/michelle1)\
**Post date:** [November 17, 2009, 9:07pm UTC](https://www.ruby-forum.com/t/meaning-of-eg-myobject-parse-argv/179073/9 "2009-11-17T21:07:09Z")

</div>

On 17.11.2009 09:48, Michelle P. wrote:

> Sorry Ryan… I’m still not getting it ☹
> 
> What is that exclamation mark doing? Could I substitute lets say a ‘w’  
> for the exclamation mark?

Yes. The exclamation mark is just more visible than the “w”. In the  
particular case of OptionParser the difference between #parse and  
#parse! is that the latter actually modifies the argument (typically  
ARGV), which btw. is a nice thing because what it leaves behind in ARGV  
are only file names and so using ARGF works well together with  
OptionParser. I typically do

OptionParser.new do |opts|  
opts.on …  
end.parse! ARGV

Then, depending on what the program does

ARGF.each do |line|  
puts “read a line: #{line}”  
end

Kind regards

robert