Checking whether a list contains a particular element

Checking whether a list contains a particular element

[Peter Dyballa]

Hello!

How can I check whether load-path contains a particular path (so that  
I could add it if necessary)?

--
Greetings

   Pete

Real Time, adj.:
Here and now, as opposed to fake time, which only occurs there and then.

Re: Checking whether a list contains a particular element

[Barry Margolin]

In article <mailman.4145.1153076026.9609.help-gnu-emacs@gnu.org>,
 Peter Dyballa <Peter_Dyballa@Web.DE> wrote:

> Hello!
> 
> How can I check whether load-path contains a particular path (so that  
> I could add it if necessary)?

The built-in 'add-to-list' function already performs this check, so you 
shouldn't need to do it yourself.  But if you do, you can use the 
function 'member'.

-- 
Barry Margolin, barmar@alum.mit.edu
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***
*** PLEASE don't copy me on replies, I'll read them in the group ***

Re: Checking whether a list contains a particular element

[B. T. Raven]

"Peter Dyballa" <Peter_Dyballa@Web.DE> wrote in message
news:mailman.4145.1153076026.9609.help-gnu-emacs@gnu.org...
> Hello!
>
> How can I check whether load-path contains a particular path (so that
> I could add it if necessary)?
>
> --
> Greetings
>
>    Pete

Since load-path variable is a list of strings, you could try:

(setq my-path "x:/my/subdir")
(setq found (car (member-ignore-case my-path load-path))
(if (not found) ;; do whatever )

Ed.

>
> Real Time, adj.:
> Here and now, as opposed to fake time, which only occurs there and then.
>
>
>
>

Re: Checking whether a list contains a particular element

[Tim X]

Peter Dyballa <Peter_Dyballa@Web.DE> writes:

> Hello!
>
> How can I check whether load-path contains a particular path (so that
> I could add it if necessary)?
>
> --
> Greetings
>

there is already a function to achieve what you want 

,----[ C-h f add-to-list RET ]
| add-to-list is a compiled Lisp function in `subr.el'.
| (add-to-list LIST-VAR ELEMENT &optional APPEND)
| 
| Add ELEMENT to the value of LIST-VAR if it isn't there yet.
| The test for presence of ELEMENT is done with `equal'.
| If ELEMENT is added, it is added at the beginning of the list,
| unless the optional argument APPEND is non-nil, in which case
| ELEMENT is added at the end.
| 
| The return value is the new value of LIST-VAR.
| 
| If you want to use `add-to-list' on a variable that is not defined
| until a certain package is loaded, you should put the call to `add-to-list'
| into a hook function that will be run only after loading the package.
| `eval-after-load' provides one way to do this.  In some cases
| other hooks, such as major mode hooks, can do the job.
`----


-- 
tcross (at) rapttech dot com dot au

Re: Checking whether a list contains a particular element

[Peter Dyballa]

Am 16.07.2006 um 21:22 schrieb Barry Margolin:

> The built-in 'add-to-list' function already performs this check

Ahh! From reading the function's name it's hard to guess that it does  
such a good work. And since the name is so clear I never would have  
checked the description ...

Thank you all for the answers! It's now just a waste of time if I add  
what's already there.

--
Greetings

              ~  O
   Pete       ~~_\\_/%
              ~  O  o

Re: Checking whether a list contains a particular element

[David Kastrup]

Peter Dyballa <Peter_Dyballa@Web.DE> writes:

> Am 16.07.2006 um 21:22 schrieb Barry Margolin:
>
>> The built-in 'add-to-list' function already performs this check
>
> Ahh! From reading the function's name it's hard to guess that it does
> such a good work. And since the name is so clear I never would have
> checked the description ...

But it would be pointless if it did the same thing as `push'.

-- 
David Kastrup, Kriemhildstr. 15, 44793 Bochum