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Re: checking for nil argument in interactive
From: |
Tassilo Horn |
Subject: |
Re: checking for nil argument in interactive |
Date: |
Sun, 09 Nov 2008 22:20:18 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/23.0.60 (gnu/linux) |
Seb <spluque@gmail.com> writes:
Hi Seb,
> I don't understand why the following arg symbol doesn't get set to
> "string":
>
>
> (defun funny-function (arg)
> (interactive "sarg: ")
> (if (not arg) (setq arg "string"))
> (message "arg: %s" arg))
>
> To test:
>
> M-x funny-function RET RET
Because in that case arg is the empty string.
> I've also tried (eq "" arg) for the if boolean but get the same
> result.
Because (eq "foo" "foo") ==> nil in the general case.
See the manual for details.
,----[ (info "(elisp)Equality Predicates") ]
| -- Function: eq object1 object2
| This function returns `t' if OBJECT1 and OBJECT2 are the same
| object, `nil' otherwise.
|
| `eq' returns `t' if OBJECT1 and OBJECT2 are integers with the same
| value. Also, since symbol names are normally unique, if the
| arguments are symbols with the same name, they are `eq'. For
| other types (e.g., lists, vectors, strings), two arguments with
| the same contents or elements are not necessarily `eq' to each
| other: they are `eq' only if they are the same object, meaning
| that a change in the contents of one will be reflected by the same
| change in the contents of the other.
|
| (eq 'foo 'foo)
| => t
|
| (eq 456 456)
| => t
|
| (eq "asdf" "asdf")
| => nil
`----
So to compare stings use `string='.
Hope that helps,
Tassilo
--
The glass is neither half-full nor half-empty: it's twice as big as it
needs to be.