Designing interface of a simple elisp function

Designing interface of a simple elisp function

[gnuist006]

I want to write a function, say converting a binary to decimal in lisp.
For arbitrary length binary, I want the input as a string, which is what
we do in C. Then I want to get "car" of the string and go from there.
This is like getchar or getc in C.
Now this "car" does not apply to a string but to a list. On the other
hand the arbitary length input applies to string. Does there exist
string to list function? But even that also seems cheating. What is the
most elegant way to write such a function so that it is also readible
in use.

(b2d '(1 0 1 0 0 1)) is hardly readible or desirable.
(b2d '101001)   is most desirable
(b2d "101001")  is tolerable if this is the best that can be done

Re: Designing interface of a simple elisp function

[Henrik Motakef]

gnuist006@hotmail.com (gnuist006) writes:

> Now this "car" does not apply to a string but to a list. On the other
> hand the arbitary length input applies to string. Does there exist
> string to list function? But even that also seems cheating. What is the
> most elegant way to write such a function so that it is also readible
> in use.

The map-functions work on sequences, not only lists. So, for example

  (mapcar 'identity "foo")

returns a list of three characters. An alternative would be

  (split-string "foo" "")

which gives a list of one-character strings.

> (b2d '101001)   is most desirable
> (b2d "101001")  is tolerable if this is the best that can be done

You can convert a symbol to a name with the symbol-name function,
i.e. (symbol-name 'foo) => "foo". Unfortunatly, this will not work
with '101001, because it really is the same as 101001 - numbers are
self-quoting, so for example (eq '0001 1) is t, as is (numberp '1).
You could use '\101001 however, or just accept a number instead of a
symbol or string.

Regards
Henrik

Re: Designing interface of a simple elisp function

[Mario Lang]

gnuist006@hotmail.com (gnuist006) writes:

> I want to write a function, say converting a binary to decimal in lisp.
> For arbitrary length binary, I want the input as a string, which is what
> we do in C. Then I want to get "car" of the string and go from there.
> This is like getchar or getc in C.
> Now this "car" does not apply to a string but to a list. On the other
> hand the arbitary length input applies to string. Does there exist
> string to list function? But even that also seems cheating. What is the
> most elegant way to write such a function so that it is also readible
> in use.
You are looking for aref:

(aref "abc" 1) => 98

C-h f aref RET for more info.

-- 
CYa,
  Mario

Re: Designing interface of a simple elisp function

[Edward O'Connor]

On 12 Oct 2002, Henrik Motakef wrote:

> The map-functions work on sequences, not only lists. So, for
> example
> 
>   (mapcar 'identity "foo")
> 
> returns a list of three characters. An alternative would be
> 
>   (split-string "foo" "")
> 
> which gives a list of one-character strings.

You want to be careful with this latter option if you want to
write portable Elisp programs. Some XEmacsen will infloop on that.

-- 
Edward O'Connor
oconnor@soe.ucsd.edu