[q-lang-users] a little question

[Rob H. <hub...@gm...>]

Hello Fabian,

As you seem to have noticed, the rule
     add [X|Xs] = X+add Xs;
does not work for the list []. This is because [] does not match the
pattern [X|Xs]. The pattern [X|Xs] is only matched by non-empty lists.
Q has not been told what to do with "add []".

Your modification fails to solve the problem for a similar reason.

If you tell Q what to do with the following rule
    add [] = 0;
then all will be well.

Alternatively, if you're not happy with defining add on the empty
list, you could do this:
    add [X] = X;
    add [X|Xs] = X+add Xs;

Rob.

On 25/08/06, Fabian Boucsein <kan...@we...> wrote:
> hello,
>
> from the q manual i tried the following example:
>
> add [X|Xs] = X+add Xs;
>
> then i tried this to test the function:
>
> ==> add [1, 2, 3, 4]
> 1+(2+(3+(4+add [])))
>
> what i want to know is, how can i change this function to get
> back a result? i tried the following:
>
> add [X|Xs] = X+add Xs if Xs <> [];
>
> ==> add [1, 2, 3, 4]
> 1+(2+(3+add [4]))
>
> which is still not the result i want. by the way i am using q 7.3
> maybe there is someone who can help me.
>
> greetings,
>
> fabian