Page 61: the example with the member function. This statement in the book is actually not true: "...the member function still gives the correct answer, even when we search for nil as the member!"

[Shafique]

This seems to be true for the example given, but not universally true. Try the following:

(if (member nil '(3 4 1 nil)))

    'nil-is-in-the-list

    'nil-is-not-in-the-list)

The member function will return nil even though nil is in the list (I tested this).

Any thoughts?

 

[Archie Maskill]

On Fri, Aug 24, 2012 at 2:31 PM, Shafique <shafiqu...@gmail.com> wrote:
> (if (member nil '(3 4 1 nil)))
> 'nil-is-in-the-list
> 'nil-is-not-in-the-list)
>
> The member function will return nil even though nil is in the list (I tested
> this).

Are you sure that member is returning nil?
For me, in the example you gave, member returns '(nil), which is
different from nil.

Archie

[Shafique]

Hello Archie,

Many thanks for responding. I don't know what I'm doing wrong. When I entered that line, the output was the second of the options, and it prints FALSE (so I guess it is returning FALSE instead of '(nil) ).

I have attached a screenshot (see attached)... am I missing something simple?

[Archie Maskill]

Hi Shafique,

In your screenshot, towards the top, you have made a typo. You wrote
"nul" instead of "nil":

(if (member nil '(3 4 1 nul))
'nil-is-in-the-list
'nil-is-not-in-the-list)

Therefore, nil is not found in the list, and the second of the options
is printed.

In your second example:

(if (member nil '(3 4 1 nil))

'false
'true)

nil is found in '(3 4 1 nil), so member returns the tail, which is
'(nil). This evaluates to true (because it is not nil), and therefore
the first option is returned, which you've specified to be the symbol
'false (which might be what is confusing you!)

Archie

[Shafique]

Aha- got it now! Many thanks again, Archie, for taking the time to look through the screenshot and catching the typo. I very much appreciate it!