On 20/07/2014, 5:46 PM, David Winsemius wrote:
>
> On Jul 20, 2014, at 6:30 AM, Duncan Murdoch wrote:
>
>> On 17/07/2014, 10:00 PM, Dario Strbenac wrote:
>>> The example in the question was not inside a user function.
>>
>> The explanations you were given were slightly inaccurate. The usual
>> rule is that results returned at the top level are printed unless they
>> are marked as invisible. (There are a few cases where "top level" is
>> faked, e.g. in example code, and in Sweave.)
>>
>> When you do something like
>>
>> if(test) { a; b; c }
>>
>> you have an expression that returns NULL invisibly if the test is FALSE,
>> and returns the value of the block (i.e. c) visibly if it is TRUE. It
>> is the value of the if that is printed.
>>
>> There is no difference in the handling of a, b and c: each is an
>> expression that returns the value of the corresponding variable without
>> marking it as invisible. But none of them are top-level expressions, so
>> none of them print.
>
> I'm not sure what that last one was intended to mean but it seemed to imply
> that nothing would be printed if those expressions had values (even if the
> interpreter were able to find values in one of hte enclosing environments).
> That would not be what I expected. I think of curved-braces as a function and
> the results of the last evaluation would be expected to be returned:
I don't understand your misunderstanding. The expression "{ a; b; c }"
(ignore the quotes here and later) is found by evaluating a, then
evaluating b, then evaluating c, and the value of c is returned as the
value of the whole expression. Braces don't affect visibility, so if c
is 4 and no error occurs, the value of "{ a; b; c }" is a visible 4.
>> test <- TRUE
>> a=2;b=3;c=4
>> if(test){a;b;c}
> [1] 4
In this case, the expression isn't "{ a; b; c }", it's "if (test) { a;
b; c }". Since test is TRUE, that returns the value of "{ a; b; c }"
visibly, i.e. the value is 4.
If test had been FALSE, the value would be NULL, marked as invisible.
Braces don't affect visibility, but parens do, so
if (FALSE) 4
and
{if (FALSE) 4}
both print nothing, but
(if (FALSE) 4)
will print NULL. All three versions have the value NULL, as you could
see if you assigned them to a variable, e.g.
x <- if (FALSE) 4
x
x <- {if (FALSE) 4}
x
x <- (if (FALSE) 4)
x
which will print NULL three times.
>
> If one of them had no value, an error would be thrown.
>
>> rm(b)
>> if(test){a;b;c}
> Error: object 'b' not found
I don't see what this has to do with the previous discussion. If you
thought I was talking about errors in expressions, you misunderstood me.
Duncan Murdoch
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