[R] Using str() in a function.

[David Winsemius]

On Jul 13, 2011, at 7:31 PM, andrewH wrote:

> Thanks, David & Dennis, that is very helpful.
>
> Let me state what I think I have learned, to see if I understand it
> correctly. Then I have two remaining questions.
>
> If a function contains more than one expression in its {}, it always  
> returns
> the value of the last evaluated expression in its definition, and  
> only the
> last object -- unless you previously use the return() function on an  
> object
> before the last expression, in which case, the value of that  
> expression is
> returned instead. And in either case, explicit or implicit return(),  
> the
> returned expression is evaluated, and returned, first -- before any  
> other
> expressions are evaluated, and any side effects also occur before  
> any other
> expressions are evaluated. (Though I am unsure in what order  
> expressions are
> evaluated if  objects in the returned expression are defined by other
> expressions before it in the function.  The chain of evaluation --  
> and of
> any side effects of that evaluation -- propogates backwards, maybe?).

Not sure. There may be a console stack. I've never explored that  
question.

>
> The print() command inside a function sends the object it contains  
> to the
> currently-defined printer, as a side-effect, without returning it.

No. Read the first part of ?print.  `print`() _does_ return its  
argument,

 > b <- print("a")
[1] "a"
 > b
[1] "a"
.... but it returns it in the environment where it is called so it  
will "evaporate" after the function completes. The cat() function  
behaves as you describe. I don't have a good handle on the order of  
the side-effect and the return of objects.


>  The
> difference between return() and print() is that if something is  
> returned, R
> checks to see if the value of the function is assigned or otherwise  
> nested
> in a larger evaluated expression. Is so, a copy is moved to the  
> assigned
> object and the original is deleted. If not, it is printed to the  
> current
> device and then deleted. If you print() it, it does not check for  
> assignment
> or use before sending it to the printer and deleting it.
>
> A lot of functions, e.g. str(), have as their explicit or implict  
> return an
> expression which does not create an object. In this case, the function
> returns a NULL. If you do not want to print the NULL or other returned
> object, you make the returned argument invisible().
>
> But there are still things here I do not understand.  The function  
> that
> Dennis Murphy provided does print the str() output last instead of  
> first,
> because its final expression is invisible() rather than str(). But,  
> it still
> prints out (and returns - I checked) a NULL.  e.g.
>
> GG<-c(1:5)
> testXa <- function(X) {
>     print(summary(X))
>     print(str(X))
>     invisible()     # returns nothing
>  }
>> testXa(GG)
>   Min. 1st Qu.  Median    Mean 3rd Qu.    Max.
>      1       2       3       3       4       5
> int [1:5] 1 2 3 4 5
> NULL
>
> # Here is my latest version, of the function, which does exactly  
> what I
> want:
> testXf <- function(X) {
>     print("Summary:"); print(summary(X))
>     print("Structure:");  invisible(str(X))
>  }
> testXf(GG)
> [1] "Summary:"
>   Min. 1st Qu.  Median    Mean 3rd Qu.    Max.
>      1       2       3       3       4       5
> [1] "Structure:"
> int [1:5] 1 2 3 4 5
>
> So, two questions:
> 1. In Dennis's function, the str() results are printed last because  
> they are
> no longer returned, as invisible() is now the last expression. But  
> why does
> his function still print a visible NULL?

Because NULL was what was returned by str()

> 2. My function, above, makes the NULL value returned by str()  
> invisible. But
> invisible(str(X)) is the last expression evaluated, so why does the
> side-effect printing of str() results happen last instead of first?

That I do not know. I do know that there is console buffering and that  
some people need to flush.console() to get output before a function  
completes is computations.

-- 
David.


>
> and thanks again!
> andrewH
>
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>
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David Winsemius, MD
West Hartford, CT