[Rd] surprised matrix (1:256, 8, 8) doesn't cause error/warning

[Wolfgang Huber]

FWIW, I paste below a possible change to the warnings generating part of the do_matrix function in R/src/main/array.c that adds the kind of warning that Abby is asking for, and that IMHO would more often help users find bugs in their code than interfere with intended behaviour.

> matrix (1:6, nrow = 2, ncol = 3)

> matrix (1:12, nrow = 2, ncol = 3)
    [,1] [,2] [,3]
[1,]    1    3    5
[2,]    2    4    6
Warning message:
In matrix(1:12, nrow = 2, ncol = 3) :
 data length incompatible with size of matrix

> matrix (1:7, nrow = 2, ncol = 3)
Warning messages:
1: In matrix(1:7, nrow = 2, ncol = 3) :
 data length [7] is not a sub-multiple or multiple of the number of rows [2]
2: In matrix(1:7, nrow = 2, ncol = 3) :
 data length incompatible with size of matrix

> matrix (1:8, nrow = 2, ncol = 3)
Warning messages:
1: In matrix(1:8, nrow = 2, ncol = 3) :
 data length [8] is not a sub-multiple or multiple of the number of columns [3]
2: In matrix(1:8, nrow = 2, ncol = 3) :
 data length incompatible with size of matrix

> matrix (1:6, nrow = 0, ncol = 0)
<0 x 0 matrix>
> matrix (numeric(0), nrow = 2, ncol = 3)
    [,1] [,2] [,3]
[1,]   NA   NA   NA
[2,]   NA   NA   NA

> matrix(1:2, ncol = 8)
    [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8]
[1,]    1    2    1    2    1    2    1    2

It would be nice to combine the new warning with that about “...not a sub-multiple or multiple…” into a single warning, if appropriate (as in two of the examples above), but that would require bigger surgery way above my payscale.

Kind regards
	Wolfgang Huber


Index: array.c
===================================================================
--- array.c	(revision 79951)
+++ array.c	(working copy)
@@ -133,18 +133,19 @@
	    nc = (int) ceil((double) lendat / (double) nr);
    }

-    if(lendat > 0) {
+    if (lendat > 1) {
	R_xlen_t nrc = (R_xlen_t) nr * nc;
-	if (lendat > 1 && nrc % lendat != 0) {
+	if ((nrc % lendat) != 0) {
	    if (((lendat > nr) && (lendat / nr) * nr != lendat) ||
		((lendat < nr) && (nr / lendat) * lendat != nr))
		warning(_("data length [%d] is not a sub-multiple or multiple of the number of rows [%d]"), lendat, nr);
	    else if (((lendat > nc) && (lendat / nc) * nc != lendat) ||
		     ((lendat < nc) && (nc / lendat) * lendat != nc))
-		warning(_("data length [%d] is not a sub-multiple or multiple of the number of columns [%d]"), lendat, nc);
-	}
-	else if ((lendat > 1) && (nrc == 0)){
+		    warning(_("data length [%d] is not a sub-multiple or multiple of the number of columns [%d]"), lendat, nc);
+	if (nrc == 0)
	    warning(_("data length exceeds size of matrix"));
+        if (nrc != lendat)
+            warning(_("data length incompatible with size of matrix"));
	}
    }


------
// And here, for easy checking that part of the code in the new form:
 if (lendat > 1) {
	R_xlen_t nrc = (R_xlen_t) nr * nc;
	if ((nrc % lendat) != 0) {
	    if (((lendat > nr) && (lendat / nr) * nr != lendat) ||
		((lendat < nr) && (nr / lendat) * lendat != nr))
		warning(_("data length [%d] is not a sub-multiple or multiple of the number of rows [%d]"), lendat, nr);
	    else if (((lendat > nc) && (lendat / nc) * nc != lendat) ||
		     ((lendat < nc) && (nc / lendat) * lendat != nc))
		    warning(_("data length [%d] is not a sub-multiple or multiple of the number of columns [%d]"), lendat, nc);
	if (nrc == 0)
	    warning(_("data length exceeds size of matrix"));
       if (nrc != lendat)  
           warning(_("data length incompatible with size of matrix"));
	}
   }

> Il giorno 2feb2021, alle ore 00:27, Abby Spurdle (/əˈbi/) <spurdle.a using gmail.com> ha scritto:
> 
> So, does that mean that a clean result is contingent on the length of
> the data being a multiple of both the number of rows and columns?
> 
> However, this rule is not straightforward.
> 
>> #EXAMPLE 1
>> #what I would expect
>> matrix (1:12, 0, 0)
> <0 x 0 matrix>
> Warning message:
> In matrix(1:12, 0, 0) : data length exceeds size of matrix
> 
>> #EXAMPLE 2
>> #don't like this
>> matrix (numeric (), 2, 3)
>    [,1] [,2] [,3]
> [1,]   NA   NA   NA
> [2,]   NA   NA   NA
> 
> The first example is what I would expect, but is inconsistent with the
> previous examples.
> (Because zero is a valid multiple of twelve).
> 
> I dislike the second example with recycling of a zero-length vector.
> This *is* covered in the help file, but also seems inconsistent with
> the previous examples.
> (Because two and three are not valid multiples of zero).
> 
> Also, I can't think of any reason why someone would want to construct
> a matrix with extra data, and then discard part of it.
> And even if there was, then why not allow an arbitrarily longer length?
> 
> 
> On Mon, Feb 1, 2021 at 10:08 PM Martin Maechler
> <maechler using stat.math.ethz.ch> wrote:
>> 
>>>>>>> Abby Spurdle (/əˈbi/)
>>>>>>>   on Mon, 1 Feb 2021 19:50:32 +1300 writes:
>> 
>>> I'm a little surprised that the following doesn't trigger an error or a warning.
>>> matrix (1:256, 8, 8)
>> 
>>> The help file says that the main argument is recycled, if it's too short.
>>> But doesn't say what happens if it's too long.
>> 
>> It's somewhat subtler than one may assume :
>> 
>>> matrix(1:9, 2,3)
>>    [,1] [,2] [,3]
>> [1,]    1    3    5
>> [2,]    2    4    6
>> Warning message:
>> In matrix(1:9, 2, 3) :
>> data length [9] is not a sub-multiple or multiple of the number of rows [2]
>> 
>>> matrix(1:8, 2,3)
>>    [,1] [,2] [,3]
>> [1,]    1    3    5
>> [2,]    2    4    6
>> Warning message:
>> In matrix(1:8, 2, 3) :
>> data length [8] is not a sub-multiple or multiple of the number of columns [3]
>> 
>>> matrix(1:12, 2,3)
>>    [,1] [,2] [,3]
>> [1,]    1    3    5
>> [2,]    2    4    6
>>> 
>> 
>> So it looks to me the current behavior is quite on purpose.
>> Are you sure it's not documented at all when reading the docs
>> carefully?  (I did *not*, just now).
> 
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