[R] package expm: matrix powers within a product of matrices: operation precedence

[William Dunlap]

All operators of the form %<something>% have the same precedence,
that of %*%.    R does not look at the <something> between the percent
signs to determine the precedence.  Hence you must use parentheses
to get the order of operations that you want.

(I think that %<something>% operators are overused - providing the same
functionality in a standard functional form is often more convenient.  It
certainly would eliminate your problem.)

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com


> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf
> Of Rainer Sachs
> Sent: Monday, June 17, 2013 1:20 PM
> To: r-help at r-project.org
> Subject: [R] package expm: matrix powers within a product of matrices: operation
> precedence
> 
> 
> package expm: matrix powers within a product of matrices: operation
> precedence
> 
>     I want to double check that for M and N kxk mtrices and v a k-vector
>     M%^%2%*%N%*%v is not automatically read as (M%^%2)%*%N%*%v or
>     M%*%N%^%2%*%v is not automatically read as M%*%(N%^%2)%*%v or
>     both
> 
>     I had been assuming the usual precedence (powers first, then
> multiplication) applies for matrix powers and multiplication in expm. I was
> getting unexpected results. In desperation I tried putting in the
> parentheses and seem to be getting much more reasonable answers. But I am so
> surprised that the matrix power operation is, apparently, not automatically
> given precedence over the matrix multiplication operation that I do not
> trust that adding parentheses has cured my bug. Maybe I am still making some
> other mistake instead. Can anyone confirm that in fact the parentheses are
> (sometimes?) essential? I could not find any documentation, one way or the
> other, just definitions of the power operator and of matrix exponentials.
> 
>     TIA Rainer K. Sachs, Prof emertitus of math
> 
> 
> 
> 
> 
> --
> View this message in context: http://r.789695.n4.nabble.com/package-expm-matrix-
> powers-within-a-product-of-matrices-operation-precedence-tp4669733.html
> Sent from the R help mailing list archive at Nabble.com.
> 
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