R help - Apr 2014 - data.frame(1)*1:4 = 1?

Hello, All:


       What's the logic behind "data.frame(1)*1:4" producing a
scalar
1?  Or the following:


  data.frame(1:2, 3:4)*5:7
   X1.2 X3.4
1    5   21
2   12   20


       I stumbled over this, because I thought I was multiplying a 
scalar times a vector, and obtaining a scalar rather than the 
anticipated vector.  I learned that my "scalar" was in fact a
data.frame
with one row and one column.


       What am I missing?


       Thanks,
       Spencer

This is because your vector is recycled:

data.frame(1)*1:4 = data.frame(1)*c(1,2,3,4)

only the first element is needed since the data frame has nothing else 
to multiply with c(2,3,4)


(x<-data.frame(1:2, 3:4))
   X1.2 X3.4
1    1    3
2    2    4

(y<-x*5:7)

y[1,1] = x[1,1] * 5
y[2,1] = x[2,1] * 6
y[1,2] = x[1,2] * 7
y[2,2] = x[2,2] * 5

since you have e vector with length 3, for the 4th entry in the 
data.frame the first element in the vector is recycled.

hope this helps



On 03-04-2014 08:42, Spencer Graves wrote:
> Hello, All:
> 
> 
>       What's the logic behind "data.frame(1)*1:4" producing a
scalar
> 1?  Or the following:
> 
> 
>  data.frame(1:2, 3:4)*5:7
>   X1.2 X3.4
> 1    5   21
> 2   12   20
> 
> 
>       I stumbled over this, because I thought I was multiplying a
> scalar times a vector, and obtaining a scalar rather than the
> anticipated vector.  I learned that my "scalar" was in fact a
> data.frame with one row and one column.
> 
> 
>       What am I missing?
> 
> 
>       Thanks,
>       Spencer
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide 
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

Hi Spencer,

One piece is that a data frame of the same dimensions as went in comes out.
 The second piece is that the vector is recycled.

So in your first example:

data.frame(1) * 1:4

you only end up with the first element:

data.frame(1) * 1

If you try:

data.frame(1) * 4:1

you get a data frame with a value of 4.

Now for:

data.frame(1:2, 3:4) * 5:7

recycling kicks in again, and you get:

1 * 5, 2 * 6, 3 * 7, and 4 * 5

When working with vectors, you get recycling and it expands to the greater
length vector:

1:3 * 1:6

has length 6.  But data frames are sort of a 'higher' class and the
dimensions of the data frame trump the vector.

A slightly different behavior is observed for matrices:

matrix(1:6, ncol=2) * 1:3

Gives recycling as expected to the longer of the vectors, but

matrix(1:6, ncol=2) * 1:9

gives an error, but the error is _not_ directly in the multiplication, as
it were, but rather the results (which because matrices are stored as
vectors has expanded to be the length of the longer vector, here 1:9) do
not match the input dimensions of the matrix.  In particular, this is the
same as trying to do:

x <- 1:9
attributes(x)$dim <- c(3, 2)
Error in attributes(x)$dim <- c(3, 2) :
  dims [product 6] do not match the length of object [9]

basically, R gets the result of 1:6 * 1:9, but then cannot format it back
as a matrix, because the saved dimensions do not fit the new resulting
data.  You can verify that R does indeed to the calculations if you go
under the hood --- the multiplication is done, and then it tries to apply
the dims and it errors out.

Cheers,

Josh






On Wed, Apr 2, 2014 at 11:42 PM, Spencer Graves <
spencer.graves@structuremonitoring.com> wrote:
> Hello, All:
>
>
>       What's the logic behind "data.frame(1)*1:4" producing a
scalar 1?
>  Or the following:
>
>
>  data.frame(1:2, 3:4)*5:7
>   X1.2 X3.4
> 1    5   21
> 2   12   20
>
>
>       I stumbled over this, because I thought I was multiplying a scalar
> times a vector, and obtaining a scalar rather than the anticipated vector.
>  I learned that my "scalar" was in fact a data.frame with one row
and one
> column.
>
>
>       What am I missing?
>
>
>       Thanks,
>       Spencer
>
> ______________________________________________
> R-help@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/
> posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>


-- 
Joshua Wiley
Ph.D. Student, Health Psychology
University of California, Los Angeles
http://joshuawiley.com/
Senior Analyst - Elkhart Group Ltd.
http://elkhartgroup.com
260.673.5518

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