[R] Create a new variable and concatenation inside a "for" loop

Jeff Newmiller
Wed, 27 Apr 2016 19:57:43 -0000

"c" an extremely commonly-used function. Functions are first-class
objects that occupy the same namespaces that variables do, so they can obscure
each other. In short, don't use variables called "c" (R is case
sensitive, so "C" has no such problem).

Wherever possible, avoid incremental concatenation like the plague. If you feel
you must use it, at least concatenate in lists and then use functions like
unlist, do.call, or pre-allocate vectors or matrix-like objects with unuseful
values like NA and then overwrite each element in the vector or matrix-type
object in a loop like your first one.
--
Sent from my phone. Please excuse my brevity.

On April 27, 2016 3:25:14 PM GMT+01:00, "Gordon, Fabiana"
<fabiana.gordon at imperial.ac.uk> wrote:
>Hello,
>
>Suppose the you need a loop to create a new variable , i.e., you are
>not reading data from outside the loop. This is a simple example in
>Matlab code,
>
>for i=1:5
>r1=randn
>r2=randn
>r=[r1 r2]
>c(i,:)=r;   % creation of each row of c , % the ":" symbol
indicates
>all columns. In R this would be [i,]
>end
>
>The output of interest is c which I'm creating inside the
"for" loop
>-also the index used in the loop is used to create c. In R I had to
>create c as an  empty vector (numeric() ) outside the loop, otherwise I
>get an error message saying that c doesn't exit.
>
>The other issue is the concatenation. In each iteration I'm creating
>the rows of c by placing the new row  (r) below the previous one so
>that c becomes a 5 x 2 matrix.
>In R, it seems that I have no choice but use the function "rbind".
I
>managed to write this code in R . However, I'm not sure that if instead
>of creating a new variable  using  the index in the "for" loop , I
>wanted to use the index to read data, e.g.  suppose I have a 2 X 10
>matrix X and suppose I want to calculate the sin () for each 2 x 2
>sub-matrix of and stored in a matrix A. Then the code would be
>something like this,
>
>for i=1:5
>A(:, 2*i-1:2*i)= sin(X(:, 2*i-1:2*i))   % the ":" symbol indicates
all
>rows
>end
>
>Many Thanks,
>
>Fabiana
>
>
>Dr Fabiana Gordon
>
>Senior Statistical Consultant
>Statistical Advisory Service, School Of Public Health,
>Imperial College London
>1st Floor, Stadium House, 68 Wood Lane,
>London W12 7RH.
>
>Tel: 020 7594 1749
>Email:
>fabiana.gordon at imperial.ac.uk<mailto:fabiana.gordon at
imperial.ac.uk>
>Web:
>www.imperial.ac.uk/research-and-innovation/support-for-staff/stats-advice-service/<http://www.imperial.ac.uk/research-and-innovation/support-for-staff/stats-advice-service/>
>
>
>
>	[[alternative HTML version deleted]]
>
>______________________________________________
>R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
>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.