Create a new variable and concatenation inside a "for" loop

"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-adv
ice-service/<http://www.imperial.ac.uk/research-and-innovation/support
-for-staff/stats-advice-service/>



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