Instead of talking about how you want to do something and showing us your
Matlab code, tell us what you want to do, include reproducible data, and show
us your R code so far. Your initial assumption, that you need a loop for these
examples is incorrect if I have understood you correctly. Many functions in R
are vectorized so that loops can often be avoided.
To create a 5 x 2 matrix of random normal numbers with mean=0 and sd=1:
> set.seed(42)
> crand <- cbind(r1=rnorm(5), r2=rnorm(5)) # Don't use c as a variable since
> c() is a function
> crand
r1 r2
[1,] 1.3709584 -0.10612452
[2,] -0.5646982 1.51152200
[3,] 0.3631284 -0.09465904
[4,] 0.6328626 2.01842371
[5,] 0.4042683 -0.06271410
To compute sin() on pairs of rows and columns from a 2 x 10 matrix (this time
we'll use uniform random numbers):
> X <- matrix(runif(20), 2, 10)
> X
[,1] [,2] [,3] [,4] [,5] [,6] [,7]
[,8]
[1,] 0.9040314 0.9888917 0.08243756 0.3902035 0.4469696 0.7375956 0.3881083
0.003948339
[2,] 0.1387102 0.9466682 0.51421178 0.9057381 0.8360043 0.8110551 0.6851697
0.832916080
[,9] [,10]
[1,] 0.007334147 0.9066014
[2,] 0.207658973 0.6117786
> sin(X[, 1] - X[, 2])
[1] -0.08475853 -0.72287775
> t(sin(X[, 1:9] - X[, 2:10]))
[,1] [,2]
[1,] -0.084758528 -0.72287775
[2,] 0.787322543 0.41910237
[3,] -0.302930277 -0.38159970
[4,] -0.056735679 0.06967737
[5,] -0.286552020 0.02494653
[6,] 0.342416179 0.12555319
[7,] 0.374780442 -0.14720941
[8,] -0.003385802 0.58530576
[9,] -0.782871221 -0.39320949
-------------------------------------
David L Carlson
Department of Anthropology
Texas A&M University
College Station, TX 77840-4352
-----Original Message-----
From: R-help [mailto:r-help-boun...@r-project.org] On Behalf Of Gordon, Fabiana
Sent: Wednesday, April 27, 2016 9:25 AM
To: 'r-help@R-project.org'
Subject: [R] Create a new variable and concatenation inside a "for" loop
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.gor...@imperial.ac.uk<mailto:fabiana.gor...@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/>
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
