how to loop thru a matrix or data frame , and append calculations to a new data frame?

From: David Winsemius <dwinsemius at comcast.net>
Subject: Re: [R] how to loop thru a matrix or data frame , and append calculations to  a new data frame?
To: "John Kane" <jrkrideau at yahoo.ca>
Cc: r-help at r-project.org, "Robert Wilkins" <robstdev at gmail.com>
Received: Saturday, October 31, 2009, 2:00 PM
On Oct 31, 2009, at 12:33 PM, John
Kane wrote:

Do you mean to apply the same calculation to each

element?

? apply

mydata? <- data.frame(aa = 1:5, bb= 11:15)
mp5 <- function(x) x*5
mp5data <-? apply(mydata, 2, mp5)
mp5data

It would have been much more compact (and in the spirit of
functional programming) to just do:

mp5data <- mydata*5
# binary operations applied to dataframes give sensible
results when the data types allow.
mp5data
? ???aa bb
[1,]? 5 55
[2,] 10 60
[3,] 15 65
[4,] 20 70
[5,] 25 75

Many times the loops are implicit in the vectorized design
of R. And that solution would not result in the structure
requested, for which some further "straightening" would be
needed:

mp5data <- as.vector(as.matrix(mydata)*5)
mp5data

 [1]? 5 10 15 20 25 55 60 65 70 75

-- David

This is functionally equivelent to a double if loop.

mydata? <- data.frame(aa = 1:5, bb= 11:15)
newdata <- data.frame(matrix(rep(NA,10), nrow=5))

for (i in 1:length(mydata[1,])) {
? for (j in 1:5) {
? ? newdata[j,i] <- mydata[j,i]*5
}
? ? }

newdata



--- On Fri, 10/30/09, Robert Wilkins <robstdev at gmail.com>

wrote:

From: Robert Wilkins <robstdev at gmail.com>
Subject: [R] how to loop thru a matrix or data

frame , and append calculations to? a new data frame?

To: r-help at r-project.org
Received: Friday, October 30, 2009, 11:46 AM
How do you do a double loop through a
matrix or data frame , and
within each iteration , do a calculation and

append it to a

new,
second data frame?
(So, if your original matrix or data frame is 4 x

5 , then

20
calculations are done, and the new data frame,

which had 0

rows to
start with, now has 20 rows)

mailing list

reproducible code.

David Winsemius, MD
Heritage Laboratories
West Hartford, CT