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how to coerce part of each column of a matrix to a vector and merge them

2 messages · zhaoxing731, Michael Bedward

zhaoxing731
# 
Hello

Suppose I have a matrix mat=(1:16,2)
     [,1] [,2] [,3] [,4]
[1,]    1    5    9   13
[2,]    2    6   10   14
[3,]    3    7   11   15
[4,]    4    8   12   16

there is a vector end=c(2,3,1,3)

#coerce the 1st 2 numbers of the 1st column to a vector  [1]  1  2
#coerce the 1st 3 numbers of the 2nd column and append it to the previous vector  [1]  1  2  5  6  7
#coerce the 1st number of the 3rd column and append it to the previous vector  [1]  1  2  5  6  7  9 
#coerce the 1st 3 numbers of the 4th column and append it to the previous vector  [1]  1  2  5  6  7  9 13 14 15
#they are specified by vector end
a for loop
mat<-matrix(1:16,4)
end<-c(2,3,1,3)
vec<-numeric()
for (i in 1:4)
	{vec<-c(vec,mat[1:end[i],i])
	}
#the result
 > vec
[1]  1  2  5  6  7  9 13 14 15

 but when I want to do it to a large dataset, it's inefficiency becomes a problem, so need vectorization

 Thank you in advance
 Yours sincerely

ZhaoXing
Department of Health Statistics
West China School of Public Health
Sichuan University
No.17 Section 3, South Renmin Road
Chengdu, Sichuan 610041
P.R.China

__________________________________________________
???????????????
Michael Bedward
# 
Hello,

The answer to this one drops out of the answer to your previous question...

m <- matrix(1:16, nrow=4)
end <- c(2,3,1,3)
ii <- cbind(sequence(end), rep(1:length(end), end))
x <- m[ ii ]

Hope this helps,
Michael


2011/1/11 zhaoxing731 <zhaoxing731 at yahoo.com.cn>: