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how to use "apply" function partial to each vector of a matrix

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
I just want to use the "apply" function to the matrix partially
there is a vector end=c(2,3,1,3)
#sum the 1st 2 numbers of the 1st column
#sum the 1st 3 numbers of the 2nd column
#sum the 1st number of the 3rd column
#sum the 1st 3 numbers of the 4th column
#they are specified by vector end
a for loop
for (i in 1:4)
	{
	 outsum=sum[1:end[i],i]
	}
 but when I want to do it to a large dataset, it's inefficency becomes a problem, so need vectorization
 the "apply" function will sum all of each vector, which is not my purpose

 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


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Michael Bedward
# 
Hello,

Here is one way...

m <- matrix(1:16, nrow=4)
end <- c(2, 3, 1, 3)

ii <- cbind(sequence(end), rep(1:length(end), end))
sums <- tapply(m[ ii ], ii[ , 2], sum)

And here is another way...

sums <- mapply(function(col, lastrow) sum(m[1:lastrow, col]), 1:ncol(m), end)

Hope this helps,
Michael
On 11 January 2011 16:17, zhaoxing731 <zhaoxing731 at yahoo.com.cn> wrote: