# [R] how to use "apply" function partial to each vector of a matrix

zhaoxing731 zhaoxing731 at yahoo.com.cn
Tue Jan 11 06:17:20 CET 2011

```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

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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```