将索引分配给r中的for循环内的矩阵

Question

I have a large data set 200 rows and 5 columns in a .CSV fromat. here is part of data set:

 4.1    1.2 47.3    10954   51
 3.4    1.5 0.5 1   5316
 0.3    30.1    1.2 10  875
 0.2    0.4 119 0   0
   0    52.6    0.1 0   3.1
   0    0.3 880 0   0
   0    0.1 148 180 0
   0    0.1 490.2   0   0.4
   0    1.1 0.2 0.6 0.9
   0    0   0   0   0

I want to write a code to read each 10 rows separately and store it in a matrix(10 by 5) using for-loop. So at the end I have 20 matrices each (10*5). This is the command line:

all.data   <- read.csv("C:\\Users\\Desktop\\myarray.csv",header=FALSE)#read whole data
for (k in 1:20){   
data_temp.k <- array(NA, dim=c(10,5))
  for( i in 1:10 ){
    for( j in 1:5 ) {
        data_temp.k[i,j] <- all.data[(k-1)*10:k*10,j]
    }
  }
}
write.csv(data_temp.k,"mymatrix.k")

I know the problem is somehow related to "k" and its dual function here as both matrix index and counter.

Answer 1

Don't use a loop for this, use row indexing :

## Sample data
set.seed(1)
m <- matrix(rnorm(1000),nrow=200,ncol=5)
## Generate indices to keep
indices <- seq(1,nrow(m), by=10)
## Subset matrix rows
m[indices,]

Answer 2

This probably doesn't add much other than being a nice demonstration of how you can use array s and aperm to split a mtrix into chunks and reshape, all using base R vectorised functions. You can always apply functions to each dimension of an array using apply .

#  Sample data
m <- matrix( 1:16 , 4 , 4 )
#     [,1] [,2] [,3] [,4]
#[1,]    1    5    9   13
#[2,]    2    6   10   14
#[3,]    3    7   11   15
#[4,]    4    8   12   16

# Use array() to turn into arrays and aperm() to transpose the 3D array t0 the result you expect
out <- aperm( array( t(m) , c(4,2,2) ) , c(2,1,3) )
#, , 1
#     [,1] [,2] [,3] [,4]
#[1,]    1    5    9   13
#[2,]    2    6   10   14

#, , 2
#     [,1] [,2] [,3] [,4]
#[1,]    3    7   11   15
#[2,]    4    8   12   16

You can apply functions over the third dimension, eg using 'apply'

#  Sum all the elements in each of the third dimension of your arrays
apply( out , 3 , sum )
#[1] 60 76

Answer 3

If, though, you insist on using a for loop, you can -at least- use only one and not three nested loops.

You don't need j because you want to keep all columns in each matrix. Eg mat[1,] selects all columns and row 1; you don't need to mat[1,1:ncol(mat)] .

Also, the way you use i is unneccessary, because you subset more than one row (using k-1 * 10 etc) to pass to row i every time.

Finally, if you're trying to save each of the 20 matrices, you might need paste .

This should work (not tested):

for(k in 1:20)
 {
  data_temp.k <- all.data[((k-1)*10):(k*10),]

  write.csv(data_temp.k, paste("mymatrix", k, sep = ".")
 }