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How to extract vector using different index for columns in an R matrix?
Suppose we have a matrix and a vector containing indices of equal size as the matrix then we can extract the vector from matrix using the index vector. For this purpose, we can use cbind function as shown in the below examples.
Example1
> M1<-matrix(rpois(40,2),ncol=2) > M1
Output
[,1] [,2] [1,] 4 0 [2,] 1 1 [3,] 1 2 [4,] 2 0 [5,] 3 2 [6,] 2 2 [7,] 1 6 [8,] 1 2 [9,] 3 1 [10,] 1 2 [11,] 2 3 [12,] 2 0 [13,] 3 0 [14,] 0 1 [15,] 2 4 [16,] 1 1 [17,] 3 1 [18,] 0 2 [19,] 2 1 [20,] 2 0
Example
> Index_M1<-sample(1:2,20,replace=TRUE) > Index_M1
Output
[1] 2 1 2 1 2 2 1 1 2 1 1 2 1 1 1 1 2 2 1 1
Example
> M1[cbind(seq_along(Index_M1),Index_M1)]
Output
[1] 0 1 2 2 2 2 1 1 1 1 2 0 3 0 2 1 1 2 2 2
Example2
> M2<-matrix(rpois(80,10),ncol=4) > M2
Output
[,1] [,2] [,3] [,4] [1,] 10 9 9 11 [2,] 13 6 16 8 [3,] 11 11 8 10 [4,] 15 11 9 9 [5,] 10 8 9 9 [6,] 7 14 9 15 [7,] 8 6 8 7 [8,] 4 8 9 12 [9,] 7 12 11 10 [10,] 8 8 9 13 [11,] 9 13 11 6 [12,] 12 5 11 8 [13,] 8 6 15 8 [14,] 6 17 12 7 [15,] 8 10 9 8 [16,] 13 7 11 13 [17,] 5 10 7 7 [18,] 10 11 8 8 [19,] 5 9 9 13 [20,] 5 10 7 6
Example
> Index_M2<-sample(1:4,20,replace=TRUE) > Index_M2
Output
[1] 3 4 3 3 3 1 3 4 4 3 1 4 3 4 4 1 2 1 1 2
Example
> M2[cbind(seq_along(Index_M2),Index_M2)]
Output
[1] 9 8 8 9 9 7 8 12 10 9 9 8 15 7 8 13 10 10 5 10
Updated on: 06-Mar-2021
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