Let's say t1 is :
t1 <- array(1:20, dim=c(10,10))
[,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
[1,] 1 11 1 11 1 11 1 11 1 11
[2,] 2 12 2 12 2 12 2 12 2 12
[3,] 3 13 3 13 3 13 3 13 3 13
[4,] 4 14 4 14 4 14 4 14 4 14
[5,] 5 15 5 15 5 15 5 15 5 15
[6,] 6 16 6 16 6 16 6 16 6 16
[7,] 7 17 7 17 7 17 7 17 7 17
[8,] 8 18 8 18 8 18 8 18 8 18
[9,] 9 19 9 19 9 19 9 19 9 19
[10,] 10 20 10 20 10 20 10 20 10 20
I want to delete row 4-6 and column 7-9 from this matrix.
I know how to remove it one by one using
t2 <- t1[,-7]
t3 <- t2[,-8]
t4 <- t3[,-9]
t5 <- t4[-4,]
t6 <- t5[-5,]
t7 <- t6[-6,]
However, I believe it is the most stupid way of doing it. Could you mind to advice some smarter ways of doing it?
This question is related to
r
You can also remove rows and columns by feeding a vector of logical boolean values to the matrix. This handles the situation where you have multiple non-contiguous rows or non-contiguous columns that need to be deleted.
# TRUE = Keep a row/column
# FALSE = Delete a row/column
#
# FALSE for rows 4, 5, and 6
# Row: 1 2 3 4 5 6 7 8 9 10
rows_to_keep <- c(TRUE, TRUE, TRUE, FALSE, FALSE, FALSE, TRUE, TRUE, TRUE, TRUE)
# FALSE for columns 7, 8, and 9
# Column: 1 2 3 4 5 6 7 8 9 10
cols_to_keep <- c(TRUE, TRUE, TRUE, TRUE, TRUE, TRUE, FALSE, FALSE, FALSE, TRUE)
To remove just the rows:
t1 <- t1[rows_to_keep,]
To remove just the columns:
t1 <- t1[,cols_to_keep]
To remove both the rows and columns:
t1 <- t1[rows_to_keep, cols_to_keep]
This coding technique is useful if you don't know in advance what rows or columns you need to remove. The rows_to_keep and cols_to_keep vectors can be calculated as appropriate by your code.
You can use
t1<- t1[-4:-6,-7:-9]
or
t1 <- t1[-(4:6), -(7:9)]
or
t1 <- t1[-c(4, 5, 6), -c(7, 8, 9)]
You can pass vectors to select rows/columns to be deleted. First two methods are useful if you are trying to delete contiguous rows/columns. Third method is useful if You are trying to delete discrete rows/columns.
> t1 <- array(1:20, dim=c(10,10));
> t1[-c(1, 4, 6, 7, 9), -c(2, 3, 8, 9)]
[,1] [,2] [,3] [,4] [,5] [,6]
[1,] 2 12 2 12 2 12
[2,] 3 13 3 13 3 13
[3,] 5 15 5 15 5 15
[4,] 8 18 8 18 8 18
[5,] 10 20 10 20 10 20
> S = matrix(c(1,2,3,4,5,2,1,2,3,4,3,2,1,2,3,4,3,2,1,2,5,4,3,2,1),ncol = 5,byrow = TRUE);S
[,1] [,2] [,3] [,4] [,5]
[1,] 1 2 3 4 5
[2,] 2 1 2 3 4
[3,] 3 2 1 2 3
[4,] 4 3 2 1 2
[5,] 5 4 3 2 1
> S<-S[,-2]
> S
[,1] [,2] [,3] [,4]
[1,] 1 3 4 5
[2,] 2 2 3 4
[3,] 3 1 2 3
[4,] 4 2 1 2
[5,] 5 3 2 1
Just use the command S <- S[,-2] to remove the second column. Similarly to delete a row, for example, to delete the second row use S <- S[-2,].
Source: Stackoverflow.com