How to delete specific rows and columns from a matrix in a smarter way?

Question

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?

score 68 · Accepted Answer · answered Oct 16 '12 at 17:10

68

You can do:

t1<- t1[-4:-6,-7:-9]

answered Oct 16 '12 at 17:10

tcash21

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@tcash21 and how can I remove rows or cols by their name, but in a Matrix? – Darwin PC Aug 12 '15 at 13:25
1

@DarwinPC https://stackoverflow.com/questions/5234117/how-to-drop-columns-by-name-in-a-data-frame – untill Sep 15 '17 at 08:08
I was intending to remove the "i-th" column of a matrix: **how** to fix the error "Error in t1[,-i] : incorrect number of dimension calls" thrown by the code `t1 <- t1[,-i]` inside a for loop (where `i` is the iterator)? – mercury0114 Mar 04 '18 at 22:02

Chillar Anand · Answer 2 · 2020-08-08T08:08:34.270

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

YEG · Answer 3 · 2020-08-09T07:17:50.497

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.

score 1 · Answer 4 · edited Aug 09 '20 at 21:01

> 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,].

How to delete specific rows and columns from a matrix in a smarter way?

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