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

Question

Let s say t1 is    t1  lt - 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  lt - t1  -7  t3  lt - t2  -8  t4  lt - t3  -9  t5  lt - t4 -4   t6  lt - t5 -5   t7  lt - 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

User · Answer

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.

User · Answer

You can do   t1 lt - t1 -4 -6 -7 -9

User · Answer

You can use t1 lt - t1 -4 -6 -7 -9     or t1  lt - t1 - 4 6   - 7 9    or t1  lt - 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   gt  t1  lt - array 1 20  dim c 10 10      gt  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

User · Answer

gt  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  gt  S lt -S  -2   gt  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  lt - S  -2  to remove the second column  Similarly to delete a row  for example  to delete the second row use S  lt - S -2

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

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