Select rows of a matrix that meet a condition

Question

In R with a matrix        one two three four   1     1   6    11   16   2     2   7    12   17   3     3   8    11   18   4     4   9    11   19   5     5  10    15   20   I want to extract the submatrix whose rows have column three   11  That is         one two three four   1     1   6    11   16   3     3   8    11   18   4     4   9    11   19   I want to do this without looping  I am new to R so this is probably very obvious but the documentation is often somewhat terse

User · Answer

If the dataset is called data  then all the rows meeting a condition where value of column  pm2 5    300 can be received by -   data data  pm2 5    300

User · Answer

This is easier to do if you convert your matrix to a data frame using as data frame     In that case the previous answers  using subset or m three  will work  otherwise they will not   To perform the operation on a matrix  you can define a column by name   m m    three      11     Or by number   m m  3     11     Note that if only one row matches  the result is an integer vector  not a matrix

User · Answer

m  lt - matrix 1 20  ncol   4   colnames m   lt - letters 1 4    The following command will select the first row of the matrix above   subset m  m  4     16    And this will select the last three   subset m  m  4   gt  17    The result will be a matrix in both cases  If you want to use column names to select columns then you would be best off converting it to a dataframe with  mf  lt - data frame m    Then you can select with  mf  mf a    16      Or  you could use the subset command

User · Answer

I will choose a simple approach using the dplyr package   If the dataframe is data   library dplyr  result  lt - filter data  three    11

User · Answer

If your matrix is called m  just use    R gt  m m three    11

User · Answer

Subset is a very slow function   and I personally find it useless   I assume you have a data frame  array  matrix called Mat with A  B  C as column names  then all you need to do is    In the case of one condition on one column  lets say column A  Mat which Mat   A      10        In the case of multiple conditions on different column  you can create a dummy variable  Suppose the conditions are A   10  B   5  and C  gt  2  then we have       aux   which Mat   A      10      aux   aux which Mat aux  B      5       aux   aux which Mat aux  C    gt  2       Mat aux      By testing the speed advantage with system time  the which method is 10x faster than the subset method

[r] Select rows of a matrix that meet a condition

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