How to count how many values per level in a given factor

Question

I have a data frame mydf with about 2500 rows  These rows correspond to 69 classes of objects in colum 1 mydf V1  and I want to count how many rows per object class I have  I can get a factor of these classes with   objectclasses   unique factor mydf V1  exclude  1       What s the terse R way to count the rows per object class  If this were any other language I d be traversing an array with a loop and keeping count but I m new to R programming and am trying to take advantage of R s vectorised operations

User · Answer

Use the package plyr with lapply to get frequencies for every value  level  and every variable  factor  in your data frame   library plyr  lapply df  count

User · Answer

In case I just want to know how many unique factor levels exist in the data  I use   length unique df factorcolumn

User · Answer

Using data table   library data table   setDT dat     N  keyby ID    Using  Paul Hiemstra s  dat     Or using dplyr 0 3   res  lt - count dat  ID   head res    Source  local data frame  6 x 2       ID n   1  a 2   2  b 3   3  c 3   4  d 3   5  e 2   6  f 4   Or    dat   gt          group by ID    gt          tally     Or    dat   gt          group by ID    gt         summarise n n

User · Answer

One more  approach would be to  apply n   function which is counting the number of  observations  library dplyr  library magrittr  data   gt      group by columnName    gt     summarise Count   n

User · Answer

Using plyr package   library plyr   count mydf V1    It will return you a frequency of each value

User · Answer

Here 2 ways to do it   set seed 1  tt  lt - sample letters 100 rep TRUE      using table table tt  tt a b c d e f g h i j k l m n o p q r s t u v w x y z  2 3 3 3 2 4 6 1 6 5 6 4 7 2 2 2 5 4 5 3 8 4 5 4 3 1     using tapply tapply tt tt length  a b c d e f g h i j k l m n o p q r s t u v w x y z  2 3 3 3 2 4 6 1 6 5 6 4 7 2 2 2 5 4 5 3 8 4 5 4 3 1

User · Answer

Or using the dplyr library   library dplyr  set seed 1  dat  lt - data frame ID   sample letters 100 rep TRUE   dat   gt      group by ID    gt     summarise no rows   length ID     Note the use of   gt    which is similar to the use of pipes in bash  Effectively  the code above pipes dat into group by  and the result of that operation is piped into summarise    The result is   Source  local data frame  26 x 2      ID no rows 1   a       2 2   b       3 3   c       3 4   d       3 5   e       2 6   f       4 7   g       6 8   h       1 9   i       6 10  j       5 11  k       6 12  l       4 13  m       7 14  n       2 15  o       2 16  p       2 17  q       5 18  r       4 19  s       5 20  t       3 21  u       8 22  v       4 23  w       5 24  x       4 25  y       3 26  z       1   See the dplyr introduction for some more context  and the documentation for details regarding the individual functions

User · Answer

We can use summary on factor column   summary myDF factorColumn

[r] How to count how many values per level in a given factor?

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