Count number of rows by group using dplyr

Question

I am using the mtcars dataset  I want to find the number of records for a particular combination of data  Something very similar to the count    group by clause in SQL  ddply   from plyr is working for me   library plyr  ddply mtcars    cyl gear  nrow    has output    cyl gear V1 1   4    3  1 2   4    4  8 3   4    5  2 4   6    3  2 5   6    4  4 6   6    5  1 7   8    3 12 8   8    5  2   Using this code  library dplyr  g  lt - group by mtcars  cyl  gear  summarise g  length gear     has output    length cyl  1          32   I found various functions to pass in to summarise   but none seem to work for me  One function I found is sum G   which returned  Error in eval expr  envir  enclos    object  G  not found   Tried using n    which returned   Error in n     This function should not be called directly   What am I doing wrong  How can I get group by     summarise   to work for me

User · Answer

Another option  not necesarily more elegant  but does not require to refer to a specific column   mtcars   gt      group by cyl  gear    gt     do data frame nrow nrow

User · Answer

There s a special function n   in dplyr to count rows  potentially within groups    library dplyr  mtcars   gt      group by cyl  gear    gt      summarise n   n     Source  local data frame  8 x 3   Groups  cyl            cyl  gear     n     dbl   dbl   int   1     4     3     1  2     4     4     8  3     4     5     2  4     6     3     2  5     6     4     4  6     6     5     1  7     8     3    12  8     8     5     2   But dplyr also offers a handy count function which does exactly the same with less typing   count mtcars  cyl  gear             or mtcars   gt   count cyl  gear   Source  local data frame  8 x 3   Groups  cyl            cyl  gear     n     dbl   dbl   int   1     4     3     1  2     4     4     8  3     4     5     2  4     6     3     2  5     6     4     4  6     6     5     1  7     8     3    12  8     8     5     2

User · Answer

another approach is to use the double colons   mtcars   gt      dplyr  group by cyl  gear    gt     dplyr  summarise length gear

User · Answer

I think what you are looking for is as follows    cars by cylinders gears  lt - mtcars   gt     group by cyl  gear    gt     summarise count   n      This is using the dplyr package  This is essentially the longhand version of the count    solution provided by docendo discimus

[r] Count number of rows by group using dplyr

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