count number of rows in a data frame in R based on group

Question

I have a data frame in R like this     ID   MONTH-YEAR   VALUE   110   JAN  2012     1000   111   JAN  2012     2000                                             121   FEB  2012     3000   131   FEB  2012     4000                                                 So  for each month of each year there are n rows and they can be in any order mean they all are not in continuity and are at breaks   I want to calculate how many rows are there for each MONTH-YEAR i e  how many rows are there for JAN  2012  how many for FEB  2012 and so on  Something like this    MONTH-YEAR   NUMBER OF ROWS  JAN  2012     10  FEB  2012     13  MAR  2012     6  APR  2012     9   I tried to do this   n row  lt - nrow dat1 frame     group by MONTH-YEAR     but it does not produce the desired output How can I do that

User · Answer

Suppose we have a df data data frame as below   gt  df data    ID MONTH-YEAR VALUE 1 110   JAN 2012  1000 2 111   JAN 2012  2000 3 121   FEB 2012  3000 4 131   FEB 2012  4000 5 141   MAR 2012  5000   To count number of rows in df data grouped by MONTH-YEAR column  you can use    gt  summary df data  MONTH-YEAR    FEB 2012 JAN 2012 MAR 2012     2        2        1     summary function will create a table from the factor argument  then create a vector for the result  line 7  amp  8

User · Answer

The count   function in plyr does what you want   library plyr   count mydf   MONTH-YEAR

User · Answer

library plyr  ddply data    MONTH-YEAR   nrow    This will give you the answer  if  MONTH-YEAR  is a variable  First  try unique data MONTH-YEAR  and see if it returns unique values  no duplicates    Then above simple split-apply-combine will return what you are looking for

User · Answer

Using the example data set that Ananda dummied up  here s an example using aggregate    which is part of core R  aggregate   just needs something to count as function of the different values of MONTH-YEAR  In this case  I used VALUE as the thing to count   aggregate cbind count   VALUE    MONTH YEAR             data   mydf             FUN   function x  NROW x      which gives you      MONTH YEAR count 1  FEB  2012     2 2  JAN  2012     2 3  MAR  2012     1

User · Answer

Here s an example that shows how table     or  more closely matching your desired output  data frame table     does what it sounds like you are asking for   Note also how to share reproducible sample data in a way that others can copy and paste into their session   Here s the  reproducible  sample data   mydf  lt - structure list ID   c 110L  111L  121L  131L  141L                           MONTH YEAR   c  JAN  2012    JAN  2012                                           FEB  2012    FEB  2012                                           MAR  2012                            VALUE   c 1000L  2000L  3000L  4000L  5000L                        Names   c  ID    MONTH YEAR    VALUE                       class    data frame   row names   c NA  -5L    mydf      ID MONTH YEAR VALUE   1 110  JAN  2012  1000   2 111  JAN  2012  2000   3 121  FEB  2012  3000   4 131  FEB  2012  4000   5 141  MAR  2012  5000   Here s the calculation of the number of rows per group  in two output display formats   table mydf MONTH YEAR       FEB  2012 JAN  2012 MAR  2012            2         2         1  data frame table mydf MONTH YEAR            Var1 Freq   1 FEB  2012    2   2 JAN  2012    2   3 MAR  2012    1

User · Answer

Just for completion the data table solution    library data table   mydf  lt - structure list ID   c 110L  111L  121L  131L  141L                           MONTH YEAR   c  JAN  2012    JAN  2012                                           FEB  2012    FEB  2012                                           MAR  2012                            VALUE   c 1000L  2000L  3000L  4000L  5000L                        Names   c  ID    MONTH YEAR    VALUE                       class    data frame   row names   c NA  -5L    setDT mydf  mydf      Number of rows     N   by   MONTH YEAR      MONTH YEAR Number of rows 1   JAN  2012              2 2   FEB  2012              2 3   MAR  2012              1

User · Answer

Here is another way of using aggregate to count rows by group   my data  lt - read table text         month year    my cov       Jan 2000     apple       Jan 2000      pear       Jan 2000     peach       Jan 2001     apple       Jan 2001     peach       Feb 2002      pear    header   TRUE  stringsAsFactors   FALSE  na strings   NA   rows per group   lt - aggregate rep 1  length my data month year                                 by list my data month year   sum  rows per group       Group 1 x   1 Feb 2002 1   2 Jan 2000 3   3 Jan 2001 2

User · Answer

Try using the count function in dplyr   library dplyr  dat1 frame   gt        count MONTH YEAR    I am not sure how you got MONTH-YEAR as a variable name  My R version does not allow for such a variable name  so I replaced it with MONTH YEAR   As a side note  the mistake in your code was that dat1 frame     group by MONTH-YEAR  without  a summarise function returns the original data frame without any modifications  So  you want to use  dat1 frame   gt       group by MONTH YEAR    gt       summarise count n

[r] count number of rows in a data frame in R based on group

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