R define dimensions of empty data frame

Question

I am trying to collect some data from multiple subsets of a data set and need to create a data frame to collect the results   My problem is don t know how to create an empty data frame with defined number of columns without actually having data to put into it   collect1  lt - c       i d like to create empty df w  3 columns   id    max1  and  min1   for i in 1 10   collect1 id  lt - i ss1  lt - subset df1  df1 id    i  collect1 max1  lt - max ss1 value  collect1 min1  lt - min ss1 value      I feel very dumb asking this question  I almost feel like I ve asked it on SO before but can t find it  but would greatly appreciate any help

User · Accepted Answer

Just create a data frame of empty vectors:

collect1 <- data.frame(id = character(0), max1 = numeric(0), max2 = numeric(0))

But if you know how many rows you're going to have in advance, you should just create the data frame with that many rows to start with.

User · Answer

You can do something like   N  lt - 10 collect1  lt - data frame id     integer N                          max1   numeric N                          min1   numeric N     Now be careful that in the rest of your code  you forgot to use the row index for filling the data frame row by row  It should be   for i in seq len N       collect1 id i   lt - i    ss1  lt - subset df1  df1 id    i     collect1 max1 i   lt - max ss1 value     collect1 min1 i   lt - min ss1 value      Finally  I would say that there are many alternatives for doing what you are trying to accomplish  some would be much more efficient and use a lot less typing  You could for example look at the aggregate function  or ddply from the plyr package

User · Answer

df   data frame matrix     ncol   3  nrow   10

User · Answer

A more general method to create an arbitrary size data frame is to create a n-by-1 data-frame from a matrix of the same dimension  Then  you can immediately drop the first row    gt  v  lt - data frame matrix NA  nrow 1  ncol 10    gt  v  lt - v -1    drop FALSE   gt  v   1  X1  X2  X3  X4  X5  X6  X7  X8  X9  X10  lt 0 rows gt   or 0-length row names

User · Answer

If only the column names are available like    cnms  lt - c  Nam1   Nam2   Nam3     To create an empty data frame with the above variable names  first create a data frame object   emptydf  lt - data frame     Now call zeroth element of every column  thus creating an empty data frame with the given variable names   for  i in 1 length cnms         emptydf 0 eval cnms i

User · Answer

I have come across the same problem and have a cleaner solution  Instead of creating an empty data frame you can instead save your data as a named list  Once you have added all results to this list you convert it to a data frame after   For the case of adding features one at a time this works best   mylist   list   for column in 1 10  mylist column   rnorm 10  mydf   data frame mylist    For the case of adding rows one at a time this becomes tricky due to mixed types  If all types are the same it is easy   mylist   list   for row in 1 10  mylist row   rnorm 10  mydf   data frame do call rbind  mylist     I haven t found a simple way to add rows of mixed types  In this case  if you must do it this way  the empty data frame is probably the best solution

User · Answer

Would a dataframe of NAs work   something like   data frame matrix NA  nrow   2  ncol   3    if you need to be more specific about the data type then may prefer  NA integer   NA real   NA complex   or NA character  instead of just NA which is logical  Something else that may be more specific that the NAs is   data frame matrix vector mode    numeric  length   6   nrow   2  ncol   3    where the mode can be of any type  See  vector

User · Answer

It might help the solution given in another forum  Basically is  i e   Cols  lt - paste  A   1 5  sep     DF  lt - read table textConnection      col names   Cols colClasses    character     gt  str DF   data frame     0 obs  of  5 variables    A1  chr   A2  chr   A3  chr   A4  chr   A5  chr   You can change the colClasses to fit your needs   Original link is  https   stat ethz ch pipermail r-help 2008-August 169966 html

User · Answer

Here a solution if you want an empty data frame with a defined number of rows and NO columns   df   data frame matrix NA  ncol 1  nrow 10  -1

User · Answer

You may use NULL instead of NA  This creates a truly empty data frame

User · Answer

seq along may help to find out how many rows in your data file and create a data frame with the desired number of rows      listdf  lt - data frame ID seq along df                                 var1 seq along df   var2 seq along df

[r] R define dimensions of empty data frame

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