Populating a data frame in R in a loop

Question

I am trying to populate a data frame from within a for loop in R  The names of the columns are generated dynamically within the loop and the value of some of the loop variables is used as the values while populating the data frame  For instance the name of the current column could be some variable name as a string in the loop  and the column can take the value of the current iterator as its value in the data frame   I tried to create an empty data frame outside the loop  like this  d   data frame     But I cant really do anything with it  the moment I try to populate it  I run into an error   d 1    c 1 2  Error in    lt - data frame    tmp    1  value   c 1  2        replacement has 2 rows  data has 0   What may be a good way to achieve what I am looking to do  Please let me know if I wasnt clear

User · Answer

this works too   df   NULL for  k in 1 10               x   1        y   2        z   3        df   rbind df  data frame x y z            output will look like this  df  enter  x y z  col names 1 2 3

User · Answer

It is often preferable to avoid loops and use vectorized functions  If that is not possible there are two approaches    Preallocate your data frame  This is not recommended because indexing is slow for data frames  Use another data structure in the loop and transform into a data frame afterwards  A list is very useful here    Example to illustrate the general approach   mylist  lt - list    create an empty list  for  i in 1 5      vec  lt - numeric 5   preallocate a numeric vector   for  j in 1 5     fill the vector     vec j   lt - i j        mylist  i    lt - vec  put all vectors in the list   df  lt - do call  rbind  mylist   combine all vectors into a matrix   In this example it is not necessary to use a list  you could preallocate a matrix  However  if you do not know how many iterations your loop will need  you should use a list   Finally here is a vectorized alternative to the example loop   outer 1 5 1 5 function i j  i j    As you see it s simpler and also more efficient

User · Answer

I had a case in where I was needing to use a data frame within a for loop function  In this case  it was the  efficient   however  keep in mind that the database was small and the iterations in the loop were very simple  But maybe the code could be useful for some one with similar conditions    The for loop purpose was to use the raster extract function along five locations  i e  5 Tokio  New York  Sau Paulo  Seul  amp  Mexico city  and each location had their respective raster grids  I had a spatial point database with more than 1000 observations allocated within the 5 different locations and I was needing to extract information from 10 different raster grids  two grids per location   Also  for the subsequent analysis  I was not only needing the raster values but also the unique ID for each observations   After preparing the spatial data  which included the following tasks    Import points shapefile with the readOGR function  rgdap package  Import raster files with the raster function  raster package  Stack grids from the same location into one file  with the function stack  raster package     Here the for loop code with the use of a data frame   1  Add stacked rasters per location into a list  raslist  lt - list LOC1 LOC2 LOC3 LOC4 LOC5    2  Create an empty dataframe  this will be the output file  TB  lt - data frame VAR1 double   VAR2 double   ID character      3  Set up for loop function  L1  lt - seq 1 5 1    the location ID is a numeric variable with values from 1 to 5   for  i in 1 length L1       dat subset points LOCATION  i    select corresponding points for location  i     t data frame extract raslist  i   dat  dat ID    run extract function with points  amp  raster stack for location  i    names t  c  VAR1   VAR2   ID      TB rbind TB t

User · Answer

Thanks Notable1  works for me with the tidytextr Create a dataframe with the name of files in one column and content in other       diretorio  lt -  D  base      arquivos  lt - list files diretorio  pattern      PDF       quantidade  lt - length arquivos     df   NULL for  k in 1 quantidade           nome   arquivos k        print nome        Sys sleep 1         dados   read pdf arquivos k  ocr   T        print dados        Sys sleep 1        df   rbind df  data frame nome dados         Sys sleep 1    Encoding df text   lt -  UTF-8

User · Answer

You could do it like this    iterations   10  variables   2   output  lt - matrix ncol variables  nrow iterations    for i in 1 iterations     output i    lt - runif 2        output   and then turn it into a data frame   output  lt - data frame output   class output    what this does    create a matrix with rows and columns according to the expected growth insert 2 random numbers into the matrix convert this into a dataframe after the loop has finished

[r] Populating a data frame in R in a loop

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