R Apply function on specific dataframe columns

Question

I want to use the apply function on a dataframe  but only apply the function to the last 5 columns   B lt - by wifi  wifi Room  FUN function y  apply y  2  A      This applies A to all the columns of y  B lt - by wifi  wifi Room  FUN function y  apply y 4 9   2  A      This applies A only to columns 4-9 of y  but the total return of B strips off the first 3 columns    I still want those  I just don t want A applied to them   wifi  1 3  B    also does not do what I expected wanted

User · Accepted Answer

Using an example data frame and example function  just  1 to all values   A  lt - function x  x   1 wifi  lt - data frame replicate 9 1 4   wifi     X1 X2 X3 X4 X5 X6 X7 X8 X9  1  1  1  1  1  1  1  1  1  1  2  2  2  2  2  2  2  2  2  2  3  3  3  3  3  3  3  3  3  3  4  4  4  4  4  4  4  4  4  4  data frame wifi 1 3   apply wifi 4 9  2  A     or cbind wifi 1 3   apply wifi 4 9  2  A        X1 X2 X3 X4 X5 X6 X7 X8 X9  1  1  1  1  2  2  2  2  2  2  2  2  2  2  3  3  3  3  3  3  3  3  3  3  4  4  4  4  4  4  4  4  4  4  5  5  5  5  5  5   Or even   data frame wifi 1 3   lapply wifi 4 9   A     or cbind wifi 1 3   lapply wifi 4 9   A        X1 X2 X3 X4 X5 X6 X7 X8 X9  1  1  1  1  2  2  2  2  2  2  2  2  2  2  3  3  3  3  3  3  3  3  3  3  4  4  4  4  4  4  4  4  4  4  5  5  5  5  5  5

User · Answer

lapply is probably a better choice than apply here  as apply first coerces your data frame to an array which means all the columns must have the same type   Depending on your context  this could have unintended consequences  The pattern is  df cols   lt - lapply df cols   FUN   The  cols  vector can be variable names or indices   I prefer to use names whenever possible  it s robust to column reordering    So in your case this might be  wifi 4 9   lt - lapply wifi 4 9   A   An example of using column names  wifi  lt - data frame A 1 4  B runif 4   C 5 8  wifi c  quot B quot    quot C quot     lt - lapply wifi c  quot B quot    quot C quot     function x  -1   x

User · Answer

As mentioned  you simply want the standard R apply function applied to columns  MARGIN 2    wifi  4 9   lt - apply wifi  4 9   MARGIN 2  FUN A    Or  for short   wifi  4 9   lt - apply wifi  4 9   2  A    This updates columns 4 9 in-place using the A   function  Now  let s assume that na rm is an argument to A    which it probably should be  We can pass na rm T to remove NA values from the computation like so   wifi  4 9   lt - apply wifi  4 9   MARGIN 2  FUN A  na rm T    The same is true for any other arguments you want to pass to your custom function

User · Answer

I think what you want is mapply  You could apply the function to all columns  and then just drop the columns you don t want  However  if you are applying different functions to different columns  it seems likely what you want is mutate  from the dplyr package

[r] R Apply() function on specific dataframe columns

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