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"apply" a function that takes two or more vectors as arguments, such as cor(x, y), over a "category" or "grouping variable" or "index"?

8 messages · Richard M. Heiberger, Eric Berger, Deepayan Sarkar +2 more

Original
Kelly Thompson
# 
#Q. How can I "apply" a function that takes two or more vectors as
arguments, such as cor(x, y), over a "category" or "grouping variable"
or "index"?
#I'm using cor() as an example, I'd like to find a way to do this for
any function that takes 2 or more vectors as arguments.


#create example data

my_category <- rep ( c("a","b","c"),  4)

set.seed(12345)
my_x <- rnorm(12)

set.seed(54321)
my_y <- rnorm(12)

my_df <- data.frame(my_category, my_x, my_y)

#review data
my_df

#If i wanted to get the correlation of x and y grouped by category, I
could use this code and loop:

my_category_unique <- unique(my_category)

my_results <- vector("list", length(my_category_unique) )
names(my_results) <- my_category_unique

#start i loop
  for (i in 1:length(my_category_unique) ) {
    my_criteria_i <- my_category == my_category_unique[i]
    my_x_i <- my_x[which(my_criteria_i)]
    my_y_i <- my_y[which(my_criteria_i)]
    my_correl_i <- cor(x = my_x_i, y = my_y_i)
    my_results[i] <- list(my_correl_i)
} # end i loop

#review results
my_results

#Q. Is there a better or more "elegant" way to do this, using by(),
aggregate(), apply(), or some other function?

#This does not work and results in this error message: "Error in
FUN(dd[x, ], ...) : incompatible dimensions"
by (data = my_x, INDICES = my_category, FUN = cor, y = my_y)

#This does not work and results in this error message: "Error in
cor(my_df$x, my_df$y) : ... supply both 'x' and 'y' or a matrix-like
'x' "
by (data = my_df, INDICES = my_category, FUN = function(x, y) { cor
(my_df$x, my_df$y) } )


#if I wanted the mean of x by category, I could use by() or aggregate():
by (data = my_x, INDICES = my_category, FUN = mean)

aggregate(x = my_x, by = list(my_category), FUN = mean)

#Thanks!
Richard M. Heiberger
# 
look at
?mapply
Apply a Function to Multiple List or Vector Arguments

to see if that meets your needs
Eric Berger
# 
library(dplyr)
my_df |> group_by(my_category) |> summarise(my_z = cor(my_x, my_y))
On Sat, Apr 9, 2022 at 4:37 AM Richard M. Heiberger <rmh at temple.edu> wrote:

            

      
      
      
    

        

      
      
    

            

      
      
      
    

        

      
      
    

            

      
      
      
    

        

      
      
    

            

      
      
      
    

        

      
      
    

        

  
  


  


      

    

    

      
      

        


  

        

      Deepayan Sarkar
    

    

      
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On Sat, Apr 9, 2022 at 6:56 AM Kelly Thompson <kt1572757 at gmail.com> wrote:

            

      
      
      
    

        
split() is another generally useful function to know about: e.g.,

s <- split(my_df, ~ my_category)
lapply(s, function(d) with(d, cor(my_x, my_y)))

Best,
-Deepayan

            

      
      
      
    

        

      
      
    

  
  


  


      

    

    

      
      

        


  

        

      Rui Barradas
    

    

      
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Hello,

Another option is ?by.


by(my_df[-1], my_df$my_category, cor)
by(my_df[-1], my_df$my_category, \(x) cor(x)[1,2])


Hope this helps,

Rui Barradas

?s 02:26 de 09/04/2022, Kelly Thompson escreveu:

            

      
      
      
    

        

      
      
    

  
  


  


      

    

    

      
      

        


  

        

      Kelly Thompson
    

    

      
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Thanks. I have a clarification and a follow-up question. I should have
asked this in the original post, and I should have provided a better
example for the FUN argument, I apologize.

For use in an example, here is a "silly" example of a function that
requires arguments such as x and y to be "separately assigned" :

udf_x_plus_y <- function (x, y) { return ( x + y) }

Q. Is there a way to use by() when the argument of FUN is a function
that requires arguments such as "x" and "y" to be separately assigned
(ex. udf_x_plus_y (x = my_x , y = my_y ), rather than assigned as a
range of columns using brackets (ex. cor(x)[1,2]) ?

Something like this perhaps? (This produces an error message.)
by( data = my_df[-1], INDICES = my_df$my_category,  FUN = function(x,
y) { udf_x_plus_y (x = data$my_x, y = data$my_y) } )

Thanks again.

        
On Sat, Apr 9, 2022 at 5:32 AM Rui Barradas <ruipbarradas at sapo.pt> wrote:

            

      
      
      
    

        

      
      
    

  
  


  


      

    

    

      
      

        


  

        

      Rui Barradas
    

    

      
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Hello,

Yes, that's possible. Must by() will still pass only one object to the 
function. Then, in the function, process this object's columns.


by(my_df[-1], my_df$my_category, \(x) udf_x_plus_y(x[[1]], x[[2]]))


Hope this helps,

Rui Barradas

?s 17:36 de 09/04/2022, Kelly Thompson escreveu:

            

      
      
      
    

        

      
      
    

            

      
      
      
    

        

      
      
    

  
  


  


      

    

    

      
      

        


  

        

      Rui Barradas
    

    

      
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Hello,



?s 17:50 de 09/04/2022, Rui Barradas escreveu:

            

      
      
      
    

        
Typo: "But" ------------^^^^

Rui Barradas