Writing a R function with aggregation using data.table

Question

I'm writing a R function with aggregations using data.table package. My table looks like:

Name1   Name2   Price
  A       F      6
  A       D      5
  A       E      2
  B       F      4
  B       D      7
  C       F      4
  C       E      2

My function looks like:

MyFun <- function(Master_Table, Desired_Column, Group_By){
  Master_Table <- as.data.table(Master_Table)
  Master_Table_New <-  Master_Table[, (Master_Table$Desired_Column), by=.(Desired_Column$Group_By)]
  return(Master_Table_New)
}

I want to calculate df[, .(Group_Median = median(Price), by=.(Name1, Name2)] But when I apply it into my own function, it keeps giving me errors like: `

Error in `[.data.table`(Master_Table, , .(Med_Group = mean(Master_Table$Desired_Column)),  : 
  column or expression 1 of 'by' or 'keyby' is type NULL. Do not quote column names. Usage: DT[,sum(colC),by=list(colA,month(colB))] `

or:

Error in `[.data.table`(Master_Table, , .(Med_Group = mean(Master_Table$Desired_Column)),  : 
  column or expression 1 of 'by' or 'keyby' is type NULL. Do not quote column names. Usage: DT[,sum(colC),by=list(colA,month(colB))]

This would be the very first step of my whole work. If anyone knows anything about this, please let me know, any help would be appreciated!

The function will contain more things, but the data.table part would be the base. — Mr369
– Mr369, Commented Apr 6, 2018 at 21:49
Why don't you group with dplyr::group_by, then use mutate to calculate Group_Median? — csgroen
– csgroen, Commented Apr 6, 2018 at 21:52
@M.zz.M I understood. You need to modify your function as mentioned in my answer so that it will work properly. I have given an example as well. — MKR
– MKR, Commented Apr 6, 2018 at 22:00

MKR · Accepted Answer · 2018-04-06 21:59:18Z

2

The function should be written as:

MyFun <- function(Master_Table, Desired_Column, Group_By){
  Master_Table[, sapply(.SD, mean),  .SDcols = Desired_Column, by=Group_By]
}

#Have a close watch here how Group_By is prepared to provide multiple columns.
MyFun(DT, "Price", "Name1,Name2")
#     Name1 Name2 V1
# 1:     A     F  6
# 2:     A     D  5
# 3:     A     E  2
# 4:     B     F  4
# 5:     B     D  7
# 6:     C     F  4
# 7:     C     E  2

Data

DT <- read.table(text = 
"Name1   Name2   Price
A       F      6
A       D      5
A       E      2
B       F      4
B       D      7
C       F      4
C       E      2",
header = TRUE, stringsAsFactors = FALSE)

setDT(DT)

answered Apr 6, 2018 at 21:59

MKR

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7 Comments

Mr369 Over a year ago

Hi MKR, really appreciate for your help! It is exactly what I want! But I still have a question -- why should I modify my function before using it? Is it a special thing for data table when I want to insert it into a self-defined function?

MKR Over a year ago

@M.zz.M There are two ways. The function written by me expects a data.table type argument. Hence setDT call has been used to convert it to data.table. Another option is to call setDT within function itself. Since you had mentioned about data.table hence I kept my function expecting a data.table. Hope it clarifies your doubt.

MKR Over a year ago

@M.zz.M Also, as you can make out that there is no need of a function in your case. The one line which is part of my function can be used instead of function.

MKR Over a year ago

@M.zz.M Though you are relatively new user to SO hence wanted to inform about this link stackoverflow.com/help/someone-answers . Probably you are aware about information on link.

Mr369 Over a year ago

Thank you for the advice and explanations! As a relatively new user, it definitely helps me a lot! Also, I rewrite the Group_By part in order to merge table. Following is my code: MyFun <- function(Master_Table, Desired_Column, Group_By_A, Group_By_B) and make Group_By = c(Group_By_A, Group_By_B) I just wondering if you were me, would you have any more efficient solutions? Thank you.

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