Same function over multiple data frames in R - not over a list of data frames

Question

This Issue is almost what I wanted to do, except by the fact of an output being giving as a list of data frames. Let's reproduce the example of mentioned SE issue above.

Let's say I have 2 data frames:

df1
ID col1 col2
x  0     10
y  10    20
z  20    30

df1
ID col1 col2
a  0     10
b  10    20
c  20    30

What I want is an 4th column with an ifelse result. My rationale is:

if col1>=20 in any data.frame I could have named with the pattern "df", then the new column res=1, else res=0.

But I want to create a new column in each data.frame with the same name pattern, not put all of those data.frames in a list and apply the function, except if I could "extract" each 3rd dimension of this list back to individual data frames.

Thanks

Do you mean you want the objects df1 and df2 [I'm assuming the duplicate name in your example is a typo] in your workspace to have the new columns, but you want a one-liner instead of writing df1$...=... and df2$...=...? — Philip

Philip Philip · Accepted Answer · 2016-04-04T19:43:18

Per @Frank...if my understanding of what you are looking for is correct, consider using data.table. MWE:

library(data.table); 
addcol <- function(x) x[,res:=ifelse(col1>=20,1,0)]
df1 <- data.table(ID=c("x","y","z"),col1=c(0,10,20),col2=c(10,20,30))
df2 <- data.table(ID=c("x","y","z"),col1=c(20,10,20),col2=c(10,20,30))
#modified df2 so you can see different effects
lapply(list(df1,df2),addcol) 

> df1
   ID col1 col2 res
1:  x    0   10   0
2:  y   10   20   0
3:  z   20   30   1

> df2
   ID col1 col2 res
1:  x   20   10   1
2:  y   10   20   0
3:  z   20   30   1

This works because data.table operates by reference on tables, so inside the function you're actually updating the underlying table, not only the scoped reference to the table.

Same function over multiple data frames in R - not over a list of data frames

1 Answers