R-将功能应用于相似数据列表中的特定列

Question

I am making a list of data.frames like so:

simulation_data <- vector( mode = "list", length = length(subgroups_a))

for( A in subgroups_a) { simulation_data[['A']] <- paste0(dbGetQuery(conn, "SELECT a, b, c, date FROM t WHERE a = ", A)) }

In general, how do I apply a function to a specific column which is the same across each data.frame in the list?

My specific situation is that I need to apply ymd() to the date column of each data.frame in simulation_data . My work-around currently is to just update the column each time in the for loop like so: simulation_inv[['A']]['dt'] <- ymd(simulation_inv[['A']]['dt']) , but I'd like to vectorize it if possible.

I can't figure out how to use lapply to do this, and perhaps there is an even better solution.

Thanks for any help.

Answer 1

Something like this, perhaps -

DT1 = data.frame(A=20130101:20130103,B=letters[1:3])
DT2 = data.frame(A=20130104:20130105,B=letters[4:5])
l = list(DT1,DT2)

l2 <- lapply(l, function(x) cbind(x,as.Date(as.character(x$A),'%Y%m%d')))

Where l looks like -

> l
[[1]]
         A B
1 20130101 a
2 20130102 b
3 20130103 c

[[2]]
         A B
1 20130104 d
2 20130105 e

And l2 looks like -

> l2
[[1]]
         A B as.Date(as.character(x$A), "%Y%m%d")
1 20130101 a                           2013-01-01
2 20130102 b                           2013-01-02
3 20130103 c                           2013-01-03

[[2]]
         A B as.Date(as.character(x$A), "%Y%m%d")
1 20130104 d                           2013-01-04
2 20130105 e                           2013-01-05

Using this same basic approach, you could also overwrite your earlier column, or assign a nicer column name, etc.