如何使用占位符data.table替换列表中的空dataframe/data.tables？

Question

这篇文章如何从列表中删除空数据帧？谈论删除空数据帧.如何从列表中删除空数据帧(nrow = 0)并将其替换为1行占位符dataframes/data.tables？

M1 <- data.frame(matrix(1:4, nrow = 2, ncol = 2))
M2 <- data.frame(matrix(nrow = 0, ncol = 0))
M3 <- data.frame(matrix(9:12, nrow = 2, ncol = 2))
mlist <- list(M1, M2, M3)

placeholder  = data.table(a=1,b=1)

我试过了:

lapply(mlist, function(x) ifelse(nrow(fundslist[[x]]) == 0, placeholder, x))

Answer 1

一种选择是使用 lengths

mlist[!lengths(mlist)] <- list(placeholder)
str(mlist)
#List of 3
# $ :'data.frame':       2 obs. of  2 variables:
#  ..$ X1: int [1:2] 1 2
#  ..$ X2: int [1:2] 3 4
# $ :Classes ‘data.table’ and 'data.frame':      1 obs. of  2 variables:
#  ..$ a: num 1
#  ..$ b: num 1
#  ..- attr(*, ".internal.selfref")=<externalptr> 
# $ :'data.frame':       2 obs. of  2 variables:
#  ..$ X1: int [1:2] 9 10
#  ..$ X2: int [1:2] 11 12