I would like to use criteria in one data frame(df) to select elements from a list(mlist). Specifically, the df has ranges that define what groups of elements from the list I need.
For instance:
df = data.frame(Start=c(1,12,20), Stop=(4,16,22)) #there may be hundreds of these of arbitrary lengths.
mlist = c(a,b,c,d,e,f,....)
I can select them individually like this:
sublist <- mlist[df$Start[1]:df$Stop[1]]
and I can build a for loop to loop over all the start/stop pairs and use rbind to put them together, but I'd like to avoid the for loop because of speed. Is there syntax similar to the command I list above (sublist <- mlist[...]) that could do this or is there another function (plyr?) that can use two objects (a list and a data frame) in this manner?
The most obvious solution (though it is a hidden loop) is to use apply, like this:
df = data.frame(Start=c(1,12,20), Stop=c(4,16,22))
df
# Start Stop
# 1 1 4
# 2 12 16
# 3 20 22
mlist <- letters
apply(df, 1, function(x) mlist[x[1]:x[2]])
# [[1]]
# [1] "a" "b" "c" "d"
#
# [[2]]
# [1] "l" "m" "n" "o" "p"
#
# [[3]]
# [1] "t" "u" "v"