Subset dataframe into equal subgroup chunks - r

Question

I have df dataframe that needs subsetting into chunks of 2 names. From example below, there are 4 unique names: a,b,c,d. I need to subset into 2 one column matrices a,b and c,d.
Output format:
name1
item_value
item_value
...
END
name2
item_value
item_value
...
END
Example:
#dummy data
df <- data.frame(name=sort(c(rep(letters[1:4],2),"a","a","c")),
item=round(runif(11,1,10)),
stringsAsFactors=FALSE)
#tried approach - split per name. I need to split per 2 names.
lapply(split(df,f=df$name),
function(x)
{name <- unique(x$name)
as.matrix(c(name,x[,2],"END"))
})
#expected output
[,1]
[1,] "a"
[2,] "8"
[3,] "9"
[4,] "6"
[5,] "4"
[6,] "END"
[1,] "b"
[2,] "2"
[3,] "10"
[4,] "END"
[,2]
[1,] "c"
[2,] "6"
[3,] "6"
[4,] "2"
[5,] "END"
[1,] "d"
[2,] "4"
[3,] "1"
[4,] "END"
Note: Actual df has ~300000 rows with ~35000 unique names.

Answer 1

You may try this.
# for each 'name', "pad" 'item' with 'name' and 'END'
l1 <- lapply(split(df, f = df$name), function(x){
name <- unique(x$name)
as.matrix(c(name, x$item, "END"))
})
# create a sequence of numbers, to select two by two elements from the list
steps <- seq(from = 0, to = length(unique(df$name))/2, by = 2)
# loop over 'steps' to bind together list elements, two by two.
l2 <- lapply(steps, function(x){
do.call(rbind, l1[1:2 + x])
})
l2
# [[1]]
# [,1]
# [1,] "a"
# [2,] "6"
# [3,] "4"
# [4,] "10"
# [5,] "3"
# [6,] "END"
# [7,] "b"
# [8,] "6"
# [9,] "7"
# [10,] "END"
#
# [[2]]
# [,1]
# [1,] "c"
# [2,] "2"
# [3,] "6"
# [4,] "10"
# [5,] "END"
# [6,] "d"
# [7,] "5"
# [8,] "4"
# [9,] "END"

Answer 2

Instead of making the lists from individual names make it from the column of subsets of the data.frame
res <- list("a_b" = c(df[df$name == "a",2],"END",df[df$name == "b", 2],"END"),
"c_d" = c(df[df$name == "c",2],"END", df[df$name == "d", 2],"END"))
res2 <- vector(mode="list",length=2)
res2 <- sapply(1:(length(unique(df$name))/2),function(x) {
sapply(seq(1,length(unique(df$name))-1,by=2), function(y) {
name <- unique(df$name)
res2[x] <- as.matrix(c(name[y],df[df$name == name[y],2],"END",name[y+1],df[df$name == name[y+1],2],"END"))
})
})
answer <- res2[,1]
This is giving me a matrix of lists since there are two sapplys happening, I think everything you want is in res2[,1]