Search for index of a list entry in R

Question

Given an R list, I wish to find the index of a given list entry. For example, for entry "36", I want my output to be "2". Also, how could I do such queries in parallel using lapply?

> list

$`1`
[1] "7"  "12" "26" "29"

$`2`
[1] "11" "36"

$`3`
[1] "20" "49"

$`4`
[1] "39" "41"

Answer 1

Here's a one-liner that allows for the (likely?) possibility that more than one element of the list will contain the string for which you're searching:

## Some example data
ll <- list(1:4, 5:6, 7:12, 1:12)
ll <- lapply(ll, as.character)

which(sapply(ll, FUN=function(X) "12" %in% X))
# [1] 3 4

Answer 2

You could first turn your list into a data.frame that maps values to their corresponding index in the list:

ll <- list(c("7", "12", "26", "29"),
           c("11", "36"),
           c("20", "49"),
           c("39", "41"))

df <- data.frame(value = unlist(ll),
                 index = rep(seq_along(ll), lapply(ll, length)))
df
#    value index
# 1      7     1
# 2     12     1
# 3     26     1
# 4     29     1
# 5     11     2
# 6     36     2
# 7     20     3
# 8     49     3
# 9     39     4
# 10    41     4

Then, write a function using matchfor finding the index of the first occurrence of a given value:

find.idx <- function(val)df$index[match(val, df$value)]

You can call this function on a single value, or many at a time since match is vectorized:

find.idx("36")
# [1] 2
find.idx(c("36", "41", "99"))
# [1]  2  4 NA

Of course, you can also run it through lapply, especially if you plan to run it in parallel:

lapply(c("36", "41", "99"), find.idx)
# [[1]]
# [1] 2
# 
# [[2]]
# [1] 4
# 
# [[3]]
# [1] NA

For running this last bit in parallel, there are many, many options. I would recommend you weigh your options by searching through http://cran.r-project.org/web/views/HighPerformanceComputing.html.