All Questions
8,265
questions
675
votes
11
answers
333k
views
The difference between bracket [ ] and double bracket [[ ]] for accessing the elements of a list or dataframe
R provides two different methods for accessing the elements of a list or data.frame: [] and [[]].
What is the difference between the two, and when should I use one over the other?
- 17.5k
642
votes
27
answers
1.2m
views
Convert a list to a data frame
I have a nested list of data. Its length is 132 and each item is a list of length 20. Is there a quick way to convert this structure into a data frame that has 132 rows and 20 columns of data?
Here is ...
- 39.4k
500
votes
11
answers
358k
views
Combine a list of data frames into one data frame by row
I have code that at one place ends up with a list of data frames which I really want to convert to a single big data frame.
I got some pointers from an earlier question which was trying to do ...
- 60.3k
366
votes
20
answers
683k
views
How can I remove an element from a list?
I have a list and I want to remove a single element from it. How can I do this?
I've tried looking up what I think the obvious names for this function would be in the reference manual and I haven't ...
- 18k
346
votes
13
answers
219k
views
How to correctly use lists?
Brief background: Many (most?) contemporary programming languages in widespread use have at least a handful of ADTs [abstract data types] in common, in particular,
string (a sequence comprised of ...
- 69.6k
343
votes
9
answers
283k
views
Simultaneously merge multiple data.frames in a list
I have a list of many data.frames that I want to merge. The issue here is that each data.frame differs in terms of the number of rows and columns, but they all share the key variables (which I've ...
- 4,919
270
votes
10
answers
378k
views
How do I make a list of data frames?
How do I make a list of data frames and how do I access each of those data frames from the list?
For example, how can I put these data frames in a list ?
d1 <- data.frame(y1 = c(1, 2, 3),
...
- 20.8k
257
votes
17
answers
310k
views
Append an object to a list in R in amortized constant time, O(1)?
If I have some R list mylist, you can append an item obj to it like so:
mylist[[length(mylist)+1]] <- obj
But surely there is some more compact way. When I was new at R, I tried writing lappend(...
- 21.8k
195
votes
9
answers
660k
views
Append value to empty vector in R?
I'm trying to learn R and I can't figure out how to append to a list.
If this were Python I would . . .
#Python
vector = []
values = ['a','b','c','d','e','f','g']
for i in range(0,len(values)):
...
- 30.6k
175
votes
14
answers
255k
views
data.frame rows to a list
I have a data.frame which I would like to convert to a list by rows, meaning each row would correspond to its own list elements. In other words, I would like a list that is as long as the data.frame ...
- 70.1k
169
votes
5
answers
256k
views
How to combine two lists in R
I have two lists:
l1 = list(2, 3)
l2 = list(4)
I want a third list:
list(2, 3, 4).
How can I do it in simple way.
Although I can do it in for loop, but I am expecting a one liner answer, or maybe ...
- 2,151
151
votes
5
answers
119k
views
What are the differences between vector and list data types in R?
What are the main differences between vector and list data types in R? What are the advantages or disadvantages of using (or not) these two data types?
I would appreciate seeing examples that ...
- 1,519
124
votes
8
answers
159k
views
Creating an R dataframe row-by-row
I would like to construct a dataframe row-by-row in R. I've done some searching, and all I came up with is the suggestion to create an empty list, keep a list index scalar, then each time add to the ...
- 29.6k
117
votes
5
answers
324k
views
R: Count number of objects in list [closed]
Can someone recommend a function that can allow me to count and return the number of items in a list?
library(stringr)
l <- strsplit(words, "a")
if(# number of items in list l < 1)
...
- 5,708
115
votes
5
answers
119k
views
Select first element of nested list
Let's say I have a list like this:
x = list(list(1,2), list(3,4), list(5,6))
I would like a list that contains only the first elements of the nested list. I can do this by returning another list ...
- 19.8k
115
votes
2
answers
177k
views
Select multiple elements from a list
I have a list in R some 10,000 elements long. Say I want to select only elements, 5, 7, and 9. I'm not sure how I would do that without a for loop.
I want to do something like mylist[[c(5,7,9]] but ...
- 11.5k
109
votes
3
answers
186k
views
Read all files in a folder and apply a function to each data frame
I am doing a relatively simple piece of analysis that I have put into a function on all the files in a particular folder. I was wondering whether anyone had any tips to help me automate the process on ...
- 8,374
102
votes
7
answers
160k
views
Working with dictionaries/lists to get list of keys
I have trivial question: I couldn't find a dictionary data structure in R, so I used list instead (like "word"->number).
So, how do I get the list of keys.
- 2,199
102
votes
5
answers
56k
views
Create a data.frame where a column is a list
I know how to add a list column:
> df <- data.frame(a=1:3)
> df$b <- list(1:1, 1:2, 1:3)
> df
a b
1 1 1
2 2 1, 2
3 3 1, 2, 3
This works, but not:
> df <- data....
- 88.3k
94
votes
16
answers
106k
views
Convert a matrix to a list of column-vectors
Say you want to convert a matrix to a list, where each element of the list contains one column. list() or as.list() obviously won't work, and until now I use a hack using the behaviour of tapply :
x &...
- 108k
94
votes
4
answers
96k
views
How to flatten a list of lists?
The tm package extends c so that, if given a set of PlainTextDocuments it automatically creates a Corpus. Unfortunately, it appears that each PlainTextDocument must be specified separately.
e.g. if ...
- 20.3k
83
votes
2
answers
60k
views
What is difference between dataframe and list in R?
What is difference between dataframe and list in R? Which one should be used when? Which is easier to loop over?
Exact problem: I have to first store 3 string elements like "a", "b", "c". Later for ...
- 943
82
votes
9
answers
121k
views
Merge Two Lists in R
I have two lists
first = list(a = 1, b = 2, c = 3)
second = list(a = 2, b = 3, c = 4)
I want to merge these two lists so the final product is
$a
[1] 1 2
$b
[1] 2 3
$c
[1] 3 4
Is there a simple ...
- 7,227
80
votes
7
answers
169k
views
Appending a list to a list of lists in R
I'm having issues appending data to a list which is already in a list format. I have a program which will export results objects during a simulation loop. The data itself is stored as a list of ...
- 3,689
80
votes
5
answers
32k
views
Prevent unlist to drop NULL values
I have a vector of lists and I use unlist on them. Some of the elements in the vectors are NULL and unlist seems to be dropping them.
How can I prevent this?
Here's a simple (non) working example ...
- 51.3k
77
votes
3
answers
139k
views
Convert named list to vector with values only
I have a list of named values:
myList <- list('A' = 1, 'B' = 2, 'C' = 3)
I want a vector with the value 1:3
I can't figure out how to extract the values without defining a function. Is there a ...
- 6,265
70
votes
5
answers
65k
views
Remove empty elements from list with character(0)
How can I remove empty elements from a list that contain zero length pairlist as
character(0), integer(0) etc...
list2
# $`hsa:7476`
# [1] "1","2","3"
#
# $`hsa:656`
# character(0)
#
# $`hsa:7475`
# ...
- 1,069
69
votes
3
answers
41k
views
Sum a list of matrices [duplicate]
I have a list where each element is a 5*5 matrix. Eg
[[1]]
V1 V2 V3 V4 V5
[1,] 0.000000 46.973700 21.453500 338.547000 10.401600 ...
- 4,144
68
votes
5
answers
156k
views
How to convert a huge list-of-vector to a matrix more efficiently?
I have a list of length 130,000 where each element is a character vector of length 110. I would like to convert this list to a matrix with dimension 1,430,000*10. How can I do it more efficiently?\
My ...
- 1,111
65
votes
6
answers
71k
views
R + combine a list of vectors into a single vector
I have a single list of numeric vector and I want to combine them into one vector. But I am unable to do that. This list can have one element common across the list element. Final vector should not ...
- 3,293
64
votes
3
answers
43k
views
Creating a named list from two vectors (names, values)
Is there a way to use mapply on two vectors to construct a named list? The first vector would be of type character and contain the names used for the list while the second contains the values.
So far,...
- 2,892
64
votes
4
answers
87k
views
How to add variable key/value pair to list object?
I have two variables, key and value, and I want to add them as a key/value pair to a list:
key = "width"
value = 32
mylist = list()
mylist$key = value
The result is this:
mylist
# $key
# [1] 32
...
- 65.2k
62
votes
5
answers
50k
views
Assigning NULL to a list element in R?
I found this behaviour odd and wanted more experienced users to share their thoughts and workarounds.
On running the code sample below in R:
sampleList <- list()
d<- data.frame(x1 = letters[1:...
- 3,760
58
votes
4
answers
28k
views
preallocate list in R
It is inefficient in R to expand a data structure in a loop. How do I preallocate a list of a certain size? matrix makes this easy via the ncol and nrow arguments. How does one do this in lists? ...
- 19.8k
57
votes
3
answers
19k
views
Unpacking argument lists for ellipsis in R
I am confused by the use of the ellipsis (...) in some functions, i.e. how to pass an object containing the arguments as a single argument.
In Python it is called "unpacking argument lists", e.g.
&...
- 2,147
54
votes
8
answers
125k
views
R: Print list to a text file
I have in R a list like this:
> print(head(mylist,2))
[[1]]
[1] 234984 10354 41175 932711 426928
[[2]]
[1] 1693237 13462
Each element of the list has different number of its elements.
I ...
- 4,566
54
votes
7
answers
8k
views
How to flatten a list to a list without coercion?
I am trying to achieve the functionality similar to unlist, with the exception that types are not coerced to a vector, but the list with preserved types is returned instead. For instance:
flatten(...
- 6,012
52
votes
2
answers
25k
views
How to remove a level of lists from a list of lists
I have created some lists within a list and would like to be able have each sublist element to be an individual element at the top level.
For example to create some dummy data:
pp <- lapply(10:15,...
- 13.3k
52
votes
3
answers
81k
views
Here we go again: append an element to a list in R
I am not happy with the accepted answer to Append an object to a list in R in amortized constant time?
> list1 <- list("foo", pi)
> bar <- list("A", "B")
How can I append new element bar ...
- 7,286
50
votes
4
answers
56k
views
Convert list of vectors to data frame
I'm trying to convert a list of vectors (a multidimensional array essentially) into a data frame, but every time I try I'm getting unexpected results.
My aim is to instantiate a blank list, populate ...
- 839
50
votes
4
answers
15k
views
Fast vectorized merge of list of data.frames by row
Most of the questions about merging data.frame in lists on SO don't quite relate to what I'm trying to get across here, but feel free to prove me wrong.
I have a list of data.frames. I would like to "...
- 70.1k
48
votes
7
answers
61k
views
What is the most efficient way to cast a list as a data frame?
Very often I want to convert a list wherein each index has identical element types to a data frame. For example, I may have a list:
> my.list
[[1]]
[[1]]$global_stdev_ppb
[1] 24267673
[[1]]$...
- 5,447
48
votes
2
answers
15k
views
What is the difference between a list and a pairlist in R?
In reading the documentation for lists, I found references to pairlists, but it wasn't clear to me how they were different from lists.
- 11.4k
46
votes
4
answers
5k
views
Can lists be created that name themselves based on input object names?
It would be very helpful to me to be able to create an R list object without having to specify the names of each element. For example:
a1 <- 1
a2 <- 20
a3 <- 1:20
b <- list(a1,a2,a3, ...
- 3,945
44
votes
8
answers
62k
views
Subset elements in a list based on a logical condition
How can I subset a list based on a condition (TRUE, FALSE) in another list? Please, see my example below:
l <- list(a=c(1,2,3), b=c(4,5,6,5), c=c(3,4,5,6))
l
$a
[1] 1 2 3
$b
[1] 4 5 6 5
$c
[1] 3 ...
- 16.6k
43
votes
3
answers
20k
views
Use input of purrr's map function to create a named list as output in R
I am using the map function of the purrr package in R which gives as output a list. Now I would like the output to be a named list based on the input. An example is given below.
input <- c("a", "b"...
- 1,301
43
votes
2
answers
139k
views
Naming list elements in R
I've been doing some work with some large, complex lists lately and I've seen some behaviour which was surprising (to me, at least), mainly to do with assigning names to a list. A simple example:
Fil ...
- 2,743
43
votes
7
answers
32k
views
Is there a more efficient way to replace NULL with NA in a list?
I quite often come across data that is structured something like this:
employees <- list(
list(id = 1,
dept = "IT",
age = 29,
sportsteam = "softball"),
...
- 1,167
43
votes
1
answer
1k
views
R: Why is the [[ ]] approach for subsetting a list faster than using $?
I've been working on a few projects that have required me to do a lot of list subsetting and while profiling code I realised that the object[["nameHere"]] approach to subsetting lists was usually ...
- 1,167
42
votes
5
answers
45k
views
Recombining a list of Data.frames into a single data frame [duplicate]
I am sorry if this question has been answered already. Also, this is my first time on stackoverflow.
I have a beginner R question concerning lists , data frames and merge() and/or rbind().
I ...
- 697