R How do I extract first row of each matrix within a list?

Question

Say I have a list called mylist_1 with 2 matrices:

mylist_1

$region_1           
users   50  20  30
revenue 10000   3500    4000

$red            
users   20  20  60
revenue 5000    4000    10000

How do I extract the first row of each matrix into its own matrix?

i.e. output (first column here are rownames):

region_1    50  20  30
region_2    20  20  60

or the second row of each matrix?

region_1    10000   3500    4000
region_2    5000    4000    10000

Is there a way to reference the list/matrices to do this?

Thanks

score 12 · Accepted Answer · answered Aug 07 '14 at 09:18

12

Or,

lapply(mylist_1, `[`,1,)
lapply(mylist_1, `[`,2,)

answered Aug 07 '14 at 09:18

akrun

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@shadow. I am not entirely sure about the reason. – akrun Aug 07 '14 at 09:28
I'm just about to ask a question about this odd behavior. – shadow Aug 07 '14 at 09:29
@shadow, that would be great – akrun Aug 07 '14 at 09:31
1

It's not odd behavior. `lapply` is not `sapply`. They each have a different purpose. `lapply` is to be applied over a list. `sapply` only works on lists under specific conditions. It's all in the help file/ – Rich Scriven Aug 07 '14 at 09:32
I like this, but it keeps it as a list (I want the output to be a matrix) – shecode Aug 07 '14 at 09:33
@shecode. thanks. then `sapply()` is the better option. – akrun Aug 07 '14 at 09:34
1

`do.call("rbind", lapply(mylist, "[", 2, ))` – Rich Scriven Aug 07 '14 at 09:34
Ok, remember to transpose!! Haha – Rich Scriven Aug 07 '14 at 09:36
@RichardScriven. Can you enlighten me as to where in the help file this is. I could not find it. Feel free to answer this question: http://stackoverflow.com/questions/25179109/different-behaviors-when-passing-empty-ellipsis-arguments – shadow Aug 07 '14 at 09:45
@RichardScriven - sapply should always work on a list. Whether you get sensible results depends on what you try to return. – thelatemail Aug 07 '14 at 11:21
@akrun is there a way to modified ```lapply(mylist_1, `[`,1,)``` so you can extract instead of the first row the first cell (1,1)? – Jonathan1234 Nov 17 '21 at 19:02
@Jonathan1234 if it is a matrix, then you can just do `lapply(mylist_1, `[`,1)` – akrun Nov 17 '21 at 19:04
@Jonathan1234 example is `my_list <- list(matrix(1:6, 2, 3), matrix(1:6, 2, 3)) > lapply(my_list, `[`, 1)` – akrun Nov 17 '21 at 19:05
@akrun and we do not have to use ``` "["```? – Jonathan1234 Nov 18 '21 at 10:52
You can use both – akrun Nov 18 '21 at 16:09

score 7 · Answer 2 · answered Aug 07 '14 at 09:11

7

To extract the first row per matrix you can use:

lapply(mylist1, head, 1)

Or, if you want to rbind them:

do.call(rbind, lapply(lst, head, 1))

Or for (only) the second row per matrix:

lapply(lst, function(x) x[2,])

answered Aug 07 '14 at 09:11

talat

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OR `t(sapply(mylist_1, "[", 1, TRUE))` and `t(sapply(mylist_1, "[", 2, TRUE))` – shadow Aug 07 '14 at 09:15
@shadow, good points! `lapply(lst, `[`, 2, TRUE)` should also work then :) (for the second row) – talat Aug 07 '14 at 09:19

R How do I extract first row of each matrix within a list?

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