How to index an element of a list object in R

Question

I'm doing the following in order to import some txt tables and keep them as list:

# set working directory - the folder where all selection tables are stored hypo_selections<-list.files() # change object name according to each species hypo_list<-lapply(hypo_selections,read.table,sep="\t",header=T) # change object name according to each species 

I want to access one specific element, let's say hypo_list[1]. Since each element represents a table, how should I procced to access particular cells (rows and columns)?

I would like to do something like it:

a<-hypo_list[1]  a[1,2] 

But I get the following error message:

Error in a[1, 2] : incorrect number of dimensions 

Is there a clever way to do it?

Thanks in advance!

Answer 1

Indexing a list is done using double bracket, i.e. hypo_list[[1]] (e.g. have a look here: http://www.r-tutor.com/r-introduction/list). BTW: read.table does not return a table but a dataframe (see value section in ?read.table). So you will have a list of dataframes, rather than a list of table objects. The principal mechanism is identical for tables and dataframes though.

Note: In R, the index for the first entry is a 1 (not 0 like in some other languages).

Dataframes

l <- list(anscombe, iris)   # put dfs in list l[[1]]             # returns anscombe dataframe  anscombe[1:2, 2]   # access first two rows and second column of dataset [1] 10  8  l[[1]][1:2, 2]     # the same but selecting the dataframe from the list first [1] 10  8 

Table objects

tbl1 <- table(sample(1:5, 50, rep=T)) tbl2 <- table(sample(1:5, 50, rep=T)) l <- list(tbl1, tbl2)  # put tables in a list  tbl1[1:2]              # access first two elements of table 1  

Now with the list

l[[1]]                 # access first table from the list  1  2  3  4  5  9 11 12  9  9   l[[1]][1:2]            # access first two elements in first table  1  2  9 11