list 如何在R中索引列表对象的元素
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How to index an element of a list object in R
提问by Mohr
I'm doing the following in order to import some txt tables and keep them as list:
我正在执行以下操作以导入一些 txt 表并将它们保留为列表：
# 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)?
我想访问一个特定的元素，比如说hypo_list[1]。由于每个元素代表一个表格，我应该如何访问特定的单元格（行和列）？
I would like to do something like it:
我想做类似的事情：
a<-hypo_list[1]
a[1,2]
But I get the following error message:
但我收到以下错误消息：
Error in a[1, 2] : incorrect number of dimensions
Is there a clever way to do it?
有没有聪明的方法来做到这一点？
Thanks in advance!
提前致谢！
回答by Mark Heckmann
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.tabledoes 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.
索引列表是使用双括号完成的，即hypo_list[[1]]（例如，看看这里：http: //www.r-tutor.com/r-introduction/list）。顺便说一句：read.table不返回表而是返回数据帧（请参阅 中的值部分?read.table）。因此，您将拥有一个数据框列表，而不是一个表对象列表。不过，表和数据框的主要机制是相同的。
Note: In R, the index for the first entry is a 1(not 0like in some other languages).
注意：在 R 中，第一个条目的索引是 a 1（0与其他一些语言不同）。
Dataframes
数据帧
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
表对象
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
现在有了清单
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
