[英]R: Extracting Data Frame from list of CSV file data
So I come from a background of Matlab and Python (and several others less related). 所以我来自Matlab和Python(以及其他一些不太相关的背景)。 I'm picking up R for a Coursera course.
我正在为Coursera课程学习R。
I followed this SO answer in order to read in all my homework files into a list in a single line of code. 我遵循了这样的答案 ,以便将我的所有作业文件读入单行代码的列表中。 My code looks like this:
我的代码如下所示:
# Get a list of files
files = list.files(path = dataDir, pattern = '*.csv')
# Import the file data
setwd(dataDir)
data = lapply(files, read.csv)
This all works just fine. 这一切都很好。 However, I am getting a object back that I don't know how to access.
但是,我找回了一个我不知道如何访问的对象。 I mentioned Matlab and Python before because I've attempted to access the data in all the ways I would in those languages.
我之前提到过Matlab和Python,是因为我试图用我在那些语言中使用的所有方式来访问数据。
Here's what summary output: 这是摘要输出:
summary(data)
Length Class Mode
[1,] 4 data.frame list
[2,] 4 data.frame list
[3,] 4 data.frame list
There are actually 352 of them not just 3 but no one needs a listing of all 352. Here's what summary
of an individual index outputs: 实际上,其中有352个不仅3个,而且没有人需要列出所有352个。这是单个索引输出的
summary
:
summary(data[200])
Length Class Mode
[1,] 4 data.frame list
So if I enter data[200]
I get listing of the first 2500 rows of data. 因此,如果我输入
data[200]
则会得到前2500行数据的列表。 But data[200, 100]
returns as error as does data[200][,100]
and data[200][100,]
. 但是
data[200, 100]
以及data[200][,100]
和data[200][100,]
一样作为错误返回。 data[200][100]
returns [[1]] NULL
. data[200][100]
返回[[1]] NULL
。
While I haven't fully considered what I will need to do for this homework I'm sure it will involve calculating means/medians/maximum/etc of all non-NA values in various data columns. 尽管我还没有完全考虑完成此家庭作业需要做什么,但我确定它将涉及计算各种数据列中所有非NA值的均值/中位数/最大值/等。 This wasn't tough to do for the quizzes using something like
mean(data[which(is.na('Col1')==F), 'Col6'])
. 对于使用诸如
mean(data[which(is.na('Col1')==F), 'Col6'])
类的测验mean(data[which(is.na('Col1')==F), 'Col6'])
。
So I imagine I could use a more hackish version of what I need where I simply load the 1 file I need at the time I need it, extract only the portion of the data frame I need right then, and loop over all the data files I need to process. 因此,我想我可以在需要的地方使用更hackish的版本,只需在需要时加载所需的1个文件,然后仅提取所需的数据帧部分,然后遍历所有数据文件我需要处理。 However, I'd rather know how to access the data in the object R creates from the
lapply
line. 但是,我宁愿知道如何访问
lapply
行中R创建的对象中的数据。 I suspect this will make more complex analyses later on much easier to code. 我怀疑这将使以后更复杂的分析变得更容易编写代码。
Thanks 谢谢
When you subset, single square brackets [
always return an object of the same class as the object you are subsetting. 子集化时,单个方括号
[
总是返回与您要设置的对象相同类的对象。 So, data[200]
returns a list
of length 1 containing one dataframe because data
is a list. 因此,
data[200]
返回一个包含一个数据帧的长度为1的list
,因为data
是一个列表。 Double square brackets [[
give you the actual object contained in the list (in this case, a dataframe). 双方括号
[[
为您提供列表中包含的实际对象(在这种情况下,为数据框)。 Once you have a dataframe, you can select the first 100 rows with [100,]
, which is why the following works: 有了数据框后,就可以使用
[100,]
选择前100行,这就是以下原因的原因:
data[[200]][100,]
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