因子列上的sum(。)返回不正确的结果 [英] sum(.) on a factor column returns incorrect result
问题描述
我在这里有一个奇怪的修复。我使用 data.table
作为一个非常常规的任务,但有一些我不能解释。我已经找出了解决这个问题的方法,但我认为仍然重要的是,我明白这里发生了什么。
这段代码将数据带入工作区
库(XML)
库(data.table)
theurl < - //goo.gl/hOKW3a
tables< - readHTMLTable(theurl)
new.Res< - data.table(tables [[2]] [4:5] [ - 2),])
suppressWarnings(names(new.Res)< - c(Party,Cases))
这里有两个列, Party
和 Cases
。两者都具有默认类 factor
。虽然 Cases
应该是 numeric
。最后,我只需要 Cases
的总和 Party
。所以这样的东西应该工作:
new.Res [,sum(Cases),by = Party]
但这没有给出正确的答案。我认为如果我将 Cases
的类从因子
更改为数字
。所以我试了下面的:
new.Res [,Cases:= as.numeric(Cases)]
new .Res [,sum(Cases),by = Party]
但我得到了同样的错误答案。我意识到问题发生在将 Cases
的类从因子
更改为 numeric
。所以我尝试了一种不同的方法,它的工作原理:
Step1:重新初始化数据:
<
theurl < - http://goo.gl/hOKW3a
tables< - readHTMLTable(theurl)
new.Res< - data.table(tables [ 2]] [4:5] [ - (1:2),])
suppressWarnings(names(new.Res)< - c(Party,Cases))
步骤2:使用不同的方法将类从 factor
更改为 numeric
:
new.Res [,Cases:= strtoi ]
new.Res [,sum(Cases),by = Party]
精细!但是,我不知道前两种方法有什么问题。 c
到数字
或整数
是通过字符
。这是因为在内部,一个因素是一个整数索引(指的是 levels
向量)。当你告诉R将它转换为 numeric
时,它将简单地转换相关的索引,而不是尝试转换级别标签。
简短的答案:do Cases:= as.numeric(as.character(Cases))
。
修改:?factor
帮助页面建议 as.numeric(levels(Cases))[Cases]
更高效。 h / t @见注释。
I am in a strange fix here. I am using data.table
for a very routine task, but there is something that I am not able to explain. I have figured out a way around the problem, but I think it is still important for me to understand what is going wrong here.
This code will bring the data into workspace:
library(XML)
library(data.table)
theurl <- "http://goo.gl/hOKW3a"
tables <- readHTMLTable(theurl)
new.Res <- data.table(tables[[2]][4:5][-(1:2),])
suppressWarnings(names(new.Res) <- c("Party","Cases"))
There are two columns here, Party
and Cases
. Both of which have the default class of factor
. Although, Cases
should be numeric
. Ultimately, I just want the sum of Cases
for each Party
. So something like this should work:
new.Res[,sum(Cases), by=Party]
But this doesn't give the right answer. I thought that it'll work if I change the class of Cases
from factor
to numeric
. So I tried the following:
new.Res[,Cases := as.numeric(Cases)]
new.Res[,sum(Cases), by=Party]
But I got the same incorrect answer. I realized that the problem is happening in changing the class of Cases
from factor
to numeric
. So I tried a different method, and it worked:
Step1: Reinitialize the data:
theurl <- "http://goo.gl/hOKW3a"
tables <- readHTMLTable(theurl)
new.Res <- data.table(tables[[2]][4:5][-(1:2),])
suppressWarnings(names(new.Res) <- c("Party","Cases"))
Step2: Use a different method to change the class from factor
to numeric
:
new.Res[,Cases := strtoi(Cases)]
new.Res[,sum(Cases), by=Party]
This works fine! However, I am not sure what's wrong with the first two methods. What am I missing?
解决方案 The correct way to convert from factor
to numeric
or integer
is to go through character
. This is because internally, a factor is an integer index (that refers to a levels
vector). When you tell R to convert it to numeric
it will simply convert the underlying index, not try to convert the level label.
Short answer: do Cases:=as.numeric(as.character(Cases))
.
Edit: Alternatively the ?factor
help page suggests as.numeric(levels(Cases))[Cases]
as more efficient. h/t @Gsee in the comments.
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