英文:
In data.table in R, how can you reference columns that have `<=` in their names without making R think its an operation?
问题
假设我们在R中有以下的data.table对象，我们希望按照x<=0.5 == 1的条件对数据进行子集操作：
x=rnorm(1000)
df <- data.table(x=x, `x<=0.5`=1*(x<=0.5))
df[`x<=0.5` == 1]
这会返回一个错误信息：
在一个'on'语句中找到了多个操作符：x<=0.5==x<=0.5。请指定一个单一的操作符。
我如何在不出现这个错误的情况下引用它？我认为问题出在它将小于或等于符号视为操作符而不是名称。
英文:
Suppose we have the following data.table object in R, and we wish to subset the data by those where x<=0.5 == 1:
x=rnorm(1000)
df <- data.table(x=x, `x<=0.5`=1*(x<=0.5))
df[`x<=0.5` == 1]
This returns an error saying:
Error in .parse_on(substitute(on), isnull_inames): Found more than one operator in one 'on' statement: x<=0.5==x<=0.5. Please specify a single operator.
How can I actually reference it without this error? I assume it is because it thinks the less than or equal sign is not a name but an operation.
答案1
得分: 2
使用 get 函数可以从字符串中获取值。
df[get("x<=0.5") == 1]
输出结果：
x x<=0.5
1: 0.3903123 1
2: 0.1426477 1
3: -0.3169492 1
4: -1.3024570 1
5: -1.5403663 1
---
691: -0.4453730 1
692: -2.3406638 1
693: -0.3276438 1
694: 0.4063935 1
695: -0.5860276 1
或者将它用在 .SDcols 中：
df[df[, .SD[[1]] == 1, .SDcols = "x<=0.5"]]
英文:
We may use get on a string to return the value
df[get("x<=0.5") == 1]
-output
x x<=0.5
1: 0.3903123 1
2: 0.1426477 1
3: -0.3169492 1
4: -1.3024570 1
5: -1.5403663 1
---
691: -0.4453730 1
692: -2.3406638 1
693: -0.3276438 1
694: 0.4063935 1
695: -0.5860276 1
Or use it in .SDcols
df[df[, .SD[[1]] == 1, .SDcols = "x<=0.5"]]
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