R:如何在不声明的情况下通过数组符号对列表进行子集化? [英] R: How can I subset a list via array notation without declaring it?

问题描述

作为我所追求的例子，假设我想要数字列表中3的每一个从1到100的倍数.我知道这样做的更简单方法如下:

As an example of what I'm after, suppose that I want every multiple of 3 in the list of numbers from 1 to 100. The easier way that I know of doing this is as follows:

all<-1:100  
mod3<-all[all%%3==0]

有没有第一行的方法吗?我必须声明一个列表(并因此给它起一个名字)，然后才能对其进行子集化，这似乎很愚蠢.我可以调用 subset 函数，但是第二行中的数组表示法很自然，以至于不使用它会很可惜.在理想的世界中，我会称呼类似 1:100 [1:100 %% 3 == 0] 的代码，但这显然会带来错误.仅提及R内置的内容，我有哪些选择?

Is there a way to do this without the first line? It seems silly that I have to declare a list (and therefore give it a name) before I can subset it. I could call the subset function, but the array notation in my second line comes so naturally that it would be a shame to not use it. In an ideal world, I'd call something like 1:100[1:100%%3==0] but that obviously gives errors. Referring only to things that are built-in to R, what are my options?

推荐答案

我们可以将其包装在方括号中，以使其成为一个独立的块，因为不同的运算符具有优先权

We could wrap within brackets to make it a self-contained block as there is precedence for different operators

(1:100)[(1:100)%%3==0]

---

如果选中?Syntax ，则会给出不同的运算符优先级

---

If the check the ?Syntax, the different operator precedence are given

#:: ::: access variables in a namespace
#$ @    component / slot extraction
#[ [[   indexing
#^  exponentiation (right to left)
#- +    unary minus and plus
#:  sequence operator
#%any%  special operators (including %% and %/%)
#* /    multiply, divide
#...

在上述情况下， [] 中的表达式可以按预期工作，因为:位于优先于 %% 的优先级之上.但是，这是一个外部问题，因为与:

In the above case, the expression within the [] works as expected because the : is above on the precedence wrt to %%. But, it is the one outside having an issue because [ is higher when compared to :

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