在 R 中传递空索引 [英] Passing empty index in R

问题描述

假设我想对向量 a 进行子集化，我可以将索引的值传递给变量中的子集，例如a[idx].

Say I want to subset a vector a, I can pass the value of the indices to subset in a variable e.g. a[idx].

我应该设置什么值 idx 以获得等效于获取整个 a(即 a[] )?

What value should I set idx to get the equivalent of getting the whole a ( i.e. a[] ) ?

基本上我有一个以 idx 作为参数的函数，并且想传递一个值来处理整个数据集.我假设应该有比 1:length(a) 更好的东西.

Basically I have a function with idx as the argument, and would like to pass a value to process the whole dataset. I'm assuming there should be something better than 1:length(a).

推荐答案

子集的索引参数允许缺失"(见?"["):

The index argument in subsetting is allowed to be "missing" (see ?"["):

ff1 = function(x, i) x[i] 
ff2 = function(x, i = TRUE) x[i] 
ff3 = function(x, i = seq_along(x)) x[i]
ff4 = function(x, i = substitute()) x[i]

a = sample(10)
a
# [1]  3  8  2  6  9  7  5  1  4 10
ff1(a)
# [1]  3  8  2  6  9  7  5  1  4 10
ff2(a)
# [1]  3  8  2  6  9  7  5  1  4 10
ff3(a)
# [1]  3  8  2  6  9  7  5  1  4 10
ff4(a)
# [1]  3  8  2  6  9  7  5  1  4 10


a = runif(1e6)
identical(ff1(a), ff2(a))
#[1] TRUE
identical(ff1(a), ff3(a))
#[1] TRUE
identical(ff1(a), ff4(a))
#[1] TRUE
microbenchmark::microbenchmark(ff1(a), ff2(a), ff3(a), ff4(a), times = 25)
#Unit: milliseconds
#   expr       min        lq    median        uq       max neval
# ff1(a)  2.026772  2.131173  2.207037  2.930885  3.789409    25
# ff2(a) 12.091727 12.151931 12.318625 12.740057 16.829445    25
# ff3(a)  8.930464  9.104118  9.454557  9.643175 13.131213    25
# ff4(a)  2.024684  2.090108  2.156577  2.289166  3.496391    25

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