相当于 Rcpp 中的“哪个"函数 [英] equivalent of 'which' function in Rcpp

问题描述

我是 C++ 和 Rcpp 的新手.假设，我有一个向量

I'm a newbie to C++ and Rcpp. Suppose, I have a vector

t1<-c(1,2,NA,NA,3,4,1,NA,5)

并且我想获得 NA 的 t1 元素的索引.我可以写:

and I want to get a index of elements of t1 that are NA. I can write:

NumericVector retIdxNA(NumericVector x) {

    // Step 1: get the positions of NA in the vector
    LogicalVector y=is_na(x);

    // Step 2: count the number of NA
    int Cnt=0;
    for (int i=0;i<x.size();i++) {
       if (y[i]) {
         Cnt++;
       }
    }

    // Step 3: create an output matrix whose size is same as that of NA
    // and return the answer
    NumericVector retIdx(Cnt);
    int Cnt1=0;
    for (int i=0;i<x.size();i++) {
       if (y[i]) {
          retIdx[Cnt1]=i+1;
          Cnt1++;
       }
    }
    return retIdx;
}

然后我得到

retIdxNA(t1)
[1] 3 4 8

我在想:

(i) 在 Rcpp 中是否有任何等效的 which ?

(i) is there any equivalent of which in Rcpp?

(ii) 有没有办法让上述函数更短/更清晰?特别是，有没有什么简单的方法可以将上面的步骤 1、2、3 结合起来?

(ii) is there any way to make the above function shorter/crisper? In particular, is there any easy way to combine the Step 1, 2, 3 above?

推荐答案

RcppArmadillo 的最新版本 具有识别有限和非有限值的索引的功能.

Recent version of RcppArmadillo have functions to identify the indices of finite and non-finite values.

所以这段代码

#include <RcppArmadillo.h>
// [[Rcpp::depends(RcppArmadillo)]]

// [[Rcpp::export]]
arma::uvec whichNA(arma::vec x) {
  return arma::find_nonfinite(x);
}

/*** R
t1 <- c(1,2,NA,NA,3,4,1,NA,5)
whichNA(t1)
*/

产生您想要的答案(在 C/C++ 中将非 1 模块化，因为它们是从零开始的):

yields your desired answer (module the off-by-one in C/C++ as they are zero-based):

R> sourceCpp("/tmp/uday.cpp")

R> t1 <- c(1,2,NA,NA,3,4,1,NA,5)

R> whichNA(t1)
     [,1]
[1,]    2
[2,]    3
[3,]    7
R> 

如果您首先创建要子集的序列，Rcpp 也可以这样做:

Rcpp can do it too if you first create the sequence to subset into:

// [[Rcpp::export]]
Rcpp::IntegerVector which2(Rcpp::NumericVector x) {
  Rcpp::IntegerVector v = Rcpp::seq(0, x.size()-1);
  return v[Rcpp::is_na(x)];
}

添加到上面的代码产生:

Added to code above it yields:

R> which2(t1)
[1] 2 3 7
R> 

Rcpp 中的逻辑子集也有些新.

The logical subsetting is also somewhat new in Rcpp.

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