Rcpp 中列表的索引元素 [英] Index element from list in Rcpp

问题描述

假设我在 Rcpp 中有一个 List，这里称为 x 包含矩阵.我可以使用 x[0] 或其他东西提取其中一个元素.但是，如何提取该矩阵的特定元素?我的第一个想法是 x[0](0,0) 但这似乎不起作用.我尝试使用 * 标志但也不起作用.

Suppose I have a List in Rcpp, here called x containing matrices. I can extract one of the elements using x[0] or something. However, how do I extract a specific element of that matrix? My first thought was x[0](0,0) but that does not seem to work. I tried using * signs but also doesn't work.

这是一些打印矩阵的示例代码(显示矩阵可以轻松提取):

Here is some example code that prints the matrix (shows matrix can easily be extracted):

library("Rcpp")

cppFunction(
includes = ' 
NumericMatrix RandMat(int nrow, int ncol)
 {
  int N = nrow * ncol;
  NumericMatrix Res(nrow,ncol);
  NumericVector Rands  = runif(N);
   for (int i = 0; i < N; i++) 
  {
    Res[i] = Rands[i];
  }
  return(Res);
 }',

code = '
void foo()
{
  List x;
  x[0] = RandMat(3,3);
  Rf_PrintValue(wrap( x[0] )); // Prints first matrix in list.
}
')


foo()

如何更改 Rf_PrintValue(wrap( x[0] )); 行以打印第一行和第一列中的元素?在我想使用它的代码中，我需要提取这个元素来进行计算.

How could I change the line Rf_PrintValue(wrap( x[0] )); here to print the the element in the first row and column? In the code I want to use it for I need to extract this element to do computations.

推荐答案

快速解决方案:

1. C++ 中的复合表达式有时会咬人；模板魔法妨碍了.因此，只需将 List 对象分配给任何元素，例如 NumericMatrix.

1. Compound expression in C++ can bite at times; the template magic gets in the way. So just assign from the List object to a whatever the element is, eg a NumericMatrix.

然后从 NumericMatrix 中选择您认为合适的.我们有行、列、元素、...访问.

Then pick from the NumericMatrix as you see fit. We have row, col, element, ... access.

使用 Rcpp::Rcout << 可以更轻松地打印anElement 但请注意，我们目前无法打印整个矩阵或向量——但 int 或 double 类型很好.

Printing can be easier using Rcpp::Rcout << anElement but note that we currently cannot print entire matrices or vectors -- but the int or double types are fine.

这是一个示例实现.

#include <Rcpp.h>

// [[Rcpp::export]]
double sacha(Rcpp::List L) {
    double sum = 0;
    for (int i=0; i<L.size(); i++) {
        Rcpp::NumericMatrix M = L[i];
        double topleft = M(0,0);
        sum += topleft;
        Rcpp::Rcout << "Element is " << topleft << std::endl;
    }
    return sum;    
}

/*** R
set.seed(42)
L <- list(matrix(rnorm(9),3), matrix(1:9,3), matrix(sqrt(1:4),2))
sacha(L) # fix typo   
*/

及其结果:

R> Rcpp::sourceCpp('/tmp/sacha.cpp')

R> set.seed(42)

R> L <- list(matrix(rnorm(9),3), matrix(1:9,3), matrix(sqrt(1:4),2))

R> sacha(L)
Element is 1.37096
Element is 1
Element is 1
[1] 3.37096
R>

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