如何处理R到Rcpp中的列表 [英] How to handle list in R to Rcpp
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问题描述
我在R中有一个列表x <-list(c(1,2,3),c(4,5),c(5,5),c(6)).我想将列表输入Rcpp并将其作为平均向量c(2,4.5,5,6)返回.
I have a list in R that x<-list(c(1,2,3), c(4,5), c(5,5), c(6)). I want to input the list to Rcpp and return them as an average vector, c(2, 4.5, 5, 6).
我不确定如何在Rcpp中处理列表.我收到一条错误消息,有人可以检查我的代码吗?
I am not sure how to handle the list in Rcpp. I got an error message, so could someone check my code?
library(inline)
fx = cxxfunction(signature(x='List'), body =
'
Rcpp::List xlist(x);
int n = xlist.size();
double res[n];
for(int i=0; i<n; i++) {
Rcpp NumericVector y(xlist[i]);
int m=y.size();
res[i]=0;
for(int j=0; j<m; j++){
res[i]=res[i]+y[j]
}
}
return(wrap(res));
'
, plugin='Rcpp')
x<-list(c(1,2,3), c(4,5), c(5,5), c(6))
fx(x)
推荐答案
此处有一些小错误:
- 两个语法错误:
y
需要Rcpp::NumericVector
,并且在最后一个循环中缺少分号. - 对C ++的一个误解:您需要类似
std::vector<double> res(n);
的东西,因为n
在编译时是未知的. - 在实例化列表中的向量时,您过于积极/乐观,我在两个语句中做到了这一点.
- Two syntax errors: you need
Rcpp::NumericVector
fory
, and you lack a semicolon in the last loop. - One misunderstanding of C++: you need something like
std::vector<double> res(n);
asn
is not known at compile time. - You were too aggressive / optimistic in instantiating your vectors from the list, I did this in two statements.
此版本有效:
R> fx <- cxxfunction(signature(x='List'), plugin='Rcpp', body = '
+ Rcpp::List xlist(x);
+ int n = xlist.size();
+ std::vector<double> res(n);
+
+ for(int i=0; i<n; i++) {
+ SEXP ll = xlist[i];
+ Rcpp::NumericVector y(ll);
+ int m=y.size();
+ res[i]=0;
+ for(int j=0; j<m; j++){
+ res[i]=res[i]+y[j];
+ }
+ }
+
+ return(Rcpp::wrap(res));
+ ')
R> x<-list(c(1,2,3), c(4,5), c(5,5), c(6))
R> fx(x)
[1] 6 9 10 6
R>
这是一个更加惯用的版本:
Here is a version that is a little more idiomatic:
fx <- cxxfunction(signature(x='List'), plugin='Rcpp', body = '
Rcpp::List xlist(x);
int n = xlist.size();
Rcpp::NumericVector res(n);
for(int i=0; i<n; i++) {
SEXP ll = xlist[i];
Rcpp::NumericVector y(ll);
for(int j=0; j<y.size(); j++){
res[i] += y[j];
}
}
return(res);
')
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