在 Rcpp 的嵌套列表中提取元素

Question

我正在尝试从列表中提取一个元素并使用 Rcpp 对其值进行处理。但我无法完成任务。

这基本上是我想要在 R 代码中实现的目标：

mylist = list(steps = list(`1` = list(a = 7, b = "abc"), 
                           `2` = list(a = 3), 
                           `3` = list(a = 5, b = NULL)))
# This is the desired behavior that I can program in R
step_name = "1"
b = ifelse(is.null(mylist$steps[[step_name]][["b"]]), 
           "", mylist$steps[[step_name]][["b"]])
# Do something with the value of b

以下代码无法对 b 进行赋值。 a 的值按原样提取。我不知道我在这里错过了什么。

library(Rcpp)
cppFunction('int foo(Rcpp::List x, std::string step_name) {
  Rcpp::List steps = x("steps");
  Rcpp::List step = steps(step_name);
  int a = step("a");
  //// Not declaring b causes "not declared in this scope" error 
  //// so I initialize it with a random value.
  std::string b = "random";
  if (step.containsElementNamed("b")){
    Rcout << "b is in List!" << "\n";
    if (!Rf_isNull(step("b"))) {
      Rcout << "b is not NULL!" << "\n";
      if (TYPEOF(step("b")) == STRSXP)
        Rcout << "b is character!" << "\n";
      std::string b = step("b");
    } else {
      Rcout << "b is NULL!" << "\n";
      std::string b = "efg";
    }          
  } else {
    Rcout << "b is not in List!" << "\n";
    std::string b = "xyz";
  }
  Rcout << "The Value of b is " << b << ".\n";
  if (b == "abc") {
    //// Do something with the value of b
  }       
  return a;
}')

foo(mylist, "1")
## b is in List!
## b is not NULL!
## b is character!
## The Value of b is random.
## [1] 7
foo(mylist, "2")
## b is not in List!
## The Value of b is random.
## [1] 3
foo(mylist, "3")
## b is in List!
## b is NULL!
## The Value of b is random.
## [1] 5

Answer 1

这似乎主要是范围问题和变量类型问题。我相信 if then 语句中的 std::string b 声明是本地的。那里的任何变化都不会持久。

然后，您评论中的错误是试图将左轴 std::string 分配给右轴 Rcpp:Vector。虽然我确信有更好的方法来转换 and/or 简化，但下面的解决方案只是声明 Rcpp::CharacterVector b.

library(Rcpp)
cppFunction('int foo(Rcpp::List x, std::string step_name) {
  Rcpp::List steps = x("steps");
  Rcpp::List step = steps(step_name);
  int a = step("a");
  //// Not declaring b causes "not declared in this scope" error 
  //// so I initialize it with a random value.
  CharacterVector b; #############
  if (step.containsElementNamed("b")){
    Rcout << "b is in List!" << "\n";
    if (!Rf_isNull(step("b"))) {
      Rcout << "b is not NULL!" << "\n";
      if (TYPEOF(step("b")) == STRSXP)
        Rcout << "b is character!" << "\n";
      b = step("b");
    } else {
      Rcout << "b is NULL!" << "\n";
    }          
  } else {
    Rcout << "b is not in List!" << "\n";
  }
  Rcout << "The size of b is " << b.size() << ".\n"; #########
  if (b.size() > 0 && b[0] == "abc") { ################
    Rcout << "Do something with b.\n";
    //// Do something with the value of b
  }       
  return a;
}')

mylist = list(steps = list(`1` = list(a = 7, b = "abc"), 
                           `2` = list(a = 3), 
                           `3` = list(a = 5, b = NULL)))

foo(mylist, "1")
# b is in List!
# b is not NULL!
# b is character!
# The size of b is 1.
# Do something with b.
# [1] 7
foo(mylist, "2")
# b is not in List!
# The size of b is 0.
# [1] 3
foo(mylist, "3")
# b is in List!
# b is NULL!
# The size of b is 0.
# [1] 5

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