如果添加了print语句,则相同的Rcpp函数返回不同的输出

Question

我用Rcpp编写的C++函数给出了不同的输出,具体取决于我是否在代码中有一个Rcout或者Rprintf语句.1对于带有print语句的函数,下面的代码返回,这是正确的值H_sigma_1().但是,对于H_sigma_2()没有print语句的函数,函数返回2.我在Ubuntu 16.04.1以及CentOS 6.8上测试了这个.虽然,我无法在Windows 10上重现此错误.因此,这似乎是一个Linux问题.

Rcpp代码

library(Rcpp)

cppFunction ( 
  "double H_sigma_1(IntegerVector sigma, NumericMatrix J, NumericVector h)
  {
    double first_sum, second_sum = 0;
    int n = sigma.size();

    for(int i = 0; i < n; i++)
    {
      for(int j = 0; j < n; j++)
      {
        // skip inside loop if i >= j to stop double counting
        if(i >= j) {continue;}
        first_sum += J(i, j) * sigma[i] * sigma[j];
        Rcout << first_sum << std::endl;
      }
      second_sum += h[i] * sigma[i];
    }
    return(-1.0 * first_sum - second_sum);
  }"
)

cppFunction (
  "double H_sigma_2(IntegerVector sigma, NumericMatrix J, NumericVector h)
  {
    double first_sum, second_sum = 0;
    int n = sigma.size();

    for(int i = 0; i < n; i++)
    {
      for(int j = 0; j < n; j++)
      {
        // skip inside loop if i >= j to stop double counting
        if(i >= j) {continue;}
        first_sum += J(i, j) * sigma[i] * sigma[j];
        // Rcout << first_sum << std::endl;
      }
      second_sum += h[i] * sigma[i];
    }
    return(-1.0 * first_sum - second_sum);
  }"
)

示例电话

建立:

n = 2
params = rep(1, n) 
h = rep(params[1], n)
J = toeplitz(c(0, params[2], rep(0, n - 2)))

测试1:

H_sigma_1(c(-1, -1), J, h)

输出:

1
[1] 1

测试2

H_sigma_2(c(-1, -1), J, h)

产量

[1] 2

Answer 1

您遇到的问题是在使用它们之前没有明确声明sum的起始值.

double first_sum, second_sum = 0;

是不一样的:

double first_sum = 0, second_sum = 0;

请参阅编译警告标志:

file11d36f564b15.cpp:17:5: warning: variable 'first_sum' is uninitialized when used here [-Wuninitialized]
    first_sum += J(i, j) * sigma[i] * sigma[j];
    ^~~~~~~~~
file11d36f564b15.cpp:8:21: note: initialize the variable 'first_sum' to silence this warning
    double first_sum, second_sum = 0;

你马上使用:

first_sum += J(i, j) * sigma[i] * sigma[j];

没有设置 first_sum = ...;

此外,另一个问题是:

second_sum = 0;

使用整数值初始化double.虽然这个问题的范围很小,但为了纠正这个问题,所有人都要做的就是使用0.0而不是0.

second_sum = 0.0;

这也适用first_sum.

具有上述修复的代码:

library(Rcpp)

cppFunction ( 
    "double H_sigma_1(IntegerVector sigma, NumericMatrix J, NumericVector h)
    {
    double first_sum = 0.0, second_sum = 0.0;
    int n = sigma.size();

    for(int i = 0; i < n; i++) {
      for(int j = 0; j < n; j++) {
        // skip inside loop if i >= j to stop double counting
        if(i >= j) {continue;}
        first_sum += J(i, j) * sigma[i] * sigma[j];
        Rcout << first_sum << std::endl;
      }
      second_sum += h[i] * sigma[i];
    }
    return(-1.0 * first_sum - second_sum);
    }"
)

cppFunction ( 
    "double H_sigma_2(IntegerVector sigma, NumericMatrix J, NumericVector h)
    {
    double first_sum = 0.0, second_sum = 0.0;
    int n = sigma.size();

    for(int i = 0; i < n; i++) {
      for(int j = 0; j < n; j++) {
        // skip inside loop if i >= j to stop double counting
        if(i >= j) {continue;}
        first_sum += J(i, j) * sigma[i] * sigma[j];
      }
      second_sum += h[i] * sigma[i];
    }
    return(-1.0 * first_sum - second_sum);
    }"
)

测试:

n = 2
params = rep(1, n) 
h = rep(params[1], n)
J = toeplitz(c(0, params[2], rep(0, n - 2)))

H_sigma_1(c(-1, -1), J, h)

输出:

1
[1] 1

测试2:

H_sigma_2(c(-1, -1), J, h)

输出:

[1] 1