Understanding the Pitfalls of Rcpp Functions and Print Statements: A Common Issue on Linux Platforms?

Understanding the Issue with Rcpp Functions and Print Statements

As a developer working with C++ and R, it’s not uncommon to encounter issues that seem peculiar at first glance. In this article, we’ll delve into a specific scenario involving Rcpp functions and print statements.

A user encountered an issue where a C++ function they wrote using Rcpp gave different output depending on whether or not they had a Rcout or Rprintf statement in the code. This phenomenon seemed to occur specifically on Linux platforms, with the issue being reproducible on Ubuntu 16.04.1 and CentOS 6.8 but not Windows 10.

The Rcpp Code

To understand this issue, let’s first examine the Rcpp code provided by the user:

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];
      }
      second_sum += h[i] * sigma[i];
    }
    return(-1.0 * first_sum - second_sum);
  }"
)

The Issue

The problem lies in the way Rcpp handles variables with default values. In C++, when a variable is declared without an initial value, it will have a garbage value assigned to it. However, in Rcpp, this behavior differs from that of regular C++ code.

When you declare two variables with the same name but different data types (like double first_sum and int n) inside a function, they are treated as separate entities in Rcpp’s scope. The issue arises when these variables are used without explicit declarations.

In this case, both first_sum and second_sum have default values assigned to them, which is an integer value (0) instead of a double (0.0). This discrepancy leads to different outputs when using the functions with or without print statements.

The Solution

To fix this issue, you need to declare these variables explicitly as doubles:

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);
  }"
)

Conclusion

In conclusion, the issue with Rcpp functions and print statements is primarily due to variable declarations without explicit type definitions. By explicitly declaring variables as double, we ensure consistency in their data types, preventing unexpected behavior caused by default assignments.

The provided code example demonstrates how to correctly declare these variables as doubles, ensuring consistent outputs for both functions.

Last modified on 2024-07-22