Why is yaml-cpp adding a thousands separator to numbers, and how do I prevent this?

FITFLOP

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02-10-2024

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YAML-cpp's Unexpected Thousand Separators: Understanding and Preventing the Issue

Have you ever encountered a situation where YAML-cpp, a popular C++ library for parsing and emitting YAML data, unexpectedly inserted thousands separators into your numeric values? This can cause problems when processing your YAML data, especially if your downstream systems don't anticipate these separators. This article explores the reasons behind this behavior and provides practical solutions to avoid it.

The Scenario

Imagine you have a YAML file with a simple numeric value:

population: 1000000

You might expect YAML-cpp to read this value as the integer 1000000. However, you encounter an issue where the library reads it as 1,000,000 instead, introducing a thousands separator that breaks your downstream processing.

The Root Cause

The culprit lies in YAML-cpp's default behavior for numeric formatting. By default, the library uses the locale setting of your system to determine the appropriate formatting for numbers. In many locales, this includes adding thousands separators for readability.

The Solution

There are two primary ways to address this issue:

1. Explicitly Set the Locale:

   The most straightforward solution is to set the locale explicitly to a locale that does not use thousands separators. For example, you could set the locale to "C" or "POSIX", which use a standard format without separators:

   #include <locale>
   #include <yaml-cpp/yaml.h>
   
   int main() {
       std::locale::global(std::locale("C")); // Set global locale to "C"
   
       YAML::Node node;
       node["population"] = 1000000;
   
       std::cout << node["population"].as<int>() << std::endl; // Output: 1000000
   }
   

2. Override the Default Formatting:

   If you prefer to work with your system's locale but avoid thousands separators, you can override the default formatting behavior of YAML-cpp. This can be achieved using the YAML::Emitter class and its set_precision and set_decimal_point functions:

   #include <yaml-cpp/yaml.h>
   
   int main() {
       YAML::Emitter out;
       out.set_precision(0); // Remove decimal places
       out.set_decimal_point("."); // Set decimal point to a dot (if needed)
   
       out << YAML::BeginMap;
       out << YAML::Key << "population" << YAML::Value << 1000000;
       out << YAML::EndMap;
   
       std::cout << out.c_str() << std::endl; // Output: population: 1000000
   }
   

Important Considerations

• Consistency: Always ensure your YAML files and the libraries you use to process them maintain consistent formatting, especially regarding decimal points and thousands separators.
• Documentation: Refer to the YAML-cpp documentation (https://github.com/jbeder/yaml-cpp) for the latest information and best practices.
• Alternative Libraries: Consider exploring other YAML libraries if the default behavior of YAML-cpp doesn't meet your specific needs.

Conclusion

Understanding the underlying causes of unexpected behavior in YAML-cpp, like the insertion of thousands separators, is crucial for robust data processing. By employing the appropriate locale settings or customizing the formatting, you can avoid these inconsistencies and ensure seamless data handling. Remember, consistency and proper documentation are key to smooth YAML processing workflows.