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POR: Passing a data frame by reference
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copied!<p>R has pass-by-value semantics, which minimizes accidental side effects (a good thing). However, when code is organized into many functions/methods for reusability/readability/maintainability and when that code needs to manipulate large data structures through, e.g., big data frames, through a series of transformations/operations the pass-by-value semantics leads to a lot of copying of data around and much heap thrashing (a bad thing). For example, a data frame that takes 50Mb on the heap that is passed as a function parameter will be copied at a minimum the same number of times as the function call depth and the heap size at the bottom of the call stack will be N*50Mb. If the functions return a transformed/modified data frame from deep in the call chain then the copying goes up by another N.</p> <p>The SO question <a href="https://stackoverflow.com/questions/596819/what-is-the-best-way-to-avoid-passing-a-data-frame-around">What is the best way to avoid passing a data frame around?</a> touches this topic but is phrased in a way that avoids directly asking the pass-by-reference question and the winning answer basically says, "yes, pass-by-value is how R works". That's not actually 100% accurate. R environments enable pass-by-reference semantics and OO frameworks such as <a href="http://code.google.com/p/r-proto/" rel="nofollow noreferrer">proto</a> use this capability extensively. For example, when a proto object is passed as a function argument, while its "magic wrapper" is passed by value, to the R developer the semantics are pass-by-reference.</p> <p>It seems that passing a big data frame by reference would be a common problem and I'm wondering how others have approached it and whether there are any libraries that enable this. In my searching I have not discovered one. </p> <p>If nothing is available, my approach would be to create a proto object that wraps a data frame. I would appreciate pointers about the syntactic sugar that should be added to this object to make it useful, e.g., overloading the $ and [[ operators, as well as any gotchas I should look out for. I'm not an R expert.</p> <p>Bonus points for a type-agnostic pass-by-reference solution that integrates nicely with R, though my needs are exclusively with data frames.</p>

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