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renkun-ken committed Mar 9, 2014
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Expand Up @@ -27,7 +27,7 @@ Look at the labels on the axes. How did R know that the variable on the x axis w

The goal of this chapter is to help you understand NSE in existing R code, and to show you how to write your own functions that use it. In [capturing expressions](#capturing-expressions) you'll learn how to capture unevaluated expressions using `substitute()`. In [non-standard evaluation](#subset) you'll learn how `subset()` combines `substitute()` with `eval()` to allow you to succinctly to select rows from a data frame. [Scoping issues](#scoping-issues) will teach you about the scoping issues that arise in NSE, and show you how to resolve them.

NSE is great for interactive use, but can be hard to program with. [Calling from another function](#calling-from-another-function) shows why every function that uses NSE should have an escape hatch, a version that uses regular evaluation. Not all functions have an escape hatch, so [substitute](#substitute) shows you how you to can use `substitute()` to modify unevaluated cocde, making it possible to program with any function that uses NSE.
NSE is great for interactive use, but can be hard to program with. [Calling from another function](#calling-from-another-function) shows why every function that uses NSE should have an escape hatch, a version that uses regular evaluation. Not all functions have an escape hatch, so [substitute](#substitute) shows you how you to can use `substitute()` to modify unevaluated code, making it possible to program with any function that uses NSE.

While powerful, NSE makes code substantially harder to reason about. The chapter concludes with a look at the downsides of NSE in [the downsides](#nse-downsides).

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