Tomaz Kastrun walks through various special values in R:
R language supports several null-able values and it is relatively important to understand how these values behave, when making data pre-processing and data munging.
In general, R supports:
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NULL
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NA
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NaN
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Inf / -Inf
Read on for an explanation of each. Also be sure to read Mark van der Loo’s comment and Tomaz’s response.