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R Cookbook by Paul Teetor

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Chapter 7. Strings and Dates

Introduction

Strings? Dates? In a statistical programming package?

As soon as you read files or print reports, you need strings. When you work with real-world problems, you need dates.

R has facilities for both strings and dates. They are clumsy compared to string-oriented languages such as Perl, but then it’s a matter of the right tool for the job. I wouldn’t want to perform logistic regression in Perl.

Classes for Dates and Times

R has a variety of classes for working with dates and times; which is nice if you prefer having a choice but annoying if you prefer living simply. There is a critical distinction among the classes: some are date-only classes, some are datetime classes. All classes can handle calendar dates (e.g., March 15, 2010), but not all can represent a datetime (11:45 AM on March 1, 2010).

The following classes are included in the base distribution of R:

Date

The Date class can represent a calendar date but not a clock time. It is a solid, general-purpose class for working with dates, including conversions, formatting, basic date arithmetic, and time-zone handling. Most of the date-related recipes in this book are built on the Date class.

POSIXct

This is a datetime class, and it can represent a moment in time with an accuracy of one second. Internally, the datetime is stored as the number of seconds since January 1, 1970, and so is a very compact representation. This class is recommended for storing datetime information (e.g., in data frames). ...

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