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POR extract time components from semi-standard strings
 

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    <h3>Setup</h3> <p>I have a column of durations stored as a strings in a dataframe. I want to convert them to an appropriate time object, probably <a href="http://stat.ethz.ch/R-manual/R-devel/library/base/html/DateTimeClasses.html" rel="nofollow noreferrer">POSIXlt</a>. Most of the strings are easy to parse using <a href="https://stackoverflow.com/questions/9022908/convert-list-vector-of-strings-with-date-format-into-posix-date-class-with-r">this method</a>:</p> <pre><code>&gt; data &lt;- data.frame(time.string = c( + "1 d 2 h 3 m 4 s", + "10 d 20 h 30 m 40 s", + "--")) &gt; data$time.span &lt;- strptime(data$time.string, "%j d %H h %M m %S s") &gt; data$time.span [1] "2012-01-01 02:03:04" "2012-01-10 20:30:40" NA </code></pre> <p>Missing durations are coded <code>"--"</code> and need to be converted to <code>NA</code> - this already happens but should be preserved.</p> <p>The challenge is that <em>the string drops zero-valued elements</em>. Thus the desired value <code>2012-01-01 02:00:14</code> would be the string <code>"1 d 2 h 14 s"</code>. However this string parses to <code>NA</code> with the simple parser:</p> <pre><code>&gt; data2 &lt;- data.frame(time.string = c( + "1 d 2 h 14 s", + "10 d 20 h 30 m 40 s", + "--")) &gt; data2$time.span &lt;- strptime(data2$time.string, "%j d %H h %M m %S s") &gt; data2$time.span [1] NA "2012-01-10 20:30:40" NA </code></pre> <h3>Questions</h3> <ol> <li>What is the "R Way" to handle all the possible string formats? Perhaps test for and extract each element individually, then recombine?</li> <li>Is POSIXlt the right target class? I need duration free from any specific start time, so the addition of false year and month data (<code>2012-01-</code>) is troubling.</li> </ol> <h3>Solution</h3> <p>@mplourde definitely had the right idea w/ dynamic creation of a formatting string based on testing various conditions in the date format. The addition of <code>cut(Sys.Date(), breaks='years')</code> as the baseline for the <code>datediff</code> was also good, but failed to account for a critical quirk in <code>as.POSIXct()</code> <em>Note: I'm using R2.11 base, this may have been fixed in later versions</em>.</p> <p>The output of <code>as.POSIXct()</code> changes dramatically depending on whether or not a date component is included:</p> <pre><code>&gt; x &lt;- "1 d 1 h 14 m 1 s" &gt; y &lt;- "1 h 14 m 1 s" # Same string, no date component &gt; format (x) # as specified below [1] "%j d %H h %M m %S s" &gt; format (y) [1] "% H h % M %S s" &gt; as.POSIXct(x,format=format) # Including the date baselines at year start [1] "2012-01-01 01:14:01 EST" &gt; as.POSIXct(y,format=format) # Excluding the date baselines at today start [1] "2012-06-26 01:14:01 EDT" </code></pre> <p>Thus the second argument for the <code>difftime</code> function should be:</p> <ul> <li>The start of the first day of the current year if the input string <em>has</em> a day component</li> <li>The start of the <em>current</em> day if the input string <em>does not</em> have a day component</li> </ul> <p>This can be accomplished by changing the unit parameter on the <code>cut</code> function:</p> <pre><code>parse.time &lt;- function (x) { x &lt;- as.character (x) break.unit &lt;- ifelse(grepl("d",x),"years","days") # chooses cut() unit format &lt;- paste(c(if (grepl("d", x)) "%j d", if (grepl("h", x)) "%H h", if (grepl("m", x)) "%M m", if (grepl("s", x)) "%S s"), collapse=" ") if (nchar(format) &gt; 0) { difftime(as.POSIXct(x, format=format), cut(Sys.Date(), breaks=break.unit), units="hours") } else {NA} } </code></pre>
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    1. COThis might give some direction: http://stackoverflow.com/questions/1828206/why-is-parsing-y-m-using-strptime-in-r-giving-an-na-result-but-y-m-d-w
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    2. COThis doesn't solve your problem, but you are having problems with [strptime](http://stat.ethz.ch/R-manual/R-devel/library/base/html/strptime.html) because it is not designed to parse durations; it is intended to parse timestamps. (Some durations look like timestamps, some don't.)
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    3. CO@DavidJames Ok, that makes sense. Would you agree with @mplourde that it's better to format first, then cast using `as.difftime()`?
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