Tuesday, September 25, 2012

How to read in a vector stored as a R expression in a text file (and other things)

A Facebook friend asked:

R question: I have a file that contains a list of entries of the format "c(2.34306542445513, 1.07370083005978e-06)". How can I coax R to read such entries literally? In other words, I want to make an object from the entries. pars <- c(2.34306542445513, 1.07370083005978e-06)

My first response was to do this the hard way. Let's say we have a text file with one or multiple rows as described:



Before realizing that there is a much easier way, I pointed out that we can do what's been proposed using readLines, strsplit, paste, and as.numeric. I include this primarily to emphasize how easy it :



Anyway, shortly after posting this I realized that this could be done much more easily using the functions eval and parse:



That's it!

P.S. please let me know if you prefer this new "text box" format for R code and session results, or my previous method of font only based demarcation (e.g., shown here, and in almost any other post on this blog).

7 comments:

  1. The "other things" was supposed to be that we can obviously use eval & parse to evaluate other expressions stored as strings or in a text file as well. How to do this depends on the case, but should be fairly obvious. We can also combine these with strsplit to pull out specific expressions from a longer string.

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  2. Hey Liam,
    One limitation of the text box format is that, unlike figures and the old font-based R code, it doesn't render properly in a feed reader (at least for Google Reader in Chrome, not sure if it's generally the case).
    Keep up the good work!
    Travis

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  3. Ugghh. How bad does it render? Can you send me a screen shot? Thanks for the feedback. Liam

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    1. The line breaks aren't translating, so each block of code turns into a single big 'paragraph'. I'm emailing a screenshot.
      T

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    2. I have now switched to creating a CSS div class for code in the blog HTML, and then using that. How does it look? (E.g., here.)

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  4. It doesn't render too well in Chrome for iPad, either, because it uses a non-monospaced font.

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  5. @Travis and Roger - thanks for the feedback!

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