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https://cran.r-project.org/doc/contrib/Torfs+Brauer-Short-R-Intro.pdf Very short and compact. Must read unless have solid intermediate skills.
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http://www.biostat.jhsph.edu/~ajaffe/docs/undergradguidetoR.pdf Much longer and wordy, but more complete ok for beginners.
Search intro to R programming pdf
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http://r4ds.had.co.nz/ R for Data Science: Free online version of book
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http://ohi-science.org/news/Resources-for-R-and-Data-Science Resources list
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https://github.com/jennybc Loads of useful respositories, course materials, ideas