====== R Cheat Sheet ======

[[http://www.r-project.org/ | R]] is a free software environment for statistical computing and graphics. These notes summarize the [[http://tryr.codeschool.com/ | free R CodeSchool tutorial]].

===== Basics =====

  * ''R'' is the command-line interpreter
  * Expressions are evaluated and displayed e.g. 1, 1+1, "Hello World"
  * Booleans are e.g. ''1=1'' , ''3>4'' , ''TRUE'', T, ''FALSE'', F
  * For variable assignment ''x=1'' or ''x<-1''
  * For help on a function use ''help(sum)'' or ''example(sqrt)''
  * Operations are ''+ - * / = <-''
  * ''NA'' is used to express a missing or unknown data  value. Expressions on NA return NA.

===== Vectors =====
  * To create a vector, use the combine command ''c(4,7,9)''
  * Vectors must be of the same type, and are cast if not (e.g. to strings).
  * ''a:b'' creates a vector of integers from a to b.
  * ''seq(a,b,s)'' creates a vector of numbers from a to b in increments of s
  * ''myseq[3]'' to access third element i.e. vectors indexed starting at 1.
  * Use a vector as an index to access multiple elements e.g. ''myseq[c(1,3)]''
  * The ''names'' function can be used to assign names to vector elements. Once names are asigned, they can be used as indices e.g. <code>
names(myseq)=c('one','two','three')
myseq['two']</code>
  * ''myseq + 1'' adds one to all elements of the myseq vector.
  * Scalar operations or functions on vectors typically produce other vectors e.g. + - == sin(myseq)

===== Plotting =====

  * ''barplot[myseq]'' creates a bar plot of the ''myseq'' vector.
  * ''plot[x,y]'' plots x vs y e.g. <code>
x=seq[0,20,.1]
y=sin(x)
plot(x,y) </code>
  * ''contour(mymat)'' plots a contour map of a matrix.
  * ''persp(mymat)'' plots a contour map in perspective.
  * ''image(volcano)'' generates a heat map of the matrix.

===== Matrices =====

  * ''matrix(0,3,4)'' creates a 3x4 matrix with all elements 0.
  * ''matrix(1:12,3,4)'' creates a 3x4 matrix with numbers 1-12.
  * dim(myseq) can be used to change dimensions of a matrix
  * ''mymatrix[3,4]'' returns an element of the matrix (row,column).
  * ''mymatrix[,2]'' returns entire second column.

===== Statistics =====

===== File Handling =====

  * ''list.files()'' to list files in furrent directory
  * ''source("file.R")'' to load file of code