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====== 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
  * ''install.packages("ggplot2") '' to install additional packages

  * 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)'' ,  ''help(package='ggplot2')''  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)
  * ''head(myvec)'' , ''tail(myvec)'' to show start or end of vector

===== Plotting =====

  * ''barplot[myseq]'' creates a bar plot of the ''myseq'' vector. ''abline(h=y)'' plots a horizontal line at height y.
  * ''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.
  * ''qplot(weights, prices, color=types)'' - more attractive plotting using ggplot2 package.
===== 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.

===== Data Sets =====
  * ''factor'' is a collection type for categorized values - ''myfac=factor(myvec)''
  * ''factor''s group unique string values as ''level''s e.g. levels(myfac) shows unique levels. 
  * ''as.integer(myfac)'' shows levels as integers, can be used to set plot type
  * ''legend("topright", levels(types), pch=1:length(levels(types)))''
  * A data frame collects sets of related values (i.e. sets of columns with values in the same order) e.g. ''mydf=data.frame(weights,prices,types)''
  * To extract a column, use double-square brackets with the column index or name e.g. ''mydf%%[['weights']]%%'' or just a dollar sign e.g. ''treasure$prices''
  * ''merge'' merges data sets by joining on shared column names

===== Statistics =====

  * ''mean(myvec)  median(myvec)   sd(myvec)''
  * ''cor.test'' tests for correlation (Pearson's product-moment)
  * ''line = lm(cola ~ colb)'' calculates a linear model between cola and colb that can be plotted with ''abline(line)''
  * 
===== File Handling =====

  * ''list.files()'' to list files in furrent directory
  * ''source("file.R")'' to load file of code
  * ''read.csv('mydat.csv')'' to load a csv file
  * ''read.table'' to read text data with other separators
  * ''con<-url("http://google.com","r")'' to read a webpage
  * ''x<-readLines(con)'' to convert to a vector of lines
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