R Cheat Sheet

R is a free software environment for statistical computing and graphics. These notes summarize the free R CodeSchool tutorial.

• 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.

• 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.

  names(myseq)=c('one','two','three')
  myseq['two']

• 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

• 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.

  x=seq[0,20,.1]
  y=sin(x)
  plot(x,y) 

• 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.

• matrix(0,3,4) creates a 3×4 matrix with all elements 0.
• matrix(1:12,3,4) creates a 3×4 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.

• factor is a collection type for categorized values - myfac=factor(myvec)
• factors group unique string values as levels 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

• 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)

• 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