- Help and demos.

The home page of R is http://www.r-project.org

type help.start() to use a browser to view standard documentation

type demo() to view demos of the capabilities of R.

type ?topic to view manual pages of an R function or library.

- To recover last computed expression in R:

Use the special variable .Last.value

- How do I display floating point number to a specified number of digits?

options(digits = 10)

when you want finer control, you may use sprintf("%.nf", value) where n must be replaced by

the numerical value of the number of digits.

- Creating sequences.

X <- seq(1,100) X <- seq(1, 100, by = 3) X <- seq(1, 100, length=5) Note that the spacing in sequence X is given w = (endvalue, startvalue)/(length - 1). - What are the elementary operations on sequences or vectors.

All variables in R are vectors! Two vector operands are first set to the same length,

recycling elements if necessary.

Arithmetic operations +, -, *, / works pairwise.

Functions or operations applied to a vector result in new vector.

X^2 gives a new vector with the square of the elements in original X.

X*X gives the same results!

- To determine the number of elements in a vector

length(X)

- To extract the kth element(indexing starts at 1)

X[k]

- To return a vector with the kth element deleted.

X[-k]

- To return a vector with some elements deleted.

X[-c(1,3, 5)] will delete the elements at the first, third and fifth position.

To repeat a vector.

rep(X, ntimes) will return a new vector repeated ntimes.

X <-c(1,2) rep(X, 5) will display [1] 1 2 1 2 1 2 1 2 1 2 - To concatenate two or more vectors

c(X,Y, Z)

- To compute the product of all elements in a vector.

prod(X)

- To assign an element v at a specific position k.

X[k] <- v Not that if k is greater than length of X, X will be enlarged! To filter elements of X meeting a condition. X[condition] For example, X[X > 5] will display a new vector with element values > 5.

To sum all elements in a vector

sum(X)

To sort a vector

sort(X)

- How do you mark lines as comments in R?

Start each line (leading spaces are ignored) with a #.

- How do I edit a user defined function definition?

Method 1. use fix(function_name)

You mispelled length(X):

gm <- function(X){ exp(sum(log(X))/lenggth(X)) } use fix(gm). edit the offending line and save file. Now type gm. The erroneous definition now has been saved! Method 2. use edit(function_name) Type gm <- edit(gm) This has the same behavior as fix. Method 3. body(function_name)) for one line definitions, this may work out well from the command line. body(gm) <- expression({exp(sum(log(X))/length(X))}) Method 4. as.list(body(function_name))) for multiline definitions, use as.list, taking into note that the line 1 is always the starting {. Hence the first line of the function is body(function_name[[2]]. To change a specific line, body(function_name[[line#+1]] <- substitute(newexpression). Reference:http://stackoverflow.com/questions/2458013/what-ways-are-there-to-edit-a-function-in-r - What are reserved variables in R?

Do not use c, t, q, F, T

- In conditional tests, what are the available comparison operators?

!expr NOT

>,<, != inequalities = equality - Does R provide bitwise operators?

bitAnd(a, b)

bitOr (a, b)

bitXor(a, b)

All functions above works on 32 bitwide unsigned integers

- What are the available programming control structures in R?

conditional if

if (condition) expr

conditional if-else

if (cond) expr1 else expr2

switch

switch(expr, ...)

ifelse

ifelse(test,yes,no)

looping constructs and cotrol statements:

for (var in seq) expr

while (cond) expr

repeat expr

break

next

- How do I input a program or data file to R?

source(filename)

- How do I redirect output to a file from R?

sink(outfilename)

- What are the functions for handling libraries or packages?

library() # list all available packages

library(lib.loc = .Library) # list all packages in the default library

library(help = splines) # documentation on package 'splines'

library(splines) # load package 'splines'

require(splines) # the same

search() # "splines", too

detach("package:splines")

- How do I install contributed packages?

install.packages() interactive install

install.packages(pkgname) install a specified package.

remove.packages(pkgname) removes a package

library() # list all available packages

library(lib.loc = .Library) # list all packages in the default library

library(help = splines) # documentation on package 'splines'

library(splines) # load package 'splines'

require(splines) # the same

search() # "splines", too

detach("package:splines")

Standard repositories are "http://cran.r-project.org"and its mirror site

http://lib.stat.cmu.edu/R/CRAN.

You should be root to install the libraries systemwide.

## Wednesday, November 9, 2011

### R quick reference

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R quickref

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