3-Functions.R

# title: 3 - Functions
# author: Joseph Rickert
# date: September 28, 2015

# "A function is a group of instructions that takes inputs, 
# uses them to compute other values, and returns the result" 
# - Norm Matloff The Art of R Programming

## Get Some Data

aq <- airquality[,1:4]  	# get the first four columns of the airquality data frame
dim(aq)									
head(aq)

# Make the aq column of airquality available in the global environment
attach(aq)

# Try to get a mean for Ozone.
mean(Ozone)								# Try to get a mean for Ozone

# Eliminate NAs from the Ozone vetor and put the non-NA values into OZ
Oz <- na.omit(Ozone)
length(Oz)
mean(Oz)

# Get help with a function from the console
help(mean)								# getting help with an R function
?mean									    # Same as the above
example(mean)							# See an example of an R function

## Writing Functions  
# Let's write our own mean function  
jmean <- function(x){
m <- sum(x)/length(x)		# where is m?
return(m)
}
jmean(Oz)
# R will throw an error because m is not in the global environment
m 

#Try again and improve the formatting
jmean2 <- function(x){
m <- round(sum(x)/length(x),2)
return(m)
}
jmean2(Oz)

# Why not give the user more control of the rounding process?   
# The magic 3 dots ... enable arguments to be passed to sub functions  

jmean3 <- function(x,...){
m <- round(sum(x)/length(x),...)
return(m)
}
?round						# look to see what parameters round is expecting

jmean3(Oz,3)
jmean3(Oz)				# the default value for round is 0
jmean3(Oz,1)

## Functions calling Functions  
# How about giving the user a choice about which rounding function to use?  
# Look at the difference between round() and signif().

pi
round(pi,4)
signif(pi,4)

# This is the way to have one function call an other function
jmean4 <- function(x,FUN,...){
m <- FUN(sum(x)/length(x),...)
return(m)
}

jmean4(Oz,round,4)
jmean4(Oz,signif,4)

## Functions with Defaults
# One last try  
#   - make round the default method of rounding
#   - make the default number of decimal digits 2  
 
jmean5 <- function(x,FUN = round,digits=3){
m <- FUN(sum(x)/length(x),digits)
return(m)
}
jmean5(Oz)
jmean5(Oz,FUN=signif)
jmean5(Oz,FUN=signif,digits=4)

## How to make a function return multiple values
# A simple function that returns multiple values

aq.noMV <- na.omit(aq)
mvsd <- function(x){
m <- sapply(x,mean)
v <- sapply(x,var)
s <- sapply(x,sd)
res <- list(m,v,s)
names(res) <- c("mean","variance","sd")
return(res)
}

mvsd(aq.noMV)


## Clean up the Global Environment
detach(aq)	# remove the aq data frame variables from the global environment							
#
rm(aq)			# remove aq from the global environment
#
rm(list=ls())	# Get rid of everything. Be careful with this!!!


## Some Important,  Helpful Functions
# Missing Values
# NA is the way to designate a missing value as we saw above.

z <- c(1:3,NA)				
z

# But, the following logical expression is incomplete, and 
# therefore undecideable, so everything is NA
z == NA 

# Here is the proper way to search for NAs 
zm <- is.na(z)				
zm

# NaN is not a number
z1 <- c(z,0/0)				# NaN: not a number
z1
is.na(z1)					    # Finds NAs and NaNs
#
is.nan(z1)            # Only finds NaNs

## Getting rid of NAs
# na.omit removes the entire row containing an NA from a data frame
  
a <- 1:10
b <- letters[1:10]
c <- LETTERS[11:20]
d <- 100:91
dF <- data.frame(a,b,c,d)
names(dF) <- c("v1","v2","v3","v4") # Give names to the variables in the data frame
dF$v1[3] <- NA
dF$v2[7] <- NA
dF
na.omit(dF)