ESALQ/USP

# Introduction to R

This short-course was made by Rodrigo Rampazo Amadeu from the Statistical-Genetics Laboratory, Department of Genetics, Luiz de Queiroz College of Agriculture, University of Sao Paulo, Brazil and was first applied on a Introduction to R workshop.

You can found complete courses at:

You can found a great primer presentation about R here.

There are a lot of good tutorials files on web.

## Purpose

The idea of this course is to present the basic commands and concepts about the R language for genetic and bioinformatic students.

#Everything typable in R is be presented inside boxes.
#First lesson, if you run on R a line starting with # it will not run. # for comments.


## Installing R

Install R, we strongly advise you to use RStudio for your firsts R contact. To install it go to RStudio and follow the instructions.

## Using R

Open R, if you are using RStudio you need to create a new script pressing the green plus symbol on the top left corner. Then, you will see a script window and a console window, all the commands here is to be used and wroted on the console window. We strongly advise you to write it down on the script window and press run line by line on the top right corner (shortcut: Ctrl+Enter). In this way you can save and track all of your code. Try to write on the script and to run the following lines.

## R as calculator

Sum

3+3   #I can comment or annotate my scripts by writing after the # sign


Divide

(3+3)/3


Square Root

9^0.5 #or
sqrt(9)


Exponent

2^10


Logaritm

log(10)
log(10,base = 10)
log(10,base = 2)


Exponential e

exp(2)


Tip: Keep in mind to save your script as your work progress. To do it, just press Ctrl+s.

## Functions

There are a lot of functions in R. A function is represented by a name and can be used calling it names followed by arguments. To visualize what a function does, you can type ?functionname, for example:

?log


If you are using Rstudio this command you change a window in the right side of your screen, if not, probably you are going to see a pop-up window in your screen. This new window says what the function log does. Mainly in the help there is a description, the usage, and the arguments of it. For example, this function has the arguments x (which has to be a numeric or complex vector), and base.

Other functions good to know, try to use them!

?seq
seq(from=0, to=100, by=3) #we can write in this way, or
seq(0,100,3) #make sequence

?rep
rep(x=1,times=3) #or
rep(1,3)


## R creating objects

To create a object use the following syntax:

x = 10


Your object name ‘x’ followed by ‘=’ and then your variable. In some script you will see this ‘<-‘ symbol, ‘<-‘ gives you the same result as ‘=’.

Now, everytime you call (type) ‘x’ you we receive 10. For example,

x*2
x+x
x^x
log(x,x)
x%%3 #module operator


You can also have a character object (which can not be used in math operations):

x_name = "name"


You can call it in the same way:

x_name


To see which type your object is:

class(x_name)
class(x)


Pay attention, R is case-sensitive (i.e. ‘X’ is different than ‘x’). When in doubt, write everything of your script in lower case.

You can track all your created objects typing ‘ls()’.

Another important class is the logical whice can be TRUE or FALSE. For example:

1<0 #is 1 greater than 0? FALSE
1==0 #is 1 equal 0? FALSE
1>0 #is 1 lower than 0? TRUE


Pay attention here, ‘==’ is different than ‘=’!

This logical operation can be wrote in an R object

z=1<0


## Vectors

A vector can be create using the following syntax:

x = c(1.5,2.1,2.5,3.4,4.3,6.1) #A vector with numeric value
y = c("A","A","B","B","C","C") #A vector with character values


The above line will create: the ‘x’ vector with 6 numeric objects, and the ‘y’ vector with 6 character objects. To visualize this information in a fashion way you can use the function str. To visualize a specific object you can use:

x[6] #the sixth object of vector x
y[2] #the second object of vector y


This function is really powerful when working with big data, keep it in mind, it gives you all the structure of the object. Keep in mind, a vector can has only one type of data, for instance character or numeric.

str(x) #a vector with 6 numeric objects (1:6)
str(y) #a vector with 6 character objects (1:6)


Notice that you replace the old x object by this new one. Be careful when naming your objects. With vector, you can make more complex calculus.

sum(x) #the sum of the vector objects
mean(x) #the mean of the vector objects
var(x) #the variance between the vector objects
sum(y) #returns error, because y is not numeric


We can combine ‘y’ and ‘x’ in a more complex function structure. For example with the ‘tapply’ function. In this function first you choose your atomic object (vector x), following by your index (vector y), and then for the function you want to use. Foe example:

tapply(x,y,sum) #Compute the sums of x by each y categorie
tapply(x,y,mean) #Compute the means of x by each y categorie
tapply(x,y,var) #Compute the variance of x by each y categorie


Now, with this tools we can build a dice for example!

#Create a object called dice and put a vector with number from 1 to 6 by 1.
dice = seq(1,6,1)
#Sample 1 number from the object dice
sample(dice,1)


## Creating vectors from scratch

You can also create vectors with this following functions:

?rep
rep(1,10) #Creates a vector of 1s repeated 10 times
rep(NA,5) #Creates a vector of NAs repeated 5 times

?seq
seq(1,10,1) #Creates a vector from 1 to 10 by 1 of interval
seq(1,10,0.1) #Creates a vector from 1 to 10 by 0.1 of interval

?sample
sample(seq(1,10,0.1),4) #Sample from the seq(1,10,0.1) 4 numbers


Obviously, you can save this vector on a object, for example:

my_vector = seq(1,10,1)


## Logical Operation

We can build a logical vector, for example:

z <- x>3 #Which x is grater than 3
class(z)


If you call the object ‘z’ (writing z on console and pressing enter) you will see which values of x is grater than 3. The logical expressions is very useful when combining with another functions. For example, with function ‘which’. This function returns the exact position of the vector it has TRUE values. Combined with brackets ‘[]’, it can return which values in x is greater than 3.

which(z) #The TRUE is in position 5 and 6.
x[which(z)]


Now it is your turn, using logical operations, the which function, and brackets, extract out of vector x the numbers lower than 4.

## Data frame

We can combine vectors of same lenght in a data frame, to combine x, y, and z vectors, now renamed to Yield, Individual Name and Rust Absence:

df = data.frame(Yield=x,Ind=y,Rust=z)


Data frames is a versatile way to combine vectors of different classes in a single object. You can access the different vectors using the operator $. For example: df$Yield
df$Ind df$Rust


When combining a character vector in a data frame it will be turned as a factor. In this data frame notation, you can use the same codes earlier seen. For example for extract the mean using y as index.

tapply(df$Yield,df$Ind,mean)
tapply(df$Yield,df$Rust,mean) #see what is going on here


## Matrices

We can combine vectors of same length and same class in a matrix, we have to define the number of rows, the number of columns and the direction to read the data. Look all the differencs on the below lines.

x #visualizing your x vector
matrix(x,nrow=3,ncol=2,byrow=TRUE)
matrix(x,nrow=2,ncol=3,byrow=TRUE)
matrix(x,nrow=3,ncol=2,byrow=FALSE)
matrix(x,nrow=2,ncol=3,byrow=FALSE)


Of course, we can save this matrix as a object

X <- matrix(x,nrow=3,ncol=2,byrow=TRUE) #note X is different than x (lower case)
dim(x) #to check the dimension of X


To select a specific object inside this matrix you can use the following notation:

X[3,1] #The element of third row and first column
X[3,] #All the elements of third row
X[,1] #All the elements of first row
X[1:2,] #A matrix with all elements of first and second rows


If it is a matrix of numeric objects you can do matrix algebra. For example:

t(X) # transpose of X (X prime)
t(X) %*% X # transpose of X times X
X*X # element-wise multiplication
X + 1 #add 1 to all elements of X
2*X #multiply all elements of X by 2
solve(t(X) %*% X) # inverse of transpose of X times X
solve(t(X) %*% X) + 1 # inverse of transpose of X times X plus 1


## Lists

We can combine different objects with variable format and lengths in a list.

everything <- list(df=df,
X=X)


The object everything is a list with two objects: a data frame and a matrix, you can double-check it using str() function:

str(everything)


To access the information inside a list you can use $as in data frames. everything$x


Or you can use an indicator of the level your information is. For example, to access the first object of the first level of your list (i.e. x vector):

everything[[1]]


If you want to access the second object of the first level object of your list (i.e. the second object of x):

everything[[1]][[2]]


If you want to access the third object of the second level object of the first level object of your list (i.e. the second object of x):

everything[[1]][[2]][[3]]


And this classification goes on. Now try to access the fifth value of vector x using this approach.

To add a new object in a list, be sure in what level you are working with.

To add a new vector inside the data frame (first object of everything):

everything[[1]][[4]] = c(1,2,3,4,5,6)


To create a new third object:

everything[[3]] = c(1,2,3,4,5,6)


To add 1 on this new object:

everything[[3]] = everything[[3]]+1


An important guide to play with data frames in the function str().

str(everything)


This function print the structure of your data frame and show all the categories of it. Be familiar with lists, almost every data is handle in this format.

## Importing and exporting data

First download the data at here. To read the data, first you need to set up your working directory of R as the same of your file is. It is a good procedure to create a folder to every different R job you are working, after creater this folder and put your data there, set it as your working directory. If you are using Rstudio, it is easy go to Session -> Set Working Directory -> To Source File Location and select the folder your file is. Otherwise, you can use the function of set work directory (setwd()), the arguments of the folder file using this function can be slightly different depending upon your operational system. To make of the syntax type getwd() and follow the same syntax it shows. Use a specific folder for a specific project/course, please do not mess up your R files with different courses and projects. Additionally lets save your script, go to File -> Save As and save at the same folder of your data is. Another advise is AVOID empty space in your file names, this will save you some time. Replace them for underline *_* or dash -.

Example of ‘setwd’ function (if you used the easy way of RStudio, skip this chunk), pay attention here, it depends on your current OS. If you do not know how to declare it, go to the folder you want to work and look its properties.

getwd()
setwd("Rodrigo/Intro_R/") ##fill with your path to archives


Reading the data, if you can choose, choose csv format files, they are easy to read and understand. Before you call it in R, pay attention in what the exactly way your data is. For example in the ‘data.csv’ file, the data has 2 vectors with column names (‘header’) and our missing data (‘na.strings’) is coded as -9.

read.table("data.csv")


Saving the table in a object:

data = read.table("data.csv",header = TRUE, na.strings=NA)


If you want to save this object in a csv file:

write.table(data,file="data_export.csv")


Look at the file you create and notice the differences between it (data_export.csv) and data.csv. To let it in the same format we have to use some additional arguments of the function:

write.table(data,file="data_export.csv",row.names=FALSE,quote=FALSE,na="NA")


Now we have a copy of the file originated created. The default field separator is “ “ (single space), if you want comma as separator, just add ‘sep=”,”’ on the arguments. Pay attention in what you are writing and reading, this can drag a lot of minor errors in your data analysis.

## Libraries

Library is a set of functions and dataset you can add to your base R. Some libraries were already installed with your base R and someothers have to be installed. The most famous package repository is the CRAN, to install a package from there you need to type install.packages("packagename"). In bioinformatics, the repository Bioconductor is a very useful, to install a package of it please go to the webpage of the repository and follow the instruction. After installed, you have to call the library to use its functions and datasets. On the next section we will use data from the datasets package, which should be already available with your R. To use this package type:

library(datasets)


## Graphic Plotting

There are several types of graphics in R. We will present some of the most used ones. We will use the data mtcars from the library datasets. To look all the data from this package go to https://stat.ethz.ch/R-manual/R-devel/library/datasets/html/00Index.html.

data(mtcars) #Now mtcars is an object
?mtcars #To look the data description
mtcars #To look the data


## Scatterplot

To plot a scatterplot graphic (x × y graphic) where x will be mpg (Miler/Gallon) and y will be number of hp (Gross horsepower)

plot(x=mtcars$mpg,y=mtcars$hp)
#Changing title (main) and axis label (xlab and ylab)
plot(x=mtcars$mpg,y=mtcars$hp, main="Scatterplot mpg x hp", xlab="mpg", ylab="hp")


hist(x=mtcars$mpg, main="Histogram of mpg", xlab="mpg")  ## Density plot(density(mtcars$mpg), main="Density", xlab="mpg")


boxplot(x=mtcars$mpg, main="Boxplot of mpg", xlab="mpg")  ## Boxplot by cylindrade In this case we use x ~ y which means x as function of y. For example: boxplot(mtcars$mpg ~ mtcars$cyl, main="Boxplot of mpg", xlab="mpg")  ## Costumizing your graphics You can see some examples on demo(graphics)  ## Exporting a graphic You can save a graphic using the RStudio interface, after plot it just click on export at the same window of the plot, and follow the instructions. Another way to do it, it is from the command line, for example to save a png graphic first you need to create a png file with the function png, then plot your graphic, and at last close your png file with dev.off(). For example: png(filename = "Rplot.png", width = 480, height = 480) boxplot(mtcars$mpg ~ mtcars$cyl, main="Boxplot of mpg", xlab="mpg") dev.off()  There are several ways and formats to save a R graphic, all the ways follow those paths, create a file, plot, close the device. ?png to more information about it. The graphics here presented are the basic ones, you can make better ones with the package ggplot2 and interactive ones with package shiny. ## Exploring your data Beyond the graphical exploration we can do some descriptive analysis in our data: ‘summary’ function to get the quartiles and mean values. summary(mtcars$mpg)
summary(mtcars$cyl) tapply(mtcars$mpg,mtcars$cyl,summary) #summary of mpg within cyl  To regress mpg against hp you can use the ‘lm’ function which fit linear models. If your objective is fit a posterior analysis of variance for stratum (factors), you have to use the ‘aov’ function. model = lm(mtcars$mpg ~ mtcars$hp) # '~' means 'is function of' summary(model) # ANOVA of the model plot(model) # To look the residual graphics plot(x=mtcars$hp,y=mtcars$mpg) #plotting the points abline(model) #to plot the curve cor(mtcars$cyl,mtcars$mpg) #correlation between x and y  With you want compare mpg with cyl (as factor) model = aov(mtcars$mpg ~ as.factor(mtcars$cyl)) # '~' means 'is function of' summary(model) #ANOVA plot(model) # To look the residual graphics  For multiple comparision (e.g. Tukey test), you can use the ‘TukeyHSD’ function. TukeyHSD(model)  If you want a more presentable Tukey test, you can use this same test of other packages. For example, the ‘agricolae’ package. To use it, you need first to install it. After installed, you need to call it with ‘library’ function. Note that is a quite different to use compared with ‘TukeyHSD’ function, everytime you will use a new function look at its help or google about it. In some cases it can be confuse to use new functions! In the case of Tukey function of ‘agricolae’ package, we need to declare de mean square of residuals and its degree of freedom. install.packages("agricolae") library(agricolae) (HSD.test(y=mtcars$mpg,trt="mtcars\$cyl",MSerror=10.39,DFerror=29))


## Exploring matrices

To look a covariance matrix within a data frame

x=cov(mtcars)
heatmap(x, Rowv = NA, Colv = NA, scale = "row")


## Fitting Linear Models: a genetic simple example

On the next chunk we fit a model using genotypes and phenotypes information. On the first model we fit a model considering genotype as factor, on the second model we fit the genotype as numeric, try to understand what is going on on both examples. On regresion analysis, the default on R is to consider the first level of the factor as the intercept. From http://augusto-garcia.github.io/R-Introduction/.

genotypes = c("AA", "aa", "aa", "Aa", "AA", "Aa")
phenotypes = c(9, 4, 3, 7, 10, 8)

genotypes = as.factor(genotypes)
phenotypes = as.numeric(phenotypes)

str(genotypes)
str(phenotypes)

model = lm(phenotypes ~ genotypes)
model
summary(model)

genotypes = as.numeric(genotypes)
model = lm(phenotypes ~ genotypes)
summary(model)
tapply(phenotypes, INDEX = genotypes, FUN = mean)

plot(y=phenotypes,x=genotypes)
abline(model)

plot(y=phenotypes, x=genotypes, xaxt="n")
abline(model)
axis(side=1,at=seq(1,3,1))


## Simulating data

Here you can find several probability density functions that R can handle. The core idea on simulating data from this distributions is to know what each of this 4 family functions does (r,p,q,and d). Knowing this, you can simulate or extract data from every probability density function. Here we present the 4 functions of the normal (Gaussian) distribution:

#Normal Distribution
?rnorm #the first argument is from the family and the following 2 are mean and standard deviation

#Distribution Family functions
rnorm(10,0,1) #“r”: random, randomly generated numbers
pnorm(0,0,1) #“p”: probability, cumulative density function
qnorm(0.5,0,1) #“q”: quantiles, cumulative density function (quantiles)
dnorm(1,0,1) #"d": density, height of the probability density function


## Simulating data

Plotting it in a histogram.

hist(rnorm(1000,0,1))


If we use set.seed function, before a sample, we fix the next sample, so every time we run this script it will sample the exact same values if set.seed has the same seed (in this case 100)

set.seed(100) #set seed you will fix the next sample
hist(rnorm(1000,0,1))


## Correlation plot

Here we present how to simulate data, to build and to export a correlation plot. First we sample from a Normal Distribution 50 values and saved them on vector A. Then, based on this vector we sum or substract values with the objective to create some correlated data. The vector F has no priori correlation with the others vectors. With this data we computed the correlation and made the plot.

## Correlation Plot
library(corrplot)
A = rnorm(50,10,6)
B = -(A-rnorm(50,10,2))
plot(A,B)

C = B+rnorm(50,2,3)
plot(A,C)
plot(B,C)

D = -(C+rnorm(50,1,3))
plot(C,D)

E = D+rnorm(50,3,5)
plot(D,E)

F = rnorm(50,10,10)
plot(F,D)

#Getting the correlation between A and B, full formula:
var(A,B)/sqrt((var(A)*var(B)))

#Getting the correlation A and B, reduced function
?cor
cor(A,B)

#Getting the correlation matrix (A,B,C,D, 'n F):
cor.matrix = cor(data)
cor.matrix

#Changing the method
corrplot(cor.matrix, method="color")

#Corr value inside the cell (black for the color of the coef.)


We can export a graphic with high quality determining how many dot per inches we want (dpi). In the following chunk we export a graphic in TIFF format with 300 dpis:

## Exporting a graphic with high quality
tiff(filename = "Rplot.tiff", width = 5, height = 5, units="in", res=300)
#Corr value inside the cell (black for the color of the coef.)
dev.off()


## “for”

Using for is a great way to do repetitive tasks in R, it shorts your script and saves your time. We will present some basic loop ideas and concepts. In the following for, you have a counter letter i which starts on 1 and goes to 10, 1 by 1. Each loop will add 1 to your object x. If your x is 0 as here stated, after the loop will be

1. In other words, you will add one on x ten times.

x=0 for( i in 1:10){ x = x+1 } x

A big help of the for structure is the function print, you can use it to print on console what is going on on your loop and let you track your loop. For example:

x=0
for( i in 1:10){
x = x+1
print(i)
}
x


In the for above, you print the x value of after adding 1 inside each loop. You can also print your object within the loop (if your object is not too big). For example:

x=0
for( i in 1:10){
x = x+1
print(i)
print(x)
}
x


Notice that the print order is first i and then x (on this case they are equal). You can also use a explicit print way using the function paste:

x=0
for( i in 1:10){
x = x+1
print(paste("i=",i))
print(paste("x=",x))
}
x


## “if”

Using if is a great way to make alternative routes in your script. Attention, inside a if structure has to have a logical object, if it has a non-logical object it will be interpreted as TRUE. In the following example we verify if the x number is 0, if yes, print x is equal to 0.

## If 1
x=0
if(x==0){
print("x is equal to 0")
}


In the following example we verify if the x number is 0, if yes, print x is equal to 0, if not, print x is different to 0.

## If 2
x=0
if(x==0){
print("x is equal to 0")
}else{
print("x is different to 0")
}


In the following 2 examples we scan a vector from x to x+10 and verify each one is even and each one is odd, and print the condition. The first example uses 2 ifs and the second uses if and else.

# For + If
x=0
for(i in 1:10){
if(x%%2==0){
print(paste(x,"is even"))
}
if(x%%2==1){
print(paste(x,"is odd"))
}
x=x+1
}

x=0
for(i in 1:10){
if(x%%2==0){
print(paste(x,"is even"))
}else{
print(paste(x,"is odd"))
}
x=x+1
}


## “for” with vectors

In the following for we are checking which number of a given vector is even and which is odd, if it is even multiply by 10, else multiply by 100.

x = c(1:20) #1:20 means build a vector with numbers from 1 to 20
j = length(x) #function length return the length of the object, in this case 20
for( i in 1:j){
if( x[i]%%2 == 0){
x[i] = 10*x[i]
}else{
x[i] = 100*x[i]
}
}
x #looking the result


When using for and if structures pay attention on the opening and closing brackets. On RStudio you can track the opening/closing brackets letting your text cursor on it. It is a good programming skill to keep the indentation of the script growing up as it goes deeper on the if/for structures and, of course, comment your code!

## One simple way to apply on genetics: Recoding a genotype vector

Here we have a vector with AA, Aa, aa, and . (missing data) and we want to recode it to 2, 1, 0, and NA. First we extrach how many objects are in our genotype vector, then we create a vector of NAs which will receive the new values, and, finally, we do the for with ifs.

genotypes = c("AA", "aa", "aa", "Aa", "AA", "Aa",".")
n = length(genotypes)
genotypes_recoded = rep(NA,n)

for(i in 1:n){
if(genotypes[i]=="aa"){
genotypes_recoded[i] = 0
}
if(genotypes[i]=="Aa"){
genotypes_recoded[i] = 1
}
if(genotypes[i]=="AA"){
genotypes_recoded[i] = 2
}
}


Clearly there are many other ways to do the calculations above presented, in better and fasters ways, this post presents some tips to speedup our loops, however some basic knowledge is required.

If you have to stop your work or are afraid to lose the current progress, save a image of all your R objects. To do it. If your Rdata files are not too big, try to not overwrite them as you work on your script and to named in a sequential way so you can track your progress and keep backups.

save.image("today_date.Rdata")



Keep in mind to save your script as your work progress. To do it, just press Ctrl+s.

We are currently writing this course, if you find any mistake (including misspelling ones) or want to add something else please drop us an email at rramadeu at gmail dot com. This material is written using RMarkdown.

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