# Matrices, Data Frames, Functions, Conditionals, Loops with R

Guest post by Slaviana Pavlovich Microsoft Student Partner

My name is Slaviana Pavlovich. I am an IT and Management student at University College London with a passion for data science. I recently completed the Microsoft Professional Program for Data Science, where I developed core skills to work with data. If you are also interested in this career, but not sure where to start - I strongly encourage you to check it out. I also have a wide range of interests including 3D bioprinting, public speaking, and politics. Additionally, I enjoy swimming and photography to balance out my studies. I became a Microsoft Student Partner at the end of my first year and I absolutely enjoy being part of such a vibrant community. If you have any questions, feel free to ask!

### Introduction

In today’s article, I am going to continue talking about R. In the second part of this two-part introduction to R (the first part is available here), we are going to consider:

• Matrices

• Data Frames

• Functions

• Conditionals

• Loops

### Matrices

Matrix is another data type that we are going to look at. Matrix is a two-dimensional data set. A matrix is created using the function matrix() :

`````` > # creating a matrix
> example <- matrix(c(99,45,4,47,2,5), nrow = 3, ncol = 2, byrow = TRUE)
> example
[,1] [,2]
[1,]   99   45
[2,]    4   47
[3,]    2    5
``````

As you can see in the example above, nrow and ncol are used to define the values for rows and columns. Also, byrow = TRUE means that the matrix is filled by rows, while byrow=FALSE – by columns. Let’s look at the following example:

`````` > # creating a 2x3 matrix that contains the numbers from 1 to 6 and filled by columns
> example.2 <- matrix(1:6, nrow = 2, ncol = 3, byrow = FALSE)
> example.2
[,1] [,2] [,3]
[1,] 1 3 5
[2,] 2 4 6
``````

To change the names of rows and columns of the matrix use dimnames:

`````` > # creating a matrix
> A <- matrix(1:6, nrow = 3, byrow = TRUE)
> # setting row and column names
> A
[,1] [,2]
[1,] 1 2
[2,] 3 4
[3,] 5 6
> dimnames(A) = list(c("1row", "2row", "3row"), c("1col", "2col"))
> A
1col 2col
1row 1 2
2row 3 4
3row 5 6
``````

There are certain operations you can do with matrices. You can transpose a matrix, using a function t() :

`````` > M <- matrix(c(14,2,4,3,2,5), nrow = 2, ncol = 3, byrow = TRUE)
> M
[,1] [,2] [,3]
[1,]   14    2    4
[2,]    3    2    5
> t(M)
[,1] [,2]
[1,]   14    3
[2,]    2    2
[3,]    4    5
``````

Furthermore, use solve() function to find an inverse of a square matrix:

`````` > X <- matrix(1:4, nrow = 2, byrow = TRUE)
> X
[,1] [,2]
[1,]    1    2
[2,]    3    4
> solve(X)
[,1] [,2]
[1,] -2.0  1.0
[2,]  1.5 -0.5
``````

Arithmetic operations are done element-wise:

`````` > A <- matrix(1:6, nrow = 3, byrow = TRUE)
> B <- matrix(1:6, nrow = 2, byrow = TRUE)
> A
[,1] [,2]
[1,]    1    2
[2,]    3    4
[3,]    5    6
> B
[,1] [,2] [,3]
[1,]    1    2    3
[2,]    4    5    6
> A + 2
[,1] [,2]
[1,]    3    4
[2,]    5    6
[3,]    7    8
> B / 2
[,1] [,2] [,3]
[1,]  0.5  1.0  1.5
[2,]  2.0  2.5  3.0
``````

For matrix multiplication use “%*%”:

`````` > A %*% B
[,1] [,2] [,3]
[1,]    9   12   15
[2,]   19   26   33
[3,]   29   40   51
``````

In R, to select elements of the matrix, do the following:

`````` > K <- matrix(4:7, nrow = 2, byrow = TRUE)
> K
[,1] [,2]
[1,]    4    5
[2,]    6    7
> K[1,2] # element at 1st row and 2rd column
[1] 5
> K[1,] # first row
[1] 4 5
> K[,2] # second column
[1] 5 7
``````

Finally, we can always modify a matrix:

`````` > V <- matrix(1:9, ncol = 3)
> V
[,1] [,2] [,3]
[1,]    1    4    7
[2,]    2    5    8
[3,]    3    6    9
> V[1,3] <- 0; V    # modify a single element at 1st row and 3rd column to 0
[,1] [,2] [,3]
[1,]    1    4    0
[2,]    2    5    8
[3,]    3    6    9
> V[V>2] <- 1; V    # change all elements greater than 2 to 1
[,1] [,2] [,3]
[1,]    1    1    0
[2,]    2    1    1
[3,]    1    1    1
``````

### Data Frames

After looking at matrices, I suggest learning about data frames. A data frame is a special case of a list (another data object in R that was previously considered in the first part of the article). Data frames are used for storing tables. Unlike matrices, each column, also known as a vector, can store different types of data (logical, numeric, character, complex, etc.). The function data.frame() is used to create a data frame:

`````` > # creating a data frame
> table <- data.frame(name=c("Jack", "Karan", "Thomas", "Vito", "Kristine"),
+                     age=c(19, 20, 19, 19, 19),
+                     sex=c("M", "M", "M", "M", "F"),
+                     colour=c("yellow", "red", "green", "blue", "pink"))
> table
name age sex colour
1     Jack  19   M yellow
2    Karan  20   M    red
3   Thomas  19   M  green
4     Vito  19   M   blue
5 Kristine  19   F   pink
> typeof(table)
[1] "list"
> class(table)
[1] "data.frame"
> # function of a data frame
> names(table)
[1] "name"   "age"    "sex"    "colour"
> nrow(table)
[1] 5
> ncol(table)
[1] 4
``````

There are several ways of accessing an element of a data frame:

`````` > table[2:4] # columns starting from 2nd to 4th of data frame
age sex colour
1  19   M yellow
2  20   M    red
3  19   M  green
4  19   M   blue
5  19   F   pink
> table[c("colour","age")] # columns with the titles colour and age from data frame
colour age
1 yellow  19
2    red  20
3  green  19
4   blue  19
5   pink  19
``````

In a similar way to matrices, it is possible to change the values of the elements:

`````` > table[3,"age"] <- 20; table # modify the element at 3st row and column age to 20
name age sex colour
1     Jack  19   M yellow
2    Karan  20   M    red
3   Thomas  20   M  green
4     Vito  19   M   blue
5 Kristine  19   F   pink
``````

### Functions

There is a straightforward way of creating own functions in R. Let’s consider an example where our function is going to find the difference between two integers:

`````` > example <- function (a, b) {
+          c <- a - b
+          c
+      }
>  example(15, 1)
[1] 14
``````

As shown above, the word function is used to declare a function in R. Now we are going to create a function that prints a type and class of an argument:

`````` example<-function(X){
+     print(typeof(X))
+     print(class(X))

> example <- function (a, b) {
+          c <- a - b
+          c
+      }
>  example(15, 1)
[1] 14

+     print(paste("The type is", typeof(X) , "and class is", class(X)))
+ }
> Y<-c("Vito")
> example(Y)
[1] "character"
[1] "character"
[1] "The type is character and class is character"
> Z<-c(11)
> example(Z)
[1] "double"
[1] "numeric"
[1] "The type is double and class is numeric"
``````

If you want to take an input from the user, use the function readline() in R:

`````` read.example <- function()
+ {
+     return(as.character(str))
+ }
> print(paste("Nice to meet you,", read.example(), "!"))
[1] "Nice to meet you, Dre !"
``````

### Conditionals

To use conditional execution in R, we are going to use if…else statement:

`````` > x <- 4
> if (x < 0) {
+     print("It is a negative number!")
+ } else if (x > 0) {
+     print("It is a positive number!")
+ } else
+     print("Zero!")
[1] "It is a positive number!"

> x <- -10
> if (x < 0) {
+     print("It is a negative number!")
+ } else if (x > 0) {
+     print("It is a positive number!")
+ } else
+     print("Zero!")
[1] "It is a negative number!"

> x <- 0
> if (x < 0) {
+     print("It is a negative number!")
+ } else if (x > 0) {
+     print("It is a positive number!")
+ } else
+     print("Zero!")
[1] "Zero!
``````

Loops

Now we are going to consider the control statements in R, such as for{}, repeat{} and while{}.

· A for{} loop in the example below is going to print the first three numbers in the vector Y:

`````` > Y <- c(17, 25, 19, 33, 11, 51, 55)
> for(i in 1:3) {
+ print(Y[i])
+ }
[1] 17
[1] 25
[1] 19
``````

· In this example, a repeat{} loop is going to print “task” and after 3 loops it is going to break:

`````` > task <- c("R is great!")
> i <- 3
> repeat {
+     i <- i + 1
+     if(i > 5) {
+         break
+     }
+ }
[1] "R is great!"
[1] "R is great!"
[1] "R is great!"
``````

· A while{} loop is going to follow the commands as long as the condition is true:

`````` > i <- 1
> while(i < 5) {
+     print(i)
+     i <- i + 1
+ }
[1] 1
[1] 2
[1] 3
[1] 4
``````

Resources

There are so many interesting resources online that can help you further with R. I strongly recommend checking them out. In the following article, I am going to cover data visualisation, stay updated!

https://academy.microsoft.com/en-us/professional-program/ Microsoft professional programmes, Big Data, Data Science