# Day1 Learning Notes : R Nuts and Bolts

## R symbols:

The <- symbol is the assignment operator.

The # character indicates a comment. Anything to the right of the # (including the # itself) is ignored.

When an R vector is printed you will notice that an index for the vector is printed in square brackets [] on the side. For example, see this integer sequence of length 20.

The  shown in the output indicates that x is a vector and 11 is its first element.

## R Objects:

R has five basic or “atomic” classes of objects:

• character
• numeric (real numbers)
• integer
• complex
• logical (True/False)

The most basic type of R object is a vector.
Empty vectors can be created with the vector() function.
A vector can only contain objects of the same class. Exception:  list object

### Attributes

R objects can have attributes, which are like metadata for the object. These metadata can be very useful in that they help to describe the object.

For example, column names on a data frame help to tell us what data are contained in each of the columns. Some examples of R object attributes are

• names, dimnames
• dimensions (e.g. matrices, arrays)
• class (e.g. integer, numeric)
• length

Attributes of an object (if any) can be accessed using the attributes() function.
Not all R objects contain attributes, in which case the attributes() function returns NULL.

### Numbers

Numbers in R are generally treated as numeric objects (i.e. double precision real numbers).

The numbers represented behind the scenes as numeric objects (so something like “1.00” or “2.00”).

If you explicitly want an integer, you need to specify the L suffix. So entering 1 in R gives you a numeric object; entering 1L explicitly gives you an integer object.

There is also a special number Inf which represents infinity. This allows us to represent entities like 1 / 0. This way, Inf can be used in ordinary calculations;
e.g.   1/Inf   is 0.

The value NaN represents an undefined value (“not a number”); e.g. 0 / 0; NaN can also be thought of as a missing value (more on that later)

### Creating Vectors

The c() function can be used to create vectors of objects by concatenating things together.

Note that in the above example, T and F are short-hand ways to specify TRUE and FALSE. However, in general one should try to use the explicit TRUE and FALSE values when indicating logical values. The T and F values are primarily there for when you’re feeling lazy.

You can also use the vector() function to initialize vectors.

### Mixing Objects

There are occasions when different classes of R objects get mixed together. Sometimes this happens by accident but it can also happen on purpose.

When different objects are mixed in a vector, coercion occurs so that every element in the vector is of the same class.

#### Implicit Coercion

What R tries to do is find a way to represent all of the objects in the vector in a reasonable fashionSometimes this does exactly what you want and…sometimes not.

For example, combining a numeric object with a character object will create a character vector, because numbers can usually be easily represented as strings.

#### Explicit Coercion

Objects can be explicitly coerced from one class to another using the as.* functions, if available.

Sometimes, R can’t figure out how to coerce an object and this can result in NAs being produced.

When nonsensical coercion takes place, you will usually get a warning from R.

### Matrices

Matrices are vectors with a dimension attribute. The dimension attribute is itself an integer vector of length 2 (number of rows, number of columns)

Matrices are constructed column-wise, so entries can be thought of starting in the “upper left” corner and running down the columns.

Matrices can also be created directly from vectors by adding a dimension attribute.

Matrices can be created by column-binding or row-binding with the cbind() and rbind() functions.

### Lists

Lists are a special type of vector that can contain elements of different classes. Lists are a very important data type in R and you should get to know them well. Lists, in combination with the various “apply” functions discussed later, make for a powerful combination.

Lists can be explicitly created using the list() function, which takes an arbitrary number of arguments.

We can also create an empty list of a prespecified length with the vector() function

### Factors

Factors are used to represent categorical data and can be unordered or ordered. One can think of a factor as an integer vector where each integer has a label. Factors are important in statistical modeling and are treated specially by modeling functions like lm() and glm().

Using factors with labels is better than using integers because factors are self-describing. Having a variable that has values “Male” and “Female” is better than a variable that has values 1 and 2.

Factor objects can be created with the factor() function.

Often factors will be automatically created for you when you read a dataset in using a function like read.table(). Those functions often default to creating factors when they encounter data that look like characters or strings.

The order of the levels of a factor can be set using the levels argument to factor(). This can be important in linear modelling because the first level is used as the baseline level.

### Missing Values

Missing values are denoted by NA (for ‘not available’) or NaN for q undefined mathematical operations.

• is.na() is used to test objects if they are NA
• is.nan() is used to test for NaN
• NA values have a class also, so there are integer NA, character NA, etc.
• A NaN value is also NA but the converse is not true

### Data Frames

Data frames are used to store tabular data in R.
They are an important type of object in R and are used in a variety of statistical modeling applications.
Hadley Wickham’s package dplyr has an optimized set of functions designed to work efficiently with data frames.

Data frames are represented as a special type of list where every element of the list has to have the same length. Each element of the list can be thought of as a column and the length of each element of the list is the number of rows.

Unlike matrices, data frames can store different classes of objects in each column. Matrices must have every element be the same class (e.g. all integers or all numeric).

In addition to column names, indicating the names of the variables or predictors, data frames have a special attribute called row.names which indicate information about each row of the data frame.

Data frames are usually created by reading in a dataset using the read.table() or read.csv(). However, data frames can also be created explicitly with the data.frame() function or they can be coerced from other types of objects like lists.

Data frames can be converted to a matrix by calling data.matrix(). While it might seem that the as.matrix() function should be used to coerce a data frame to a matrix, almost always, what you want is the result of data.matrix().

### Names

R objects can have names, which is very useful for writing readable code and self-describing objects. Here is an example of assigning names to an integer vector.

Matrices can have both column and row names.

Column names and row names can be set separately using the colnames() and rownames() functions.

Note that for data frames, there is a separate function for setting the row names, the row.names() function. Also, data frames do not have column names, they just have names (like lists). So to set the column names of a data frame just use the names() function. Yes, I know its confusing. Here’s a quick summary:

Object              Set column names         Set row names
data frame      names()                            row.names()
Matrix              colnames()                       rownames()

Summary

There are a variety of different builtin-data types in R. In this chapter we have reviewed the following

• atomic classes: numeric, logical, character, integer, complex
• vectors, lists
• factors
• missing values
• data frames and matrices

All R objects can have attributes that help to describe what is in the object.

Perhaps the most useful attribute is names, such as column and row names in a data frame, or simply names in a vector or list.

Attributes like dimensions are also important as they can modify the behavior of objects, like turning a vector into a matrix.

Copied from R Programming for Data Science – Roger D. Peng