R Variables, Constants and Reserved Words

R Variables

Variables are very similar to an open box, where you can put any value of your wish. We can change the values as well. Variables are used to store data.

A variable in R is used to store an atomic vector, group of atomic vectors or a combination of many R-objects.

• A valid variable name can be a combination of numbers, letters, and the dot or underline character.
• A valid variable name must start with a letter or a dot and if it starts with a dot, it cannot be followed by a number.
• Reserved words in R cannot be used as a variable name. 

Variable Name	Validity
total	Valid
Sum	Valid
.this.is.ok	Valid
this _is_acceptable	Valid
tot@l	Invalid
var_name%	Invalid
_var_name	Invalid
5um	Invalid
TRUE	Invalid

We can separate the variables by the types of values they contain:

Boolean Variables: This is the simplest type of variable. It contains a single bit, and indicate a binary result (0 and 1, yes and no, or true and false).

Examples of Boolean variables are:

a = TRUE
b = FALSE

Integer variables: Numbers with no floating point are called integers. In R programming, sometimes it is difficult to declare a single integer. In most cases, try to do so will actually declare a numeric value.

Numeric Variables: Numeric variables are used to store numbers. It can contain floating point numbers.

Examples of Numeric variables are:

a = 1
b = 3.14

Characters Variables:  Character variables are used to store non-numeric data. Unlike other programming languages, there are a no differences between characters and strings in R.  Characters are assigned using quotation mark. R does not care if you use double or single quotes. Examples are:

a = “x”
c = “6”

String Variables:  String variables are those variables which contain one or more characters.  Examples are:

a = “abcd2”
b = “Hello World”

How to Create, Rename and Recode variables in R

In R, it is simple task to create a new variable or to convert an old variable into a new one.

Let's understand these all operations of variable through the given example:

First of all create a data set:

> hospital <- c("India", "US")
> patients <- c(200, 250)
> costs <- c(1000, 2000)
> df <- data.frame(hospital, patients, costs)
> df

Here, the name of dataset is df. When you run the above command, the output will be:

      hospital patients costs
1    India      200  1000
2       US      250  2000

Now we will create a new variable called totalcost as shown below:

> df$totalcost <- df$patients * df$costs
> df

Let's see the dataset again:

  hospital patients costs totalcost
1    India      200   2.5       500
2       US      250   3.1       775

Now, if we want to rename and recode a variable “costs” to “costs_rs” then:

> df$costs_rs <- df$costs
> df

Now the data set is:

  hospital patients costs totalcost costs_rs
1    India      200   2.5       500      2.5
2       US      250   3.1       775      3.1

We can also delete the variable through the NULL command, if we want to delete the costs variable then:

> df$totcosts <- NULL
> df

Then the dataset will be changed:

  hospital patients totalcost costs_rs
1    India      200       500      2.5
2       US      250       775      3.1

Now, let’s see that how to recode a variable “patients”. Here we use ifelse(), but you can use other functions as well:

df$patients <- ifelse(df$patients==200, 100, ifelse(df$patients==250, 300, NA))

Let's see the dataset again:

     hospital patients totalcost costs_rs
1    India      100       500      2.5
2       US      300       775      3.1

R Constants

Constants are the entities whose value cannot be changed. Basically there are two types of constants:

2. Numeric Constants: numeric constants are the numbers which can be integer, double or complex. You can check the type of constant through the typeof() function. Numeric constant suffix with L are the integer type and suffix with i are called complex type.

Example:

> typeof(10)
[1] "double"
> typeof(10L)
[1] "integer"
> typeof(10i)
[1] "complex"

2. Character Constant: Character constant can be declared using either single quote (‘ ’) or double quote (“ ”).

> typeof(“nikita”)
[1] “character”
> typeof(‘hello’)
[1] “character”

Built-in Constants: Some of the built-in constants of R along with their values are shown below:

> LETTERS
[1] "A" "B" "C" "D" "E" "F" "G" "H" "I" "J" "K" "L" "M" "N" "O" "P" "Q" "R" "S" "T"
[21] "U" "V" "W" "X" "Y" "Z"
> letters
[1] "a" "b" "c" "d" "e" "f" "g" "h" "i" "j" "k" "l" "m" "n" "o" "p" "q" "r" "s" "t"
[21] "u" "v" "w" "x" "y" "z"
> month.name
[1] "January"   "February"  "March"     "April"     "May"       "June"     
[7] "July"      "August"    "September" "October"   "November"  "December"
> month.abb
[1] "Jan" "Feb" "Mar" "Apr" "May" "Jun" "Jul" "Aug" "Sep" "Oct" "Nov" "Dec"
> pi
[1] 3.141593

R Reserved Words

The set of words that have special meaning and cannot be used as an identifier (variable name, function name, etc.) are called Reserved words.

Following is the list of reserved words in R:

Reserved Words	Purpose
else	Conditional statements
repeat	Loop
While	Loop
function	User defined function
for	Loop
in	Loop
break	Loop
If	Conditional statements
TRUE	Logical constant
FALSE	Logical constant
NULL	Represents the absence of a value or an undefined value
Inf	Infinity
NaN	“Not a number”, e.g. when 0 is dived by 0
NA	Not Available

This list of reserved words can be viewed by the command help (reserved) or ?reserved.

> ?reserved

Note: Since R is a case sensitive language therefore TRUE and True are not the same. Here, TRUE is a reserved word and True can be a variable name. Reference: https://www.datamentor.io/r-programming/variable-constant/ https://intellipaat.com/tutorial/r-programming/basic-syntax-data-types-and-variables/