Factors are data structures which are implemented to categorize the data or represent categorical data and store it on multiple levels.
They can be stored as integers with a corresponding label to every unique integer. Though factors may look similar to character vectors, they are integers and care must be taken while using them as strings.
The factor accepts only a restricted number of distinct values. For example, a data field such as gender may contain values only from female, male or transgender.
In the above example, all the possible cases are known beforehand and are predefined. These distinct values are known as levels. After a factor is created it only consists of levels that are by default sorted alphabetically.
Attributes of a Factor
- x: It is the vector which needs to be converted into a factor.
- Levels: It is a set of distinct values which are given to the input vector x.
- Labels: It is a character vector corresponding to the number of labels.
- Exclude: This will mention all the values you want to exclude.
- Ordered: This logical attribute decide whether the levels are ordered.
- nmax: It will decide the upper limit for the maximum number of levels.
Creating a Factor
The command used to create or modify a factor in R language is –
factor() with a vector as input.
The two steps to creating a factor are:
- Creating a vector
- Converting the vector created into a factor using function factor()
Examples: Let us create a factor gender with levels female, male and transgender.
 "female" "male" "male" "female"  female male male female Levels: female male
Levels can also be predefined by the programmer.
 female male male female Levels: female transgender male
Further one can check the levels of a factor by using function
Checking for a Factor
is.factor() is used to check whether the variable is a factor and returns “TRUE” if it is a factor.
class() is also used to check whether the variable is a factor and if true returns “factor”.
Accessing elements of a Factor
Like we acess elements of a vector, same way we acess the elements of a factor. If gender is a factor then gender[i] would mean acessing i th element in the factor.
 male Levels: female male
More than one element can be accessed at a time.
 male female Levels: female male
 female male female Levels: female male
Modification of a Factor
After a factor is formed, its components can be modified but the new values which needs to be assigned must be in the predefined level.
 female female male female Levels: female male
For selecting all the elements of the factor gender except ith element, gender[-i] should be used.
So if you want to modify a factor and add value out of predefines levels, then first modify levels.
 female male other female Levels: female male other
Factors in Data Frame
The Data frame is similar to a 2D array with the columns containing all the values of one variable and the rows having one set of values from every column. There are four things to remember about data frames:
- column names are compulsory and cannit be empty.
- Unique names should be assigned to each row.
- The data frame’s data can be only of three types- factor, numeric and character type.
- The same number of data items must be present in each column.
In R language when we create a data frame, its column is a categorical data and hence a factor is automatically created on it.
We can create a data frame and check if its column is a factor.
age salary gender 1 40 103200 male 2 49 106200 male 3 48 150200 transgender 4 40 10606 female 5 67 10390 male 6 52 14070 female 7 53 10220 transgender  TRUE
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