How to Convert Factor to Numeric With the as.numeric Function in R

1. Use the as.numeric Function to Convert Factor to Numeric in R
2. Combine as.character and as.numeric Functions to Convert Factor to Numeric in R

This article will demonstrate multiple methods about how to convert factor to numeric in R.

Use the as.numeric Function to Convert Factor to Numeric in R

The as functions are generally used to convert data type to another type explicitly. The conversion process is called coercion in R terminology, and it denotes the concept of casting present in other programming languages. When we call to function as.logical, it tries to coerce the passed argument to the logical type. Similarly, as.character and as.numeric convert the given arguments to the corresponding types. Notice that any number (except zero) converted to the logical type represents a TRUE value, even negatives.

> as.logical(31)
[1] TRUE

> as.logical(-31)
[1] TRUE

> as.character(31)
[1] "31"

> as.numeric(FALSE)
[1] 0

> as.numeric(TRUE)
[1] 1

R also implements implicit type coercion rules, which are needed when arithmetic operations are done on the vectors consisting of different types. As demonstrated in the following code snippet, if an atomic vector contains a character string, every other element is converted to the string as well. If the vector includes mixed elements of logic, numbers, and strings, they are coerced to string types. Finally, when the vector contains numbers and logic, the latter ones are converted to numbers.

> v1 <- c(1, "two", 3, 4, 5, 6)

> typeof(v1)
[1] "character"

> v1 <- c(T, "two", 3, 1, F, T)

> typeof(v1)
[1] "character"

> v1 <- c(1, T, 3, F, 5, 6)

> typeof(v1)
[1] "double"

When converting a factor to numeric, it’s important to note that as.numeric will only return the underlying integer representation, which is mostly meaningless and does not correspond to the factor levels. One can examine how a factor object is stored internally using the unclass function. Notice that the f2 factor has an index of 2 1 2 3, which is generally returned using the as.number call as shown in the next code sample:

> f2 <- factor(c(3, 2, 3, 4))

> f2
[1] 3 2 3 4
Levels: 2 3 4

> f2.n <- as.numeric(f2)

> unclass(f2)
[1] 2 1 2 3
attr(,"levels")
[1] "2" "3" "4"

> f2.n
[1] 2 1 2 3

Combine as.character and as.numeric Functions to Convert Factor to Numeric in R

The factor levels can be converted to the integer type by combining as.character and as.numeric functions. as.character returns factor levels as character strings. Then we can call the as.numeric function to coerce string to numbers.

> f2 <- factor(c(3, 2, 3, 4))

> f2.c <- as.character(f2)

> f2.c
[1] "3" "2" "3" "4"

> f2.c <- as.numeric(as.character(f2))

> f2.c
[1] 3 2 3 4