How to Convert a Factor to Integer/Numeric Without Loss of Information

R Casting R Faq

Introduction

When working with the R programming language, there may be situations where you need to convert a factor variable to integer/numeric without losing any information. By default, when you try to convert a factor to a numeric or integer, you get the underlying level codes instead of the actual values as numbers.

Problem Description

Let's consider a scenario where you have a factor variable called 'f', which has random values generated using the 'sample' function:

            
                f <- factor(sample(runif(5), 20, replace = TRUE))
                ##  [1] 0.0248644019011408 0.0248644019011408 0.179684827337041 
                ##  [4] 0.0284090070053935 0.363644931698218  0.363644931698218 
                ##  [7] 0.179684827337041  0.249704354675487  0.249704354675487 
                ## [10] 0.0248644019011408 0.249704354675487  0.0284090070053935
                ## [13] 0.179684827337041  0.0248644019011408 0.179684827337041 
                ## [16] 0.363644931698218  0.249704354675487  0.363644931698218 
                ## [19] 0.179684827337041  0.0284090070053935
                ## 5 Levels: 0.0248644019011408 0.0284090070053935 ... 0.363644931698218

When you try to convert this factor variable to numeric using the 'as.numeric' function:

            
                as.numeric(f)
                ##  [1] 1 1 3 2 5 5 3 4 4 1 4 2 3 1 3 5 4 5 3 2

You can see that it returns the underlying level codes of the factor variable as numbers, not the actual values. The same happens when you try to convert it to integer using the 'as.integer' function:

            
                as.integer(f)
                ##  [1] 1 1 3 2 5 5 3 4 4 1 4 2 3 1 3 5 4 5 3 2

The real values of the factor variable can be obtained by using the 'paste' function:

            
                as.numeric(paste(f))
                ##  [1] 0.02486440 0.02486440 0.17968483 0.02840901 0.36364493 0.36364493
                ##  [7] 0.17968483 0.24970435 0.24970435 0.02486440 0.24970435 0.02840901
                ## [13] 0.17968483 0.02486440 0.17968483 0.36364493 0.24970435 0.36364493
                ## [19] 0.17968483 0.02840901

Solution

Method 1: Using as.numeric(as.character(...))

A better way to convert a factor to numeric or integer without losing information is to use the 'as.character' function along with 'as.numeric'. This method first converts the factor to a character and then to numeric/integer:

            
                as.numeric(as.character(f))

Using this approach will give you the correct numeric values:

            
                ##  [1] 0.02486440 0.02486440 0.17968483 0.02840901 0.36364493 0.36364493
                ##  [7] 0.17968483 0.24970435 0.24970435 0.02486440 0.24970435 0.02840901
                ## [13] 0.17968483 0.02486440 0.17968483 0.36364493 0.24970435 0.36364493
                ## [19] 0.17968483 0.02840901

Method 2: Using the 'levels' and 'as.numeric' functions

Another approach is to use the 'levels' and 'as.numeric' functions to convert the factor levels to numeric:

            
                as.numeric(levels(f))[f]

This method also gives the correct numeric values:

            
                ##  [1] 0.02486440 0.02486440 0.17968483 0.02840901 0.36364493 0.36364493
                ##  [7] 0.17968483 0.24970435 0.24970435 0.02486440 0.24970435 0.02840901
                ## [13] 0.17968483 0.02486440 0.17968483 0.36364493 0.24970435 0.36364493
                ## [19] 0.17968483 0.02840901

Conclusion

When you need to convert a factor to numeric or integer without loss of information, using the 'as.character' function in conjunction with 'as.numeric' is a reliable and recommended approach. Additionally, the 'levels' and 'as.numeric' functions can also be used to achieve the desired result. By following these methods, you can successfully convert a factor variable to numeric/integer without any loss of information in R.

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