Convert factor to integer while maintaining factor level ordering

问题

I have an R dataframe where one of the columns is a factor whose levels have an implicit ordering. How can I convert the factor levels to specific integers in the following manner:

"Strongly disagree" --> 1
"Somewhat disagree" --> 2
"Neutral" --> 3
"Somewhat agree" --> 4
"Strongly agree" --> 5

For example, here is my data frame:

agree <- c("Strongly agree", "Somewhat disagree", "Somewhat agree",
           "Neutral", "Strongly agree", "Strongly disagree", "Neutral")
age <- c(41, 35, 29, 42, 31, 22, 58)

df <- data.frame(age, agree)
df
#   age             agree
# 1  41    Strongly agree
# 2  35 Somewhat disagree
# 3  29    Somewhat agree
# 4  42           Neutral
# 5  31    Strongly agree
# 6  22 Strongly disagree
# 7  58           Neutral

str(df)
# 'data.frame': 7 obs. of  2 variables:
#  $ age  : num  41 35 29 42 31 22 58
#  $ agree: Factor w/ 5 levels "Neutral","Somewhat agree",..: 4 3 2 1 4 5 1

Now, I would like to convert the agree column to be an integer column using the mapping that I showed above.

I already searched these other questions about converting factor to integer, but they do not related to maintaining the factor ordering.

"How to convert a factor to an integer\numeric without a loss of information?"

"Convert factor to integer"

"Convert factor to integer in a data frame"

回答1:

You need to define the order of factors first:

ordering <- c("Strongly disagree", "Somewhat disagree", "Neutral", "Somewhat agree", "Strongly agree")

Then, when you first create your factor, you should use that definition:

agreeFactor <- factor(agree, levels = ordering)

Then, you should be able to get your ordered factor:

as.numeric(agreeFactor)

You can also just apply the order when using as.numeric(), but this can lead to inconsistencies if you decide to later retrieve your numeric vector and forget to apply the "levels = " argument.

e: If you want to directly import the numeric into your dataframe, simply use:

df$agree <- as.numeric(factor(df$agree, levels = ordering))

回答2:

The dplyr library has a useful revalue function for this type of operation:

library(plyr)
df$agree<-as.numeric( revalue(df$agree, c("Strongly disagree" = 1,
                     "Somewhat disagree" = 2,
                     "Neutral" = 3,
                     "Somewhat agree" = 4,
                     "Strongly agree" = 5)) )

Overall @tluh method of ordering the factors is a better approach since it maintains the original input and sets the factors into the correct order.

回答3:

If your factor is already ordered with levels you can use the following function to convert that factor into the numerical order.

Convert_Numeric = function(X) {
        L = levels(X)
        Y = as.numeric(factor(X, labels = seq(1:length(L))))
        return(Y)
}

This can be particularly useful with functions or with dplyr:

df %>%
       mutate(Numeric_version = Convert_Numeric(agree))

来源：https://stackoverflow.com/questions/38621334/convert-factor-to-integer-while-maintaining-factor-level-ordering

标签

dataframe

r-factor