如何从数字转换为R中的因子

Question

I have the following dataframe:

df_raw <- cbind( P1=c(1,1,2,2,3), P2=c(2,3,4,4,4) )

How can I convert these numbers to factors with different levels, so that I have something like:

head(df_factor)
    P1  P2
1   "alpha" "beta"
2   "alpha" "gamma"
3   "beta"  "delta"

ie where 1 gets converted to "alpha", 2 to "beta", 3 to "gamma" and so on. I know I could use an ifelse statement but this would be more tedious than having some way of just converting the factor levels.

If I try for example:

df$P1 <- factor(df$P1, levels=c("alpha","beta","gamma" ))

I get NAs for values.

Answer 1

Firstly, using cbind gives a matrix , not a data.frame - try:

df <- data.frame( P1=c(1,1,2,2,3), P2=c(2,3,4,4,4) )

Then use labels for your labels instead of levels , and set levels to 1:4 to cover all the possible options in df$P1 and df$P2

df$P1 <- factor(df$P1, levels=1:4, labels=c("alpha","beta","gamma","delta"))
df

#     P1 P2
#1 alpha  2
#2 alpha  3
#3  beta  4
#4  beta  4
#5 gamma  4

df$P1
#[1] alpha alpha beta  beta  gamma
#Levels: alpha beta gamma delta

You could use lapply to tackle all the variables in one step:

df <- data.frame( P1=c(1,1,2,2,3), P2=c(2,3,4,4,4) )
data.frame(lapply(df,factor,levels=1:4,labels=c("alpha","beta","gamma","delta")))

#     P1    P2
#1 alpha  beta
#2 alpha gamma
#3  beta delta
#4  beta delta
#5 gamma delta