Mapping arbitrary strings to to arbitrary numbers

gistfile1.txt

# I have a vector of strings that map to known numeric values. 
# What's the cleanest/most reader-friendly R idiom for this conversion?

# sample data
df = expand.grid(
    x_str = c("string1", "secondstring", "blah", "garbagestring"),
    replicate=1:3,
    stringsAsFactors=FALSE)

# Approach 1: Encode the look-up table as its own dataframe
string_map = read.csv(strip=TRUE, stringsAsFactors=FALSE, text="
    x_str, x_num
    string1, 10
    secondstring, 256
    blah, 3.9
")
df = merge(df, string_map, all.x=TRUE)

# Approach 2: Enum-like
mapit = function(s){
    switch(
        s,
        "string1"=10,
        "secondstring"=256,
        "blah"=3.9,
        NA)
}
df$x_num2 = sapply(df$x_str, mapit)

# Approach 3: As a factor
df$x_num3 = factor(
    df$x_str,
    levels=c("string1", "secondstring", "blah"),
    labels=c(10, 256, 3.9))
df$x_num3 = as.numeric(levels(df$x_num3))[df$x_num3]

# Approach 4 (current winner): A simple named vector
# Credit to mrflick
mapper<-c(string1=10, secondstring=256, blah=3.9)
df$x_num4<-mapper[df$x_str]

# Are the results equivalent? Yes, they ought to be.
identical(df$x_num, df$x_num2, df$x_num3, df$x_num4)