Recode categorical factor with N categories into N binary columns

Question

Original data frame:

v1 = sample(letters[1:3], 10, replace=TRUE)
v2 = sample(letters[1:3], 10, replace=TRUE)
df = data.frame(v1,v2)
df
         


        

Answer 1

A fairly direct approach is to just use table on each column, tabulating the values in the column by the number of rows in the data.frame:

allLevels <- levels(factor(unlist(df)))
do.call(cbind, 
        lapply(df, function(x) table(sequence(nrow(df)), 
                                     factor(x, levels = allLevels))))
#    a b c a b c
# 1  0 1 0 0 0 1
# 2  1 0 0 1 0 0
# 3  0 0 1 0 0 1
# 4  0 1 0 1 0 0
# 5  0 0 1 0 0 1
# 6  0 0 1 0 1 0
# 7  1 0 0 1 0 0
# 8  1 0 0 0 1 0
# 9  1 0 0 0 0 1
# 10 1 0 0 0 1 0

I've used factor on "x" to make sure that even in cases where there are, say, no "c" values in a column, there will still be a "c" column in the output, filled with zeroes.