Convert all character variables in your data.frame to factors

df.make.factors <- function(df)
{ 
    ## Purpose: Convert all character variables in your data.frame to factors 
    ## Arguments:
    ##   df: a data frame
    ## Return: a data frame with converted factors on those character variables.
    ## Author: Feiming Chen, Date: 17 Oct 2017, 14:26
    ## ________________________________________________

    character_vars <- sapply(df, class) == "character"
    df[, character_vars] <- lapply(df[, character_vars], factor)
    df
}
if (F) {                                # Unit Test
    df <- data.frame(x=c("x", "x", "y"), y=1:3, z=c("1", "3", "3"))
    df.make.factors(df)
}