I want to `droplevels` a dataframe (please do not mark this question as duplicate :)). 
Given all the methods available only one works. What am I doing wrong?
Example: 

```
> df = data.frame(x = (c("a","b","c")),y=c("d","e","f"))
> class(df$x)
[1] "factor"
> levels(df$x)
[1] "a" "b" "c"

```

Method 1 not working:

```
> df1 = droplevels(df)
> class(df1$x)
[1] "factor"
> levels(df1$x)
[1] "a" "b" "c"

```

Method 2 not working:

```
> df2 = as.data.frame(df, stringsAsFactors = FALSE) 
> class(df2$x)
[1] "factor"
> levels(df2$x)
[1] "a" "b" "c"

```

Method 3 not working:

```
> df3 = df
> df3$x = factor(df3$x) 
> class(df3$x)
[1] "factor"
> levels(df3$x)
[1] "a" "b" "c"

```

Method 4 finally works:

```
> df4 = df
> df4$x = as.vector(df4$x)
> class(df4$x)
[1] "character"
> levels(df4$x)
NULL

```

While working, I think method 4 is the least elegant. Can you help me to debug this? Many thanks

EDIT: Following comments and answers: I want to remove the factor structure from a data frame and not only `droplevels`