Difference between as.data.frame(x) and data.frame(x)

Question

What is the difference between as.data.frame(x) and data.frame(x)

In this following example, the result is the same at the exception of the columns names.

Answer 1

Looking at the code, as.data.frame fails faster. data.frame will issue warnings, and do things like remove rownames if there are duplicates:

> x <- matrix(data=rep(1,9),nrow=3,ncol=3)
> rownames(x) <- c("a", "b", "b")
> data.frame(x)
  X1 X2 X3
1  1  1  1
2  1  1  1
3  1  1  1
Warning message:
In data.row.names(row.names, rowsi, i) :
  some row.names duplicated: 3 --> row.names NOT used

> as.data.frame(x)
Error in (function (..., row.names = NULL, check.rows = FALSE, check.names =        
TRUE,  : 
  duplicate row.names: b