R help - Jul 2010 - What does `_data` mean in transform()?

Hi All,

I meant to take the min row by row. But the result is apparently not
what I want. Changing min to pmin solve the problem.
> df=data.frame(X=1:10, Y=1:10)
> transform(df, Z=min(X,10-Y))
    X  Y Z
1   1  1 0
2   2  2 0
3   3  3 0
4   4  4 0
5   5  5 0
6   6  6 0
7   7  7 0
8   8  8 0
9   9  9 0
10 10 10 0

I try to look at the source code to understand what transform() does.
I know that ` can be used to refer to a column in a data.frame. But I
don't understand what the usage of `_data` in transform.data.frame.
Could you please help me understand what `_data` means and min doesn't
work row by row?

> transform.data.frame
function (`_data`, ...)
{
    e <- eval(substitute(list(...)), `_data`, parent.frame())
    tags <- names(e)
    inx <- match(tags, names(`_data`))
    matched <- !is.na(inx)
    if (any(matched)) {
        `_data`[inx[matched]] <- e[matched]
        `_data` <- data.frame(`_data`)
    }
    if (!all(matched))
        do.call("data.frame", c(list(`_data`), e[!matched]))
    else `_data`
}
<environment: namespace:base>


-- 
Tom

On Jul 7, 2010, at 6:32 PM, thmsfuller066 at gmail.com wrote:
> Hi All,
>
> I meant to take the min row by row. But the result is apparently not
> what I want. Changing min to pmin solve the problem.
>
>> df=data.frame(X=1:10, Y=1:10)
>> transform(df, Z=min(X,10-Y))
>    X  Y Z
> 1   1  1 0
> 2   2  2 0
> 3   3  3 0
> 4   4  4 0
> 5   5  5 0
> 6   6  6 0
> 7   7  7 0
> 8   8  8 0
> 9   9  9 0
> 10 10 10 0
>
> I try to look at the source code to understand what transform() does.
> I know that ` can be used to refer to a column in a data.frame. But I
> don't understand what the usage of `_data` in transform.data.frame.
> Could you please help me understand what `_data` means
It is a function argument that gets assigned a fvalue after a function  
call.

> and min doesn't
> work row by row?
?pmin
>
>
>> transform.data.frame
> function (`_data`, ...)
> {
>    e <- eval(substitute(list(...)), `_data`, parent.frame())
>    tags <- names(e)
>    inx <- match(tags, names(`_data`))
>    matched <- !is.na(inx)
>    if (any(matched)) {
>        `_data`[inx[matched]] <- e[matched]
>        `_data` <- data.frame(`_data`)
>    }
>    if (!all(matched))
>        do.call("data.frame", c(list(`_data`), e[!matched]))
>    else `_data`
> }
> <environment: namespace:base>
>
>
> -- 
> Tom
-- 

David Winsemius, MD
West Hartford, CT

thmsfuller066 at gmail.com wrote:
> Could you please help me understand what `_data` means and min doesn't
> work row by row?
`_data` is just a name (once upon a time, it was called "x" but then
someone wanted to create a new variable called x).

min() calculates the scalar minimum of everything it gets. It doesn't
work by row, because that's what pmin() does.... (It's a design choice,
you just have to believe that it is intentional and that min(X,Y) =min(c(X,Y))
has useful applications).

-- 
Peter Dalgaard
Center for Statistics, Copenhagen Business School
Phone: (+45)38153501
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com