[R] apply problem

[Prof Brian Ripley]

On Fri, 13 Apr 2007, aedin culhane wrote:

> Dear R-Help
> I am running apply on a data.frame containing factors and numeric
> columns.  It appears to convert are columns into as.character? Does it
> convert data.frame into matrix? Is this expected? I wish it to recognise

Yes, and quite explicit on the help page

Arguments:

        X: the array to be used.
               ^^^^^
      If 'X' is not an array but has a dimension attribute, 'apply'
      attempts to coerce it to an array via 'as.matrix' if it is
      two-dimensional (e.g., data frames) or via 'as.array'.

I am baffled as to how you managed to miss this, as it is part of the 
homework the posting guide asked you to do *before* posting.



> numerical columns and round numbers.  Can I use another function instead
> of apply, or should I use a for loop in the case?

You haven;t told us _what_ you want to do, but lapply works by column.

> > summary(xmat)
>        A               B             C             D
>  Min.   :  1.0   414    :  1   Stage 2:  5   Min.   :-0.075369
>  1st Qu.:113.8   422    :  1   Stage 3:  6   1st Qu.:-0.018102
>  Median :226.5   426    :  1   Stage 4:441   Median :-0.003033
>  Mean   :226.5   436    :  1                 Mean   : 0.008007
>  3rd Qu.:339.2   460    :  1                 3rd Qu.: 0.015499
>  Max.   :452.0   462    :  1                 Max.   : 0.400578
>                  (Other):446
>        E                F                G
>  Min.   :0.2345   Min.   :0.9808   Min.   :0.01558
>  1st Qu.:0.2840   1st Qu.:0.9899   1st Qu.:0.02352
>  Median :0.3265   Median :0.9965   Median :0.02966
>  Mean   :0.3690   Mean   :1.0079   Mean   :0.03580
>  3rd Qu.:0.3859   3rd Qu.:1.0129   3rd Qu.:0.03980
>  Max.   :2.0422   Max.   :1.3742   Max.   :0.20062
>
> > for(i in 1:7) print(class(xmat[,i]))
> [1] "integer"
> [1] "factor"
> [1] "factor"
> [1] "numeric"
> [1] "numeric"
> [1] "numeric"
> [1] "numeric"

Better, sapply(xmat, class).

> > apply(xmat, 2, class)
>           A           B           C           D           E           F
> "character" "character" "character" "character" "character" "character"
>           G
> "character"

Well, all columns of a matrix are of the same class.


-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595