John, Thank you for your comment.
There is no secret. But the actual function I need to call is rather irrelevant. However don't take it as the "abs" function. If you like to know, it is a function that converts 4 kinds of old ids from several old database tables into a new id in a new database. Again, I don't think providing such detail is better than saying MyDummyFunc maps a number into a number but doesn't work with vectors. All I need to do, is to call DummyFunc for every element in a column of a data.frame and returns the resulted vector. But, I cannot change DummyFunc. Correct me if I am wrong: this is rather common in a group collaboration enviroment. Person A may be responsible for writing a function and person B who needs to use that function cannot or better not change it. Obviously, I could write a loop. Michael in a previous post suggested using vectorize which works perfectly. As a newbie of R, I would wish to learn more ways to achieve my goal (sorry, it automatically involves "how" not just "what" ;). Is there a way using a "*apply" function to do it where * stands for any function. Thanks a lot! ________________________________ From: John Fox <j...@mcmaster.ca> To: 'Alex Zhang' <alex.zh...@ymail.com> Cc: r-help@r-project.org Sent: Tuesday, December 27, 2011 4:06 PM Subject: RE: [R] sapply Call Returning " the condition has length > 1" Error Dear Alex, > -----Original Message----- > From: Alex Zhang [mailto:alex.zh...@ymail.com] > Sent: December-27-11 3:34 PM > To: John Fox > Cc: r-help@r-project.org > Subject: Re: [R] sapply Call Returning " the condition has length > 1" > Error > > John, > > Thanks for the pointers. > > The DummyFunc is just a made-up example. The true function I need to > use is more complicated and would be distractive to include. You'll probably get a better answer if you don't keep what you want to do a secret. > > Do you mean that sapply would take columns in the input data.frame and > feed them into "FUN" as "whole" vectors? That explains the behavior. Yes. As I said, a data frame is a list of columns, so FUN is called with each column as its argument. > Is there an "*apply" function that will fee elements of the input > data.frame into "FUN" instead of whole columns? Thanks. I'm afraid that I don't know what you mean. Do you want to deal with the columns of the data frame separately (in general, they need not all be of the same class), and within each column, apply a function separately to each element? You could nest calls to lapply() or sapply(), as in sapply(D, function(DD) sapply(DD, abs)) assuming, of course, that D is an entirely numeric data frame. But in this case, abs(as.matrix(D)) would be more sensible, and using sapply() like this isn't necessarily better than a loop. Again, not knowing what you want to do makes it hard to suggest a solution. Best, John > > ________________________________ > > From: John Fox <j...@mcmaster.ca> > To: 'Alex Zhang' <alex.zh...@ymail.com> > Cc: r-help@r-project.org > Sent: Tuesday, December 27, 2011 3:10 PM > Subject: RE: [R] sapply Call Returning " the condition has length > 1" > Error > > Dear Alex, > > > -----Original Message----- > > From: r-help-boun...@r-project.org [mailto:r-help-bounces@r- > > project.org] On Behalf Of Alex Zhang > > Sent: December-27-11 2:14 PM > > To: r-help@r-project.org > > Subject: [R] sapply Call Returning " the condition has length > 1" > > Error > > > > Dear all, > > > > Happy new year! > > > > I have a question re using sapply. Below is a dummy example that > would > > replicate the error I saw. > > > > ##Code Starts here > > DummyFunc <- function(x) { > > > > if (x > 0) { > > return (x) > > } else > > { > > return (-x) > > } > > > > } > > > > Y = data.frame(val = c(-3:7)) > > sapply(Y, FUN = DummyFunc) > > ##Code ends here > > > > When I run it, I got: > > val > > [1,] 3 > > [2,] 2 > > [3,] 1 > > [4,] 0 > > [5,] -1 > > [6,] -2 > > [7,] -3 > > [8,] -4 > > [9,] -5 > > [10,] -6 > > [11,] -7 > > Warning message: > > In if (x > 0) { : > > the condition has length > 1 and only the first element will be > used > > > > The result is different from what I would expect plus there is such > an > > error message. > > This is a warning, not really an error message. A data frame is > essentially a list of variables (columns), and sapply() applies its > FUN argument to each list element, that is, each variable -- the one > variable val in your case. > That produces a warning because val > 0 is a vector of 11 elements, > and the first comparison, 3 > 0, which is TRUE, controls the result. > > > > > I guess if the DummyFunc I provided is compatible with vectors, the > > problem would go away. But let's suppose I cannot change DummyFunc. > Is > > there still a way to use sapply or alike without actually writing a > > loop? Thanks. > > Well, you could just use > > > abs(Y$val) > [1] 3 2 1 0 1 2 3 4 5 6 7 > > but I suppose that you didn't really want to write your own version of > the absolute-value function as something more than an exercise. > > An alternative is > > > with(Y, ifelse(val > 0, val, -val)) > [1] 3 2 1 0 1 2 3 4 5 6 7 > > I hope this helps, > John > > -------------------------------- > John Fox > Senator William McMaster > Professor of Social Statistics > Department of Sociology > McMaster University > Hamilton, Ontario, Canada > http://socserv.mcmaster.ca/jfox > > > > > > > - Alex > > [[alternative HTML version deleted]] > > > > [[alternative HTML version deleted]]
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