On 12/05/2009 6:18 AM, Titus von der Malsburg wrote:
Hi list,
I have a function that detects saccadic eye movements in a time series
of eye positions sampled at a rate of 250Hz. This function needs
three vectors: x-coordinate, y-coordinate, trial-id. This information
is usually contained in a data frame that also has some other fields.
The names of the fields are not standardized.
head(eyemovements)
time x y trial
51 880446504 53.18 375.73 1
52 880450686 53.20 375.79 1
53 880454885 53.35 376.14 1
54 880459060 53.92 376.39 1
55 880463239 54.14 376.52 1
56 880467426 54.46 376.74 1
There are now several possibilities for the signature of the function:
1. Passing the columns separately:
detect(eyemovements$x, eyemovements$y, eyemovements$trial)
or:
with(eyemovements,
detect(x, y, trial))
I'd choose this one, with one modification described below.
2. Passing the data frame plus the names of the fields:
detect(eyemovements, "x", "y", "trial")
I think this is too inflexible. What if you want to temporarily change
one variable? You don't want to have to create a whole new dataframe,
it's better to just substitute in another variable.
3. Passing the data frame plus a formula specifying the relevant
fields:
detect(eyemovements, ~x+y|trial)
4. Passing a formula and getting the data from the environment:
with(eyemovements,
detect(~x+y|trial))
Rather than 3 or 4, I would use the more common idiom
detect(~x+y|trial, data=eyemovements)
(and the formula might be x+y~trial). But I think the formula interface
is too general for your needs. What would ~x+y+z|trial mean?
I'd suggest something like 1 but using the convention plot.default()
uses, where you have x and y arguments, but y can be skipped if x is a
matrix/dataframe/formula/list. It uses the xy.coords() function to do
the extraction.
Duncan Murdoch
I saw instances of all those variants (and others) in the wild.
Is there a canonical way to tell a function which fields in a data
frame are relevant? What other alternatives are possible? What are
the pros and cons of the alternatives?
Thanks, Titus
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