R help - Feb 2012 - question about user written function (newb question)

Quick newb question about R relating to the line of code below:

rawCool = read.zoo("cooling.txt", FUN = as.chron, format =
"%m/%d/%Y %H:%M",
sep = "\t", aggregate = function(x) tail(x, 1))

I'm wondering what the specifics are for the argument where it has
"aggregate = function(x) tail(x, 1)". I understand that it removes the
last
row of duplicates/aggregates in the zoo series. I'm confused as to why
"tail(x, 1)", a built in function in the utils package, requires the
coder
to treat it as a user written function thus defining the assignment, in this
case an argument, with "function(x)". Why can't the coder just
write
"tail(x, 1)" instead? Also, with the argument 'x', within
tail, I'm assuming
it's looking at all columns simultaneously within the zoo series? Is that
correct to say? Thanks.

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Short answer to a very good question: one has to use "function(x)
tail(x, 1)"  syntax to avoid using the default tail(x, 6). There are
some other ways to achieve the same thing, but I think this syntax is
generally preferred for its clarity.

Other question: yes I believe so.

Michael

On Sun, Feb 26, 2012 at 9:06 AM, knavero <knavero at gmail.com>
wrote:
> Quick newb question about R relating to the line of code below:
>
> rawCool = read.zoo("cooling.txt", FUN = as.chron, format =
"%m/%d/%Y %H:%M",
> sep = "\t", aggregate = function(x) tail(x, 1))
>
> I'm wondering what the specifics are for the argument where it has
> "aggregate = function(x) tail(x, 1)". I understand that it
removes the last
> row of duplicates/aggregates in the zoo series. I'm confused as to why
> "tail(x, 1)", a built in function in the utils package, requires
the coder
> to treat it as a user written function thus defining the assignment, in
this
> case an argument, with "function(x)". Why can't the coder
just write
> "tail(x, 1)" instead? Also, with the argument 'x', within
tail, I'm assuming
> it's looking at all columns simultaneously within the zoo series? Is
that
> correct to say? Thanks.
>
> --
> View this message in context:
http://r.789695.n4.nabble.com/question-about-user-written-function-newb-question-tp4422187p4422187.html
> Sent from the R help mailing list archive at Nabble.com.
>
> ______________________________________________
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> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

On Sun, Feb 26, 2012 at 9:06 AM, knavero <knavero at gmail.com>
wrote:
> Quick newb question about R relating to the line of code below:
>
> rawCool = read.zoo("cooling.txt", FUN = as.chron, format =
"%m/%d/%Y %H:%M",
> sep = "\t", aggregate = function(x) tail(x, 1))
>
> I'm wondering what the specifics are for the argument where it has
> "aggregate = function(x) tail(x, 1)". I understand that it
removes the last
> row of duplicates/aggregates in the zoo series. I'm confused as to why
> "tail(x, 1)", a built in function in the utils package, requires
the coder
> to treat it as a user written function thus defining the assignment, in
this
> case an argument, with "function(x)". Why can't the coder
just write
> "tail(x, 1)" instead? Also, with the argument 'x', within
tail, I'm assuming
> it's looking at all columns simultaneously within the zoo series? Is
that
> correct to say? Thanks.
1.  tail(x, 1) is not a function.  Its the operation of executing the
function tail with the values x and 1.  Even if x existed it would be
an error since aggregate= requires a function, not a value.   One
could have written aggregate = tail except that the default is 6
elements whereas we want 1.

One could write this where tail1 is defined to be the same as tail
except its hard coded to use the last element of x only:

tail1 <- function(x) tail(x, 1)
read.zoo(...whatever..., aggregate = tail1)

2.  aggregate= ultimately calls aggregate in the core of R and that
works by looking at the columns one by one.   For a particular column
it splits it into groups (in the case of aggregate.zoo the groups are
defined by rows having the same times) and then for each group
applying the indicated function.

See ?read.zoo and the vignette("zoo-read") for more.

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