[R] Data Structure to Code

[Gabor Grothendieck]

On Sun, Jan 29, 2012 at 11:36 AM, Ajay Askoolum <aa2e72e at yahoo.co.uk> wrote:
> Thank you. I need some clarification.
>
> dput(AirPassengers)
>
> gives:
>
> structure(c(112, 118, 132, 129, 121, 135, 148, 148, 136, 119,
> 104, 118, 115, 126, 141, 135, 125, 149, 170, 170, 158, 133, 114,
> 140, 145, 150, 178, 163, 172, 178, 199, 199, 184, 162, 146, 166,
> 171, 180, 193, 181, 183, 218, 230, 242, 209, 191, 172, 194, 196,
> 196, 236, 235, 229, 243, 264, 272, 237, 211, 180, 201, 204, 188,
> 235, 227, 234, 264, 302, 293, 259, 229, 203, 229, 242, 233, 267,
> 269, 270, 315, 364, 347, 312, 274, 237, 278, 284, 277, 317, 313,
> 318, 374, 413, 405, 355, 306, 271, 306, 315, 301, 356, 348, 355,
> 422, 465, 467, 404, 347, 305, 336, 340, 318, 362, 348, 363, 435,
> 491, 505, 404, 359, 310, 337, 360, 342, 406, 396, 420, 472, 548,
> 559, 463, 407, 362, 405, 417, 391, 419, 461, 472, 535, 622, 606,
> 508, 461, 390, 432), .Tsp = c(1949, 1960.91666666667, 12), class = "ts")
>
> and AirPassengers looks as follows:
>
> AirPassengers
>      Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
> 1949 112 118 132 129 121 135 148 148 136 119 104 118
> 1950 115 126 141 135 125 149 170 170 158 133 114 140
> 1951 145 150 178 163 172 178 199 199 184 162 146 166
> 1952 171 180 193 181 183 218 230 242 209 191 172 194
> 1953 196 196 236 235 229 243 264 272 237 211 180 201
> 1954 204 188 235 227 234 264 302 293 259 229 203 229
> 1955 242 233 267 269 270 315 364 347 312 274 237 278
> 1956 284 277 317 313 318 374 413 405 355 306 271 306
> 1957 315 301 356 348 355 422 465 467 404 347 305 336
> 1958 340 318 362 348 363 435 491 505 404 359 310 337
> 1959 360 342 406 396 420 472 548 559 463 407 362 405
> 1960 417 391 419 461 472 535 622 606 508 461 390 432
>
> Where are the column headers in the result of dput(AirPassengers)?
>
> I guess it must be something to do with the '12' in the final argument of .Tsp.
>
> So, If I just wanted 'Jan' ... 'Jun' and specified the following:
>

AirPassengers is a "ts" class object.  "ts" class objects represent
regularly spaced series.  If the frequency of the regularly spaced
series is 12 then the elements of the series are assumed to represent
monthly values -- that assumption is where the month names magically
come from.

If you want to take the first 6 months of each year then the resulting
series is no longer regularly spaced since some elements are now one
month later than the prior element and others are 6 months later.
Not being regularly spaced we can't use "ts" any more.  We could use
"zoo" class from the zoo package which is intended to represent series
which are not necessarily regularly spaced:

library(zoo)
z <- as.zoo(AirPassengers)
z6 <- z[cycle(z) <= 6]  # first 6 mos in each yr

# optionally use yearmon class for the time index
time(z6) <- yearmon(time(z6))

Read the vignettes and help files/reference manual in the zoo package
for more info.
http://cran.r-project.org/package=zoo


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