R - Getting the AirPassenger data set into numeric matrix fomat

Question

I want to use the AirPassenger set for exploration, e.g., making a heatmap. Unfortunately I don't get it into a numeric matrix format. It doesn't even have columns

>ncol(AirPassengers)
NULL

Could someone help me to get this data set into a numeric matrix format? Usually, would use the as.matrix function, but in this case it doesn't work.

This is the data set

>data(AirPassengers)
>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

Answer 1

AirPassengers is a time series (see ?AirPassengers or str(AirPassengers)

You can convert to a matrix by

apm <- matrix(AirPassengers, ncol = 12, byrow =TRUE,  dimnames = list( as.character(1949:1960),month.abb)))

note you have tell matrix that it needs to be 12 columns, and to fill by row, so the columns are organized as you see in the display of the time series. The dimnames will ensure it has similar information.

Answer 2

It's worth noting that some of this confusion stems from the print method associated with the AirPassengers data, which is a timeseries and thus calls print.ts

Compare the results of print.default using a ts (timeseries) object:

> test <- ts(1:24,start=2008,frequency=12)

> test #this invokes print.ts
     Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
2008   1   2   3   4   5   6   7   8   9  10  11  12
2009  13  14  15  16  17  18  19  20  21  22  23  24

> print.ts(test)
     Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
2008   1   2   3   4   5   6   7   8   9  10  11  12
2009  13  14  15  16  17  18  19  20  21  22  23  24

> print.default(test)
 [1]  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
attr(,"tsp")
[1] 2008.000 2009.917   12.000
attr(,"class")
[1] "ts"