[Rd] head.ts, tail.ts loses time

[Martin Maechler]

>>>>> Spencer Graves 
>>>>>     on Mon, 10 Jun 2024 07:50:13 -0500 writes:

    > Hi, Gabor et al.: Thanks for this. I should change my
    > current application to use either zoo or xts, as Gabor
    > suggests.


    > 	  However, I was surprised to learn that "[.ts" does NOT
    > return an object of class "ts". I see that "head.default"
    > and "head.matrix" both call "[", so "head" cannot return a
    > ts object, because "[" doesn't.

Yes, the default head() and tail() are built on  `[` very much
on purpose.
Note that   `[`  should *not* keep the "ts"  property  in
general, e.g.,  
	 lynx[c(1:3, 7)]
cannot be a regular time series 

I think I'd consider using  windows() for a head.ts() and tail.ts(),
but in any case, I am sympathetic adding such methods to "base R"'s
utils package.


Martin

    > 	  Best Wishes, Spencer Graves


    > On 6/9/24 8:40 PM, Gabor Grothendieck wrote:
    >> zoo overcomes many of the limitations of ts:
    >> 
    >> library(zoo) as.ts(head(as.zoo(presidents))) ## Qtr1 Qtr2
    >> Qtr3 Qtr4 ## 1945 NA 87 82 75 ## 1946 63 50
    >> 
    >> xts also works here.
    >> 
    >> On Sun, Jun 9, 2024 at 12:04 PM Spencer Graves
    >> <spencer.graves using prodsyse.com> wrote:
    >>> 
    >>> Hello, All:
    >>> 
    >>> 
    >>> The 'head' and 'tail' functions strip the time from a
    >>> 'ts' object.  Example:
    >>> 
    >>> 
    >>> > head(presidents) [1] NA 87 82 75 63 50
    >>> 
    >>> 
    >>> > window(presidents, 1945, 1946.25) Qtr1 Qtr2 Qtr3 Qtr4
    >>> 1945 NA 87 82 75 1946 63 50
    >>> 
    >>> 
    >>> Below please find code for 'head.ts' and 'tail.ts' that
    >>> matches 'window'.
    >>> 
    >>> 
    >>> Comments?  Spencer Graves
    >>> 
    >>> head.ts <- function(x, n=6L, ...){ tmx <-
    >>> as.numeric(time(x))
    >>> #
    >>> utils:::checkHT(n, d <- dim(x)) if(is.na(n[1]) ||
    >>> n[1]==0)ts(NULL)
    >>> #
    >>> firstn <- head(tmx, n[1]) if(is.null(d)){
    >>> return(window(x, firstn[1], tail(firstn, 1))) } else{
    >>> if(length(n)<2){ return(window(x, firstn[1],
    >>> tail(firstn, 1))) } else { Cols <- head(1:d[2], n[2])
    >>> xn2 <- x[, Cols[1]:tail(Cols, 1)] return(window(xn2,
    >>> firstn[1], tail(firstn, 1))) } } }
    >>> 
    >>> 
    >>> tail.ts <- function (x, n = 6L, ...)  {
    >>> utils:::checkHT(n, d <- dim(x)) tmx <-
    >>> as.numeric(time(x))
    >>> #
    >>> if(is.na(n[1]) || n[1]==0)ts(NULL)
    >>> #
    >>> lastn <- tail(tmx, n[1]) if(is.null(d)){
    >>> return(window(x, lastn[1], tail(lastn, 1))) } else{
    >>> if(length(n)<2){ return(window(x, lastn[1], tail(lastn,
    >>> 1))) } else { Cols <- head(1:d[2], n[2]) xn2 <- x[,
    >>> Cols[1]:tail(Cols, 1)] return(window(xn2, lastn[1],
    >>> tail(lastn, 1))) } } }
    >>> 
    >>> 
    >>> # examples head(presidents)
    >>> 
    >>> head(presidents, 2)
    >>> 
    >>> npresObs <- length(presidents) head(presidents,
    >>> 6-npresObs)
    >>> 
    >>> try(head(presidents, 1:2)) # 'try-error'
    >>> 
    >>> try(head(presidents, 0)) # 'try-error'
    >>> 
    >>> # matrix time series str(pres <-
    >>> cbind(n=1:length(presidents), presidents)) head(pres, 2)
    >>> 
    >>> head(pres, 2-npresObs)
    >>> 
    >>> head(pres, 1:2) head(pres, 2:1) head(pres, 1:3)
    >>> 
    >>> # examples tail(presidents)
    >>> 
    >>> tail(presidents, 2)
    >>> 
    >>> npresObs <- length(presidents) tail(presidents,
    >>> 6-npresObs)
    >>> 
    >>> try(tail(presidents, 1:2)) # 'try-error'
    >>> 
    >>> try(tail(presidents, 0)) # 'try-error'
    >>> 
    >>> # matrix time series str(pres <-
    >>> cbind(n=1:length(presidents), presidents)) tail(pres, 2)
    >>> 
    >>> tail(pres, 2-npresObs)
    >>> 
    >>> tail(pres, 1:2) tail(pres, 2:1) tail(pres, 1:3)
    >>> 
    >>> # for unit testing: headPres <- head(presidents) pres6
    >>> <- ts(presidents[1:6], time(presidents)[1],
    >>> frequency=frequency(presidents))
    >>> stopifnot(all.equal(headPres, pres6))
    >>> 
    >>> headPres2 <- head(presidents, 2) pres2 <-
    >>> ts(presidents[1:2], time(presidents)[1],
    >>> frequency=frequency(presidents))
    >>> stopifnot(all.equal(headPres2, pres2))
    >>> 
    >>> npresObs <- length(presidents) headPres. <-
    >>> head(presidents, 6-npresObs)
    >>> stopifnot(all.equal(headPres., pres6))
    >>> 
    >>> headPresOops <- try(head(presidents, 1:2))
    >>> stopifnot(class(headPresOops) == 'try-error')
    >>> 
    >>> headPres0 <- try(head(presidents, 0))
    >>> stopifnot(class(headPres0) == 'try-error')
    >>> 
    >>> str(pres <- cbind(n=1:length(presidents), presidents))
    >>> headP2 <- head(pres, 2)
    >>> 
    >>> p2 <- ts(pres[1:2, ], time(presidents)[1],
    >>> frequency=frequency(presidents))
    >>> stopifnot(all.equal(headP2, p2))
    >>> 
    >>> headP2. <- head(pres, 2-npresObs)
    >>> stopifnot(all.equal(headP2., p2))
    >>> 
    >>> 
    >>> #############
    >>> 
    >>> 
    >>> sessionInfo() R version 4.4.0 (2024-04-24) Platform:
    >>> aarch64-apple-darwin20 Running under: macOS Sonoma 14.5
    >>> 
    >>> Matrix products: default BLAS:
    >>> /System/Library/Frameworks/Accelerate.framework/Versions/A/Frameworks/vecLib.framework/Versions/A/libBLAS.dylib
    >>> 
    >>> LAPACK:
    >>> /Library/Frameworks/R.framework/Versions/4.4-arm64/Resources/lib/libRlapack.dylib;
    >>> LAPACK version 3.12.0
    >>> 
    >>> locale: [1]
    >>> en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8
    >>> 
    >>> time zone: America/Chicago tzcode source: internal
    >>> 
    >>> attached base packages: [1] stats graphics grDevices
    >>> utils datasets [6] methods base
    >>> 
    >>> loaded via a namespace (and not attached): [1]
    >>> compiler_4.4.0 tools_4.4.0
    >>> 
    >>> ______________________________________________
    >>> R-devel using r-project.org mailing list
    >>> https://stat.ethz.ch/mailman/listinfo/r-devel
    >> 
    >> 
    >> 

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