“as.numeric”和“as.POSIXct.numeric”之間的區別（只有第一個似乎有效）？

我正在苦苦掙扎as.POSIXct.numeric。以下代碼有效：

dates <- structure(c(1649285787, 1651134684), class = c("POSIXct", "POSIXt"), 
                   tzone = "UTC")
lims <- as.numeric(dates)
limits <- as.POSIXct.numeric(lims, origin = as.POSIXct("1970-01-01", tz = "UTC"))
plot(stepfun(x = dates, # OK
             y = c(0, 1, 2)),
     do.points = FALSE)
plot(stepfun(x = dates, # OK 
             y = c(0, 1, 2)),
     do.points = FALSE, 
     xlim = lims)

但是，以下失敗，我不明白為什么（我需要指定限制）：

plot(stepfun(x = dates, # Not OK
             y = c(0, 1, 2)),
     do.points = FALSE, 
     xlim = dates)
# Error in as.POSIXct.numeric(e) : 'origin' must be supplied
plot(stepfun(x = dates, # Not OK
             y = c(0, 1, 2)),
     do.points = FALSE, 
     xlim = limits)
# Error in as.POSIXct.numeric(e) : 'origin' must be supplied

有人理解這種行為嗎？我明確指定了原點as.POSIXct.numeric(lims, origin = ...)！

uj5u.com熱心網友回復：

第一個塊起作用的原因是因為stepfun(x = dates, y = c(0, 1, 2))回傳一個類“stepfun”的物件（您可以通過運行來檢查它class(stepfun(x = dates, y = c(0, 1, 2)))）并且 stepfun 物件的 plot 方法將繪制一個將 X 和 Y 軸設定為數字的圖。換句話說，您的第一個塊有效，因為它xlim被設定為數字（這是 stepfun 物件的繪圖方法所期望的）。相反，當您xlim使用POSIX向量（dates或limits）設定時，繪圖方法根本不會對它們做什么，因為它需要一個數字向量。

如果您需要更改軸標簽，我建議在外部進行如下操作：

plot(stepfun(x = dates, 
             y = c(0, 1, 2)),
     do.points = FALSE, 
     xlim = lims, axes = FALSE)

xLabels <- pretty(limits)
axis(side = 1, at = as.numeric(xLabels), labels = xLabels)
axis(side = 2)
box()

pretty函式只是找出“漂亮”（四舍五入）斷點向量的一種快速方法，但可以使用另一種方法來設定該值。

uj5u.com熱心網友回復：

我們可能會嘗試plot.stepfun通過從第 30 行開始做一些日期轉換來破解這個函式。更具體地說，我們將xlim=in"POSIXt"格式轉換seq為日期的影響，并在第 63 行將其用作axis.

plot_stepfun <- function(x, xval, xlim, ylim=range(c(y, Fn.kn)), xlab="x", 
                         ylab="f(x)", main=NULL, add=FALSE, verticals=TRUE, 
                         do.points=(n < 1000), pch=par("pch"), col=par("col"), 
                         col.points=col, cex.points=par("cex"), col.hor=col, 
                         col.vert=col, lty=par("lty"), lwd=par("lwd"), ...) {
  if (!is.stepfun(x)) {
    if (is.numeric(x)) {
      sarg <- substitute(x)
      x <- ecdf(x)
      attr(x, "call") <- call("ecdf", sarg)
    }
    else stop("'plot.stepfun' called with wrong type of argument 'x'")
  }
  if (missing(main)) {
    cl <- attr(x, "call")
    main <- if (is.null(cl)) sys.call() else cl
  }
  knF <- knots(x)
  xval <- if (missing(xval)) 
    knF
  else sort(xval)
  if (missing(xlim)) {
    rx <- range(xval)
    dr <- if (length(xval) > 1L) 
      max(0.08 * diff(rx), median(diff(xval)))
    else abs(xval)/16
    xlim <- rx   dr * c(-1, 1)
  }
  else {
    xlim1 <- xlim
    xlim <- as.numeric(xlim)
    if (inherits(xlim1, c('Date', "POSIXt"))) {
      tz <- attributes(as.POSIXlt(xlim1))$tzone
      xlim2 <- as.POSIXct(paste(strftime(xlim1, '%F')), tz=tz[length(tz)])
      if (as.numeric(difftime(xlim2[2], xlim2[1], units='secs')) < 86400) {
        stop('will be implemented soon :)')
      } else {
        xseq <- seq.POSIXt(xlim2[1], xlim2[2], by='days')
        if (length(xseq) > 6) {
          xseq <- xseq[(seq_along(xseq) - 5) %% ceiling(length(xseq)/6) == 0]
        }
      }
    }
    dr <- diff(xlim)
  }
  xval <- xval[xlim[1L] - dr <= xval & xval <= xlim[2L]   dr]
  ti <- c(xlim[1L] - dr, xval, xlim[2L]   dr)
  ti.l <- ti[-length(ti)]
  ti.r <- ti[-1L]
  y <- x(0.5 * (ti.l   ti.r))
  n <- length(y)
  Fn.kn <- x(xval)
  dev.hold()
  on.exit(dev.flush())
  if (add) 
    segments(ti.l, y, ti.r, y, col=col.hor, lty=lty, 
             lwd=lwd, ...)
  else {
    if (missing(ylim)) 
      ylim <- range(c(y, Fn.kn))
    plot(NA, NA, type="n", xlim=xlim, ylim=ylim, xlab=xlab, 
         ylab=ylab, main=main, xaxt='n', ...)
    if (exists('xseq')) {
      axis(1, xseq, xseq)
    } else {
      axis(1, at=axTicks(1), labels=axTicks(1))
    }
    segments(ti.l, y, ti.r, y, col=col.hor, lty=lty, 
             lwd=lwd)
  }
  if (do.points) 
    points(xval, Fn.kn, pch=pch, col=col.points, cex=cex.points)
  if (verticals) 
    segments(xval, y[-n], xval, y[-1L], col=col.vert, lty=lty, 
             lwd=lwd)
  invisible(list(t=ti, y=y))
}

plot_stepfun(stepfun(x=dates, y=c(0, 1, 2)), do.points=FALSE)  ## works!
plot_stepfun(stepfun(x=dates, y=c(0, 1, 2)), do.points=FALSE, xlim=lims)  ## works!
plot_stepfun(stepfun(x=dates, y=c(0, 1, 2)), do.points=FALSE, xlim=dates)  ## works!
plot_stepfun(stepfun(x=dates, y=c(0, 1, 2)), do.points=FALSE, xlim=limits)  ## works!

資料：

dates <- structure(c(1649285787, 1651134684), class = c("POSIXct", "POSIXt"
), tzone = "UTC")
lims <- as.numeric(dates)
limits <- as.POSIXct.numeric(lims, origin=as.POSIXct("1970-01-01", tz="UTC"))

標籤：r 日期 posixct

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