R/sm_auc.R

In smin95/sesR: Create Standalone and Composite Plots in 'ggplot2' for Publications

#' Calculation of the Area under a Curve (Trapezoidal numerical integration)
#'
#' This is equivalent to Matlab's trapz function.
#'
#' @param x
#' This is the scalar spacing of the coordinate.
#' When the argument for 'x' is not provided, it will calculates the
#' approximate integral value for ‘Y’ with unit spacing based on the length of ‘y’.
#'
#' @param y
#' Numerical data. The length of x and y must be equal.
#'
#' @importFrom zoo rollmean
#'
#' @export
#' @return
#' A vector that is the value from the trapezoidal integration is returned.
#' @examples
#' library(smplot2)
#' X <- c(1, 2, 3, 4, 5)
#' Y1 <- c(2, 3, 4, 2, 3)
#' Y2 <- c(3, 3, 3, 3, 3)
#'
#' sm_auc(X, Y2)
#' sm_auc(X, Y1)
sm_auc <- function(x, y) {
  if (is.null(x)) {
    n <- length(y)
    res <- 0.5 * (y[1] + y[n] + 2 * sum(y[-c(1, n)]))
  } else {
    id <- order(x)
    res <- sum(diff(x[id]) * rollmean(y[id], 2))
  }
  return(res)
}