cumulative standard deviation

[spsanderson]

Function:

#' Cumulative Standard Deviation
#' 
#' @family Vector Function
#' 
#' @author Steven P. Sanderson II, MPH
#' 
#' @details
#' A function to return the cumulative standard deviation of a vector.
#' 
#' @description 
#' A function to return the cumulative standard deviation of a vector.
#' 
#' @parm .x A numeric vector
#' 
#' @examples 
#' x <- mtcars$mpg
#' 
#' csd(x)
#' 
#' @return 
#' A numeric vector
#' 
#' @export
#' 

csd <- function(.x){
  sapply(seq_along(.x), function(k, z) stats::sd(z[1:k]), z = .x)
}

Example:

x <- mtcars$mpg

> csd(x)
 [1]        NA 0.0000000 1.0392305 0.8544004 1.4737707 1.7663522 2.8445436
 [8] 3.1302385 3.0524580 2.9070986 2.8647069 2.9366416 2.8975233 3.0252418
[15] 3.7142967 4.1476098 4.1046423 5.2332053 5.7405452 6.4594362 6.3029736
[22] 6.2319940 6.1698105 6.1772007 6.0474457 6.1199296 6.1188444 6.3166405
[29] 6.2611772 6.1530527 6.1217574 6.0269481