R/posmod.R

In danjamesadams/Dantools: A Veritable Plethora of Helper Functions

#' Modify Specific Elements of a Vector in Different Ways
#'
#' Carry out alternative arithmetic operations on different elements of a vector. Intended for use when specifying plot annotations using objects.
#' @param vector The vector to alter.
#' @param positions The indices of the elements of the vector to alter.
#' @param operations Mathematical operations to carry out (in conjunction with numbers) on the specified vector elements. Some degree of work-around may be needed (i.e. to square a number, set operations to '*' and the numbers element to the same as the vector element).
#' @param numbers The numbers that are used at the end of the operation.
#' @keywords position modify add subtract multiply divide
#' @export
#' @examples
#' vector <- rep(1, times = 5)
#' posmod(vector = vector, positions = c(2, 4), operations = c('+', '-'), numbers = c(1, 1))
#' [1] 1 2 1 0 1

posmod <- function(vector, positions, operations, numbers) {

  if ( !identical(length(positions), length(operations), length(numbers)) ) { stop('The number of positions, operations and numbers must be equal.') }
  if ( any(positions > length(vector)) || any(positions < 1) ) { stop('Positions outside the vector range cannot be specified.') }
  # Maybe an error that checks if all position and number vector elements are actually numbers?

  vector[positions] <-

    { lapply(X = 1:length(positions),
             FUN =  function(pm) {

        p <- positions[pm]
        o <- operations[pm]
        n <- numbers[pm]

        eval(parse(text = paste0(vector[p], o, n)))

             }) %>%
        unlist }

  return(vector)

  }