cond.R

#' R implementation of `cond` from Lisp
#' allows for arbitrary numbers of conditionals without ugly nested if statements
#' conditionals are entered as pairs of expressions (clauses), 
#' first the expression to be evaluated and second the return value if the expression is true
#' @param ... an even number of expressions as pairs (see example)
#' @param true the `else` expression (Taken from the Lisp (T resultN) see http://www.cis.upenn.edu/~matuszek/LispText/lisp-cond.html)
#' @return The paired value of the first true conditional expression or the value of true
#' @examples
#' x <- runif(1)
#' cond(x < 0.2, "lower tail",
#' x < 0.8, "middle",
#' true = "upper tail")
cond <- function(..., true = NULL) {
    dots <- eval(substitute(alist(...)), envir=parent.frame())
    if(length(dots) %% 2 != 0) stop("Must have an even number of expressions")
    for(condition in seq(1, length(dots), by = 2)){
        if(eval(dots[[condition]], 
                envir = parent.frame())) return(eval(dots[[condition + 1]], 
                                                     envir = parent.frame()))
    }
    true
}

x <- runif(1)
x
cond(x < 0.025, "lower tail",
     x < 0.975, "middle",
     true = "upper tail")


nums <- runif(100)
cbind(nums, sapply(nums, 
                   function(x) cond(x < 0.27, "Group_1",
                                    x < 0.33, "Group_2",
                                    x < 0.36, "Group_3",
                                    x < 0.42, "Group_4",
                                    x < 0.44, "Group_5",
                                    x < 0.58, "Group_6",
                                    x < 0.85, "group_7",
                                    true = "Group_8")))