lc: List comprehension

In mailund/lc: List Comprehension

Description

This function implements list comprehension, an alternative to Filter and Map for working with lists. The syntax is as follows: lc(expr, lists, predicates) where expr is some expression to be evaluated for all elements in the lists, where lists are one or more named lists, where these are specified by a name and an expression name = list_expr, and where predicates are expressions that should evaluated to a boolean value. For example, to get a list of all even numbers, squared, from a list x we can write lc(x ** 2, x = x, x %% 2 == 0). The result of a call to lc is a list constructed from the expressions in expr, for all elements in the input lists where the predicates evaluate to true.

Usage

lc(expr, ...)

Arguments

expr	An expression that will be evaluated for each element in the input lists.
...	Additional list or predicate arguments.

Examples

qsort <- function(lst) {
    n <- length(lst)
    if (n < 2) return(lst)
    pivot <- lst[[sample(n, size = 1)]]
    smaller <- lc(x, x = lst, x < pivot)
    equal <- lc(x, x = lst, x == pivot)
    larger <- lc(x, x = lst, x > pivot)
    c(qsort(smaller), equal, qsort(larger))
}
(lst <- sample(1:10))
unlist(qsort(lst))

mailund/lc documentation built on May 29, 2019, 6:53 p.m.