nest_while: Apply a function recursively until the condition is satisfied

In parksw3/wolframR: Functions from the Wolfram language

Description

Implementation of the NestWhile function from the Wolfram langauge. Syntax order has been changed so that the function is compatible with the pipe operator (see %>%).

Usage

nest_while(x, expr, cond, m = 1, maxit = 1e+05, n = 0, xname = "x")

Arguments

x	an object or a variable name.
expr	an expression or a function to be evaluated recursively.
cond	a condition to be evaluated.
m	return results of the last m iterations. Setting m = "all" returns a list of all results; This is equivalent to NestWhileList function from the Wolfram language.
maxit	maximum number of iterations.
n	additional iterations to be completed after the condition has been satisfied.
xname	character string containing the name of the argument that will be used recursively.

Examples

## Some of these examples are directly taken from the Wolfram documentation

## number of terms required in taylor expansion to achieve a target error.
nest_while(0, x+1, exp(1) - sum(1/factorial(0:x)) > 1e-5)

## collatz sequence
nest_while(1214, ifelse(x %% 2 == 0, x/2, 3*x+1), x != 1, m = "all")

## root finding algorithms
f <- function(x) x^5 - 4 * x^3 + 159
secant <- function(x) c(x[2], x[2] - f(x[2]) * diff(x)/diff(f(x)))
nest_while(c(2, 3), secant, abs(f(x[2])) > 10^-5)

## Happy number
## http://stackoverflow.com/questions/18675285/digit-sum-function-in-r
digitsum2 <- function(x) sum((floor(x / 10^(0:(nchar(x) - 1))) %% 10)^2)
res <- sapply(1:1000, function(x) nest_while(x, digitsum2, x!= 1 & x!= 4))
which(res == 1)

parksw3/wolframR documentation built on May 24, 2019, 6:16 p.m.