Relooping a function over its own output

问题

I have defined a function which I want to reapply to its own output multiple times. I tried

   replicate(1000,myfunction)

but realised that this is just applying my function to my initial input 1000 times, rather than applying my function to the new output each time. In effect what I desire is:

    function(function(...function(x_0)...))

1000 times over and being able to see the changes at each stage.

I have previous defined b as a certain vector of length 7.

        b_0=b
        C=matrix(0,7,1000)
        for(k in 1:1000){
            b_k=myfun(b_(k-1))
            }
        C=rbind(b_k)
        C

Is this the right idea behind what I want?

回答1:

A loop would work just fine here.

apply_fun_n_times <- function(input, fun, n){
  for(i in 1:n){
    input <- fun(input)
  }
  return(input)
}

addone <- function(x){x+1}

apply_fun_n_times(1, addone, 3)

which gives

> apply_fun_n_times(1, addone, 3)
[1] 4

回答2:

You could use Reduce for this. For example

add_two <- function(a) a+2
ignore_current <- function(f) function(a,b) f(a)

Reduce(ignore_current(add_two), 1:10, init=4)
# 24

Normally Reduce expects to iterate over a set of new values, but in this case I use ignore_current to drop the sequence value (1:10) so that parameter is just used to control the number of times we repeat the process. This is the same as

add_two(add_two(add_two(add_two(add_two(add_two(add_two(add_two(add_two(add_two(4))))))))))

回答3:

Pure functional programming approach, use Compose from functional package:

library(functional)

f = Reduce(Compose, replicate(100, function(x) x+2))
#> f(2)
#[1] 202

But this solution does not work for too big n ! Very interesting.

回答4:

you can try a recursive function:

rec_func <- function(input, i=1000) {
                if (i == 0) {
                   return(input)
                } else {
                   input <- myfunc(input)
                   i <- i - 1
                   rec_func(input, i)
                }
            }

example

myfunc <- function(item) {item + 1}
> rec_func(1, i=1000)
[1] 1001

来源：https://stackoverflow.com/questions/29237136/relooping-a-function-over-its-own-output

标签

function

loops

repeat