A function within a function in the R language

Question

I have a simple function in the R language, for instance:

f<-function(x){
y=(x+2)/3
return(y)
}

I want to evaluate this function five times on its own outputs, i.e., y1=f(x), y2=f(f(x)), y3=f(f(f(x))), y4=f(f(f(f(x)))), y5=f(f(f(f(f(x))))). Is there a simpler way to do it in the form of one function, for instance the function with two arguments when the first argument is x and the second argument is the number of evaluation times, i.e., n. For instance, for x=3 and n=5 I would like to have the function f2<-(x=3,n=5) whose output would be in the form of a vector or a list of the length equal to n with the values:

y1=f(3)=1.666667
y2=f(f(3))=1.222222
y3=f(f(f(3)))=1.074074
y4=f(f(f(f(3))))=1.024691
y5=f(f(f(f(f(3)))))=1.00823

How to write such one function in the R ?

Rui Barradas · Accepted Answer · 2019-06-12 17:46:52Z

4

Using the accepted answer in another post, just use Reduce to create the composition, g.

composite <- function(g, h) {function(...)g(h(...))}
g <- Reduce(composite, list(f, f, f, f, f))

g(2)
f(f(f(f(f(2)))))

identical(g(2), f(f(f(f(f(2))))))
#[1] TRUE

answered Jun 12, 2019 at 17:46

Rui Barradas

78k8 gold badges41 silver badges75 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Dason Over a year ago

Using replicate(5, f) to create the list of the function replicated would make this a bit more easily extendable.

Rui Barradas Over a year ago

@Dason Right, thanks, I hadn't thought of that. I had tried rep(f, 5) but it doesn't work.

John Coleman · Accepted Answer · 2019-06-12 17:45:17Z

3

You can write a helper function like:

iterate <- function(f,s,n){
  iterates <- vector(length = n+1)
  iterates[1] <- s
  for(i in 1:n){
    iterates[i+1] <- f(iterates[i])
  }
  iterates
}

Then, for example,

> iterate(function(x){(x+2)/3},3,5)
[1] 3.000000 1.666667 1.222222 1.074074 1.024691 1.008230

You can of course modify the above code so that the vector would start with f(s) rather than s.

edited Jun 12, 2019 at 17:45

answered Jun 12, 2019 at 17:35

John Coleman

52.1k7 gold badges59 silver badges127 bronze badges

Comments

M-- · Accepted Answer · 2019-06-12 17:45:23Z

2

You can also tweak your original function like below:

f2<-function(x, steps){
             ans<-c(x)
              for (i in 1:steps) {
                   y=(x+2)/3
                   ans=append(ans,y)
                   x = y
  }
    return(ans)
}

f2(3,5)
#> [1] 3.000000 1.666667 1.222222 1.074074 1.024691 1.008230

answered Jun 12, 2019 at 17:45

M--

33.7k12 gold badges74 silver badges115 bronze badges

Comments

IceCreamToucan · Accepted Answer · 2019-06-12 18:26:18Z

1

Reduce will do this if you add a "junk" argument to your f. If you just want the last result set accumulate = F or just remove the accumulate argument from the function

f<-function(x, junk){
y=(x+2)/3
return(y)
}

Reduce(f, x = numeric(5), init = 3, accumulate = T)
# [1] 3.000000 1.666667 1.222222 1.074074 1.024691 1.008230

Or with purrr

purrr::accumulate(numeric(5), f, .init = 3)
# [1] 3.000000 1.666667 1.222222 1.074074 1.024691 1.008230

answered Jun 12, 2019 at 18:26

IceCreamToucan

28.8k2 gold badges27 silver badges48 bronze badges

Collectives™ on Stack Overflow

A function within a function in the R language

4 Answers 4

2 Comments

Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

2 Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related