Passing the output of a function back to the input multiple times

Question

I have a function that takes a numeric value, and produces a numeric value. My code is as follows:

p<-function(a){
  sp(bp(a))
}

The sp and bp functions take a numeric entry, and produce a numeric outcome (sp and bp follow different operations). I want to run this function many times. The following code:

sp(bp(sp(bp(sp(bp(a))))))

has the effect I want, but obviously this isn't a practical approach for high numbers of iterations. How can I repeatedly feed the output of p back in to p as the input?

Can you share your function and tell which argument should be passed back to which input? — Liman
– Liman, Commented Sep 11, 2020 at 18:57
I've edited the question to provide more details on my specific problem. — dslater0
– dslater0, Commented Sep 11, 2020 at 19:21
Agree with @Liman. But what's your base case? When do you want to stop calling the fucntion on its outpu. — SmokeyShakers
– SmokeyShakers, Commented Sep 11, 2020 at 19:45

Valeri Voev · Accepted Answer · 2020-09-11 19:39:14Z

1

EDIT: After clarification

So, if it is some known (limited) number of times you would like to apply p in succession, this would be one way (I have assumed some simple functions for sp and bp for illustration):

sp <- function(x) return(x+1)
bp <- function(x) return(2*x)
p <- function(x) return(sp(bp(x)))

# Applying p 3 times in succession:
p_old <- 1
for (i in 1:3){
  p_new =  p(p_old)
  p_old = p_new
}

p_new
#> [1] 15
# Which is the same as
p(p(p(1)))
#> [1] 15

Created on 2020-09-11 by the reprex package (v0.3.0)

I am not quite sure what use case you have in mind (because that can easily lead to an infinite "loop", but here is one (admittedly contrived) example of a function which sums some numbers regardless in how many lists they can be nested which is using a call of the same function in the function definition itself:

sum_of_c_or_list <- function(x){
  
  if (!is.list(x)) return(sum(x))
  else {
    x = unlist(x)
    x = sum_of_c_or_list(x)
    return(x)
  }
}
sum_of_c_or_list(1:3)
#> [1] 6
sum_of_c_or_list(list(1,2,3))
#> [1] 6
sum_of_c_or_list(list(list(1,2,3)))
#> [1] 6

Created on 2020-09-11 by the reprex package (v0.3.0)

edited Sep 11, 2020 at 19:39

answered Sep 11, 2020 at 19:16

Valeri Voev

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2 Comments

dslater0 Over a year ago

Thanks - I don't think this approach can be used for my specific problem - I've updated the description to provide more details.

Valeri Voev Over a year ago

Just to add here: stackoverflow.com/questions/53026344/… can also be of interest.

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