How to append results from a repeated function?

Question

I have a function that repeats a function until a certain value has been reached. For each repetition, a variable X changes and so should the result Y. I want to save every value that X and Y take as a dataframe.

get_Y <- function(X) {
  Y <- Xˆ2
}

get_dataframe <- function() {
  X <- 1
  repeat {
    get_Y(X)
    X <- X*1.01
    if (X > 2)
      break
  }
}

How do I save each X and Y to a dataframe, where each row is an iteration?

Answer 1

Here's a pretty efficient answer. The efficiency gains over your set up come in 2 main ways: (1) pre-calculate the length of the result so we can optimally (2) take advantage of the fact that get_Y is vectorized, so we can call it on the entire input at once rather than one item at a time.

get_Y <- function(X) {
  Y <- X^2
}

get_dataframe <- function() {
  n = ceiling(log(2) /  log(1.01))
  X = cumprod(rep(1.01, n))
  data.frame(X, Y = get_Y(X))
}

get_dataframe()
#           X        Y
# 1  1.010000 1.020100
# 2  1.020100 1.040604
# 3  1.030301 1.061520
# 4  1.040604 1.082857
# 5  1.051010 1.104622
# 6  1.061520 1.126825
# 7  1.072135 1.149474
# ...
# 65 1.909366 3.645680
# 66 1.928460 3.718959
# 67 1.947745 3.793710
# 68 1.967222 3.869963
# 69 1.986894 3.947749
# 70 2.006763 4.027099