Question

I have found a very subtle bug in my R code just now. The following code takes a list of objects as input and create new fields for each of the objects.

Each object originally has two fields (w, p, s, u), and then I create more, beta, phi, etc.. The normal variables are OK. However the dynamic functions (Q, K, K1, K2) are not correct. Suppose I have two nigs, nigs[[1]] and nigs[[2]], the functions Q, K, K1 and K2 for nigs[[1]] would be the same as nigs[[2]]!

I just found this bug and would consult on how to get this code correct (while keeping its elegance:) Thanks!

  D <- length(nigs)

  for (i in 1:D) {
    w <- nigs[[i]]$w
    p <- nigs[[i]]$p
    s <- nigs[[i]]$s
    u <- nigs[[i]]$u

    nigs[[i]]$beta <- beta <- w / s * p * (1-p^2)^(-1/2);
    nigs[[i]]$phi <- phi <- w^2 / s^2;

    nigs[[i]]$z <- z <- (x-u)/s;
    nigs[[i]]$alpha_bar <- alpha_bar <- w * (1-p^2)^(-1/2);
    nigs[[i]]$y_bar <- y_bar <- sqrt(1+z^2);

    nigs[[i]]$Q <- Q <- function(t) { sqrt(1 - (2*beta*t+t^2)/phi) }
    nigs[[i]]$K <- K <- function(t) { u*t - w*Q(t) + w }
    nigs[[i]]$K1 <- K1 <- function(t) { (u + w * (beta+t) / (Q(t)*phi)) }
    nigs[[i]]$K2 <- K2 <- function(t) { qt = Q(t); (w/(qt * phi) + w * (beta+t)^2 / (qt^3 * phi^2)); }
  }

EDIT

The primary error I made is that I assumed that for { } introduced new scopes, in that case, w,p,s,u are different w,p,s,u every time, actually not. Only functions in R introduce new scopes. And this scoping rule is different from C/Java.

Answer 1

That is normal behavior of the lexical scope.
You can use closure instead.

f <- list()
g <- list()
for (i in 1:2) {
    j <- i * 2
    f[[i]] <- function() print(j)
    g[[i]] <- (function() {j <- j; function() print(j)}) ()
}

then,

> for (i in 1:2) f[[i]]()
[1] 4
[1] 4
> for (i in 1:2) g[[i]]()
[1] 2
[1] 4