R: How Does a Foreach Loop Find a Function That Should Be Invoked

R: how does a foreach loop find a function that should be invoked?

They behave differently because registerDoParallel registers an mclapply backend on Linux, while it registers a clusterApplyLB backend on Windows. When using an mclapply backend, there are essentially no data exporting issues, so it works on Linux. But with clusterApplyLB, you can run into problems if foreach doesn't auto-export the functions and data that are needed.

You can solve this problem by modifying FUN3 to export FUN via the .export option:

FUN3 <- function(a, b) {
  foreach(i=1:3, .export='FUN') %dopar% FUN(i, a, b)
}

This solution works on both Linux and Windows, since .export is ignored by the mclapply backend.

As pointed out by Hong Ooi, you have an error in your use of clusterExport, but I wouldn't use clusterExport to solve the problem since it is backend specific.

could not find function inside foreach loop

The short answer is that this was a bug in parallel backends such as doSNOW, doParallel and doMPI, but it has since been fixed.

The slightly longer answer is that foreach exports functions to the workers using a special "export" environment, not the global environment. That used to cause problems for functions that were created in the global environment, because the "export" environment wasn't in their scope, even though they were now defined in that same "export" environment. Thus, they couldn't see any other functions or variables defined in the "export" environment, such as "Posdef" in your case.

The doSNOW, doParallel and doMPI backends now change the associated environment from the global to the "export" environment for functions exported via ".export", and seems to have resolved these issues.

r - foreach unable to find object within function

Try defining the .GlobalEnv variables within the foreach loop in every call.

library("ramify")
library("foreach")

algor <- function (vc) {

  # initialize A, ybar, and Ia
  A <- 0

  ybar <- 0

  Ia <- 0

  # x is the first column of vc   
  x <- vc[, 1, drop = FALSE]

  # y is the second column of vc  
  y <- vc[, 2, drop = FALSE]

  # n is the length of x
  n <- length(x)

  foreach(i = 1:(n-1), .combine = 'c', .export = c("A", "ybar", "Ia", "x", "y")) %do% {
    .GlobalEnv$A <- A
    .GlobalEnv$ybar <- ybar
    .GlobalEnv$Ia <- Ia
    A <- A + 0.5 * (x[i] - x[i+1]) * (y[i] + y[i+1])
    ybar <- ybar + (1 / 6) * (x[i] - x[i+1]) * (y[i] ^ 2 + y[i] * y[i+1] + y[i+1] ^ 2)
    Ia <- Ia + (1 / 12) * (x[i] - x[i+1]) * (y[i] ^ 3 + y[i] ^ 2 * y[i+1] + y[i] * y[i+1] ^ 2 + y[i+1] ^ 3)
  }

  props <- mat("A, Ia, ybar", eval = TRUE)

  return(props)
}

inner <- mat("0, 300; 300, 300; 300, 695; 0, 695; 0, 300")

algor(inner)

This returns:

       [,1]        [,2]     [,3]
[1,] 118500 30870237500 58953750

improve readability of output list of foreach loop

The .final may be useful. According to ?foreach

.final - function of one argument that is called to return final result.

With one more nested loop

res_1 <- foreach(a = c("A", "B", "C", "D"), 
     .final = function(x) setNames(x, c("A", "B", "C", "D"))) %:% 
     foreach(c = c("a", "b", "c"),
        .final = function(x) setNames(x, c("a", "b", "c"))) %:%
      foreach(b = 1:4) %do% {paste0(rep(a, b))}

-checking

> res_1[["B"]][["c"]][[2]]
[1] "B" "B"

similar to

res_desired_1 <- list()
for(a in c("A", "B", "C", "D")) {
  for(c in c("a", "b", "c")) {
  for(b in 1:4) {
      res_desired_1[[a]][[c]][[b]] <- paste0(rep(a, b))
    }
  }
}
> res_desired_1[["B"]][["c"]][[2]]
[1] "B" "B"

How do I print the entries of a foreach loop in R?

You could use foreach on a list of function names, and get the associated functions:

library(foreach)

loop_func <- function(funcname){
  obj <- f(get(funcname))
  return(list(funcname,obj))}

a <- function(x) x
b <- function(x) 2*x
d <- function(x) 3*x

f <- function(func) {func(1)}

func_list <- c("a", "b", "d")

result <- foreach(f = func_list, .combine='rbind') %dopar% {
  loop_func(f)
}

result

         [,1] [,2]
result.1 "a"  1   
result.2 "b"  2   
result.3 "d"  3 

Function not found in R doParallel 'foreach' - Error in { : task 1 failed - could not find function raster

In the vignette of foreach and the help page of foreach, the argument .packages is pointed out as necessary to provide when using parallel computation with functions that are not loaded by default. So your code in the first example should be:

ptime<-system.time({
  r <- foreach(z = 1:length(files),
               .combine=cbind, 
               .packages='raster') %dopar% {
      # some code
      # and more code
  }
})

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Some more explanation

The foreach package does a lot of setting up behind the scenes. What happens is the following (in principle, technical details are a tad more complicated):

• foreach sets up a system of "workers" that you can see as separate R sessions that are each committed to a different core in a cluster.

• The function that needs to be carried out is loaded into each "worker" session, together with the objects needed to carry out the function

• each worker calculates the result for a subset of the data

• The results of the calculation on the different workers is put together and reported in the "master" R session.

As the workers can be seen as separate R sessions, packages from the "master" session are not automatically loaded. You have to specify which packages should be loaded in those worker sessions, and that's what the .package argument of foreach is used for.

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Note that when you use other packages (e.g. parallel or snowfall), you'll have to set up these workers explicitly, and also take care of passing objects and loading packages on the worker sessions.

Using Foreach Loop inside of Invoke

You just need an additional set of {}. Here your code is formatted so it is readable with the braces added.

public void MainLoop()
{
  while (true)
  {
    this.Invoke(new MethodInvoker(() =>
                {
                  foreach(GameObject ob in mygame.scenes[CurrentScene].objects)
                  {
                    //run code here
                  }
                }
                ));
  }
}

For and Foreach loop and return with Set. First recurring number in array

In fact the problem with .forEach() method is that its callback function always return undefined, even if you use a return statement.

If you check the forEach() method MDN reference you can see that:

forEach() executes the callback function once for each array element; unlike map() or reduce() it always returns the value undefined and is not chainable.

So in your case return el; inside forEach() callback is always ignored, and inside the forEach callback return is specific to this scope that's why the function won't return anything.

Solution:

If you want to do it with forEach(), what you can do is to store this flag in a variable so you can return it after the forEach() block:

const recurring = (arr) => {

  const set = new Set();
  let result;
  arr.forEach(el => {
    if (set.has(el)) {
       result = !result ? el : result;
      return;
    } else {
      set.add(el);
    }
  });
  return result;
}

Demo:

const arr1 = [2, 2, 3, 2, 5, 6, 6, 9];
const recurring = (arr) => {
  const set = new Set();  let result;
  arr.forEach(el => {    if (set.has(el)) {      result = !result ? el : result;      return;    } else {      set.add(el);    }  });  return result;}
console.log(recurring(arr1));

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