doParallel

doParallel

Parallel backend for foreach using parallel package.

Setup

library(doParallel)
library(foreach)

# Register parallel backend
cl <- makeCluster(4)
registerDoParallel(cl)

# Or simpler (auto-creates cluster)
registerDoParallel(cores = 4)

Basic Usage

library(doParallel)
library(foreach)

# Register backend
registerDoParallel(cores = 4)

# Parallel foreach
result <- foreach(i = 1:100) %dopar% {
  sqrt(i)
}

# Stop parallel backend
stopImplicitCluster()

Combining Results

# As list (default)
result <- foreach(i = 1:10) %dopar% { i^2 }

# As vector
result <- foreach(i = 1:10, .combine = c) %dopar% { i^2 }

# As matrix (rbind)
result <- foreach(i = 1:10, .combine = rbind) %dopar% { c(i, i^2) }

# As matrix (cbind)
result <- foreach(i = 1:10, .combine = cbind) %dopar% { c(i, i^2) }

# Sum
result <- foreach(i = 1:10, .combine = `+`) %dopar% { i }

# Custom combine
result <- foreach(i = 1:10, .combine = function(a, b) a + b) %dopar% { i }

Multiple Iterators

# Nested loops
result <- foreach(i = 1:3, .combine = rbind) %:%
  foreach(j = 1:3, .combine = c) %dopar% {
    i * j
  }

# Parallel over multiple variables
result <- foreach(a = 1:10, b = 11:20, .combine = c) %dopar% {
  a + b
}

Export Variables

my_data <- 1:100
my_func <- function(x) x^2

result <- foreach(
  i = 1:10,
  .export = c("my_data", "my_func")
) %dopar% {
  my_func(my_data[i])
}

Load Packages

result <- foreach(
  i = 1:10,
  .packages = c("dplyr", "ggplot2")
) %dopar% {
  # dplyr and ggplot2 available here
  process(i)
}

Error Handling

# Return error info instead of stopping
result <- foreach(
  i = 1:10,
  .errorhandling = "pass"  # "stop", "remove", "pass"
) %dopar% {
  if (i == 5) stop("Error!")
  i^2
}

# Remove failed iterations
result <- foreach(
  i = 1:10,
  .errorhandling = "remove"
) %dopar% {
  if (i == 5) stop("Error!")
  i^2
}

Sequential Fallback

# Use %do% for sequential execution
result <- foreach(i = 1:10) %do% { i^2 }

# Useful for debugging
result <- foreach(i = 1:10) %do% {
  print(i)  # Can see output
  i^2
}