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Fast approximate matching using greedy algorithm

Source: R/matching_core.R
greedy_couples.Rd

Performs fast one-to-one matching using greedy strategies. Does not guarantee optimal total distance but is much faster than match_couples() for large datasets. Supports blocking, distance constraints, and various distance metrics.

Usage

greedy_couples(
  left,
  right = NULL,
  vars = NULL,
  distance = "euclidean",
  weights = NULL,
  scale = FALSE,
  auto_scale = FALSE,
  max_distance = Inf,
  calipers = NULL,
  block_id = NULL,
  ignore_blocks = FALSE,
  require_full_matching = FALSE,
  strategy = c("row_best", "sorted", "pq"),
  return_unmatched = TRUE,
  return_diagnostics = FALSE,
  parallel = FALSE,
  check_costs = TRUE
)

Arguments

left

Data frame of "left" units (e.g., treated, cases)

right

Data frame of "right" units (e.g., control, controls)

vars

Variable names to use for distance computation

distance

Distance metric: "euclidean", "manhattan", "mahalanobis", or a custom function

weights

Optional named vector of variable weights

scale

Scaling method: FALSE (none), "standardize", "range", or "robust"

auto_scale

If TRUE, automatically check variable health and select scaling method (default: FALSE)

max_distance

Maximum allowed distance (pairs exceeding this are forbidden)

calipers

Named list of per-variable maximum absolute differences

block_id

Column name containing block IDs (for stratified matching)

ignore_blocks

If TRUE, ignore block_id even if present

require_full_matching

If TRUE, error if any units remain unmatched

strategy

Greedy strategy:

  • "row_best": For each row, find best available column (default)

  • "sorted": Sort all pairs by distance, greedily assign

  • "pq": Use priority queue (good for very large problems)

return_unmatched

Include unmatched units in output

return_diagnostics

Include detailed diagnostics in output

parallel

Enable parallel processing for blocked matching. Requires 'future' and 'future.apply' packages. Can be:

  • FALSE: Sequential processing (default)

  • TRUE: Auto-configure parallel backend

  • Character: Specify future plan (e.g., "multisession", "multicore")

check_costs

If TRUE, check distance distribution for potential problems and provide helpful warnings before matching (default: TRUE)

Value

A list with class "matching_result" (same structure as match_couples)

Details

Greedy strategies do not guarantee optimal total distance but are much faster:

  • "row_best": O(n*m) time, simple and often produces good results

  • "sorted": O(nmlog(n*m)) time, better quality but slower

  • "pq": O(nmlog(n*m)) time, memory-efficient for large problems

Use greedy_couples when:

  • Dataset is very large (> 10,000 x 10,000)

  • Approximate solution is acceptable

  • Speed is more important than optimality

Examples

# Basic greedy matching
left <- data.frame(id = 1:100, x = rnorm(100))
right <- data.frame(id = 101:200, x = rnorm(100))
result <- greedy_couples(left, right, vars = "x")

# Compare to optimal
result_opt <- match_couples(left, right, vars = "x")
result_greedy <- greedy_couples(left, right, vars = "x")
result_greedy$info$total_distance / result_opt$info$total_distance  # Quality ratio