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feat: Add comprehensive Queue data structure (#284)
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‎DIRECTORY.md‎

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## Data Structures
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* [Binary Search Tree](https://github.com/TheAlgorithms/R/blob/HEAD/data_structures/binary_search_tree.r)
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* Queue
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* [Queue Operations](https://github.com/TheAlgorithms/R/blob/HEAD/data_structures/Queue/queue_operations.r)
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## Dynamic Programming
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‎data_structures/Queue/queue_operations.r‎

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# Queue Data Structure Implementation in R
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#
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# A queue is a linear data structure that follows the First In First Out (FIFO) principle.
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# Elements are added at the rear (enqueue) and removed from the front (dequeue).
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#
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# Time Complexities:
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# - Enqueue: O(1) - adding element to rear
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# - Dequeue: O(1) - removing element from front
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# - Peek/Front: O(1) - viewing front element
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# - Size: O(1) - getting queue size
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# - IsEmpty: O(1) - checking if queue is empty
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#
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# Space Complexity: O(n) where n is number of elements
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#
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# Applications:
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# - CPU scheduling and process management
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# - Breadth-First Search (BFS) in graphs
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# - Handling requests in web servers
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# - Print queue management
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# - Level order traversal in trees
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# - Buffer for data streams
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# Define Queue class using Reference Classes
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Queue <- setRefClass("Queue",
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fields = list(
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items = "list",
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front_index = "numeric",
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rear_index = "numeric",
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max_size = "numeric"
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),
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methods = list(
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initialize = function(max_size = Inf) {
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"Initialize an empty queue with optional maximum size"
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.self$items <- list()
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.self$front_index <- 1
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.self$rear_index <- 0
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.self$max_size <- max_size
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cat("Queue initialized with max size:", ifelse(is.infinite(max_size), "unlimited", max_size), "\n")
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},
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enqueue = function(item) {
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"Add an element to the rear of the queue"
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if (.self$size() >= .self$max_size) {
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stop("Queue overflow: Cannot add more elements. Max size reached: ", .self$max_size)
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}
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.self$rear_index <- .self$rear_index + 1
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.self$items[[.self$rear_index]] <- item
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cat("Enqueued:", item, "| Size:", .self$size(), "\n")
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},
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dequeue = function() {
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"Remove and return the front element from the queue"
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if (.self$is_empty()) {
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stop("Queue underflow: Cannot dequeue from empty queue")
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}
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item <- .self$items[[.self$front_index]]
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.self$items[[.self$front_index]] <- NULL
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.self$front_index <- .self$front_index + 1
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# Reset indices when queue becomes empty to optimize memory
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if (.self$is_empty()) {
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.self$front_index <- 1
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.self$rear_index <- 0
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.self$items <- list()
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}
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cat("Dequeued:", item, "| Size:", .self$size(), "\n")
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return(item)
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},
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front = function() {
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"Return the front element without removing it"
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if (.self$is_empty()) {
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stop("Queue is empty: No front element")
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}
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return(.self$items[[.self$front_index]])
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},
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rear = function() {
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"Return the rear element without removing it"
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if (.self$is_empty()) {
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stop("Queue is empty: No rear element")
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}
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return(.self$items[[.self$rear_index]])
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},
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is_empty = function() {
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"Check if the queue is empty"
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return(.self$rear_index < .self$front_index)
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},
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is_full = function() {
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"Check if the queue is full (only applicable for bounded queues)"
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return(.self$size() >= .self$max_size)
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},
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size = function() {
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"Return the number of elements in the queue"
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if (.self$is_empty()) return(0)
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return(.self$rear_index - .self$front_index + 1)
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},
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clear = function() {
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"Remove all elements from the queue"
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.self$items <- list()
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.self$front_index <- 1
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.self$rear_index <- 0
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cat("Queue cleared\n")
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},
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display = function() {
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"Display all elements in the queue from front to rear"
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if (.self$is_empty()) {
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cat("Queue is empty: []\n")
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return()
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}
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elements <- character(0)
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for (i in .self$front_index:.self$rear_index) {
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if (i <= length(.self$items) && !is.null(.self$items[[i]])) {
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elements <- c(elements, as.character(.self$items[[i]]))
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}
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}
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cat("Queue: [", paste(elements, collapse = " <- "), "] (front <- rear)\n")
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},
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to_vector = function() {
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"Convert queue to vector (front to rear order)"
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if (.self$is_empty()) return(c())
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result <- c()
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for (i in .self$front_index:.self$rear_index) {
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result <- c(result, .self$items[[i]])
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}
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return(result)
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},
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search = function(item) {
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"Search for an item in the queue and return its position from front (1-indexed)"
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if (.self$is_empty()) return(-1)
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for (i in .self$front_index:.self$rear_index) {
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if (identical(.self$items[[i]], item)) {
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return(i - .self$front_index + 1)
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}
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}
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return(-1) # Item not found
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}
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)
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)
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# Utility function to demonstrate queue operations
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demonstrate_queue_operations <- function() {
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cat("\n=== Queue Data Structure Demonstration ===\n\n")
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# Create a queue with maximum size of 5
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q <- Queue$new(max_size = 5)
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cat("\n1. Basic Enqueue Operations:\n")
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q$enqueue("A")
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q$enqueue("B")
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q$enqueue("C")
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q$display()
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cat("\n2. Queue Status:\n")
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cat("Size:", q$size(), "\n")
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cat("Is Empty:", q$is_empty(), "\n")
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cat("Is Full:", q$is_full(), "\n")
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cat("Front element:", q$front(), "\n")
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cat("Rear element:", q$rear(), "\n")
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cat("\n3. Dequeue Operations:\n")
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q$dequeue()
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q$display()
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q$dequeue()
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q$display()
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cat("\n4. More Enqueue Operations:\n")
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q$enqueue("D")
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q$enqueue("E")
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q$enqueue("F")
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q$enqueue("G")
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q$display()
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cat("\n5. Search Operations:\n")
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cat("Position of 'E':", q$search("E"), "\n")
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cat("Position of 'Z':", q$search("Z"), "\n")
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cat("\n6. Queue to Vector:\n")
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vec <- q$to_vector()
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cat("As vector:", paste(vec, collapse = ", "), "\n")
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cat("\n7. Testing Queue Overflow:\n")
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tryCatch({
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q$enqueue("H") # This should cause overflow
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}, error = function(e) {
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cat("Error caught:", e$message, "\n")
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})
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cat("\n8. Clear Queue:\n")
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q$clear()
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q$display()
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cat("\n9. Testing Queue Underflow:\n")
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tryCatch({
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q$dequeue() # This should cause underflow
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}, error = function(e) {
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cat("Error caught:", e$message, "\n")
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})
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}
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# Priority Queue implementation (bonus)
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PriorityQueue <- setRefClass("PriorityQueue",
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fields = list(
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items = "list",
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priorities = "numeric"
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),
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methods = list(
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initialize = function() {
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"Initialize an empty priority queue"
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.self$items <- list()
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.self$priorities <- numeric(0)
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cat("Priority Queue initialized\n")
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},
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enqueue = function(item, priority = 0) {
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"Add an element with priority (higher number = higher priority)"
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.self$items <- append(.self$items, list(item))
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.self$priorities <- append(.self$priorities, priority)
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cat("Enqueued:", item, "with priority:", priority, "\n")
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},
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dequeue = function() {
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"Remove and return the highest priority element"
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if (length(.self$items) == 0) {
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stop("Priority queue is empty")
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}
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max_priority_index <- which.max(.self$priorities)
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item <- .self$items[[max_priority_index]]
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priority <- .self$priorities[max_priority_index]
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.self$items <- .self$items[-max_priority_index]
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.self$priorities <- .self$priorities[-max_priority_index]
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cat("Dequeued:", item, "with priority:", priority, "\n")
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return(list(item = item, priority = priority))
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},
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is_empty = function() {
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"Check if priority queue is empty"
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return(length(.self$items) == 0)
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},
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size = function() {
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"Return number of elements"
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return(length(.self$items))
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},
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display = function() {
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"Display all elements with their priorities"
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if (.self$is_empty()) {
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cat("Priority Queue is empty\n")
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return()
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}
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cat("Priority Queue:\n")
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for (i in seq_along(.self$items)) {
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cat(" ", .self$items[[i]], "(priority:", .self$priorities[i], ")\n")
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}
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}
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)
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)
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# Example usage and testing
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if (sys.nframe() == 0) {
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# Demonstrate basic queue operations
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demonstrate_queue_operations()
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cat("\n\n=== Priority Queue Demonstration ===\n")
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pq <- PriorityQueue$new()
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pq$enqueue("Low priority task", 1)
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pq$enqueue("High priority task", 5)
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pq$enqueue("Medium priority task", 3)
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pq$display()
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cat("\nDequeuing in priority order:\n")
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while (!pq$is_empty()) {
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pq$dequeue()
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}
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}

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