tasks a-s

Task/Queue-Definition/R/queue-definition-2.r

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+# The usual Scheme way : build a function that takes commands as parameters (it's like message passing oriented programming)
+queue <- function() {
+	v <- list()
+	f <- function(cmd, val=NULL) {
+		if(cmd == "push") {
+			v <<- c(v, val)
+			invisible()
+		} else if(cmd == "pop") {
+			if(length(v) == 0) {
+				stop("empty queue")
+			} else {
+				x <- v[[1]]
+				v[[1]] <<- NULL
+				x
+			}
+		} else if(cmd == "length") {
+			length(v)
+		} else if(cmd == "empty") {
+			length(v) == 0
+		} else {
+			stop("unknown command")
+		}
+	}
+	f
+}
+
+# Create two queues
+a <- queue()
+b <- queue()
+a("push", 1)
+a("push", 2)
+b("push", 3)
+a("push", 4)
+b("push", 5)
+
+a("pop")
+# [1] 1
+b("pop")
+# [1] 3