Data commit

Task/Stack/R/stack.r

@@ -0,0 +1,54 @@
+library(proto)
+
+stack <- proto(expr = {
+   l <- list()
+   empty <- function(.) length(.$l) == 0
+   push <- function(., x)
+   {
+      .$l <- c(list(x), .$l)
+      print(.$l)
+      invisible()
+   }
+   pop <- function(.)
+   {
+      if(.$empty()) stop("can't pop from an empty list")
+      .$l[[1]] <- NULL
+      print(.$l)
+      invisible()
+   }
+})
+
+stack$empty()
+# [1] TRUE
+stack$push(3)
+# [[1]]
+# [1] 3
+stack$push("abc")
+# [[1]]
+# [1] "abc"
+# [[2]]
+# [1] 3
+stack$push(matrix(1:6, nrow=2))
+# [[1]]
+#      [,1] [,2] [,3]
+# [1,]    1    3    5
+# [2,]    2    4    6
+# [[2]]
+# [1] "abc"
+# [[3]]
+# [1] 3
+stack$empty()
+# [1] FALSE
+stack$pop()
+# [[1]]
+[1] "abc"
+# [[2]]
+# [1] 3
+stack$pop()
+# [[1]]
+# [1] 3
+stack$pop()
+# list()
+stack$pop()
+# Error in get("pop", env = stack, inherits = TRUE)(stack, ...) :
+#   can't pop from an empty list