Examples explaining lexical scoping and <<- operator used in R

mareq · mareq · commit 833d801f8b19 · 2015-01-18T19:52:57.000Z
diff --git a/scopes.R b/scopes.R
@@ -0,0 +1,48 @@
+# modifying parent scope variable
+new_counter <- function() {
+	i <- 0  # this is the variable being modified
+	function() {
+		i <<- i + 1  # modifies parent scope variable
+		i  # prints parent scope variable
+	}
+}
+counter_one <- new_counter()
+counter_two <- new_counter()
+counter_one()  # [1] 1
+counter_one()  # [1] 2
+counter_two()  # [1] 1
+
+# parent scope variable overshadowed by the current scope variable
+new_counter <- function() {
+	i <- 0  # this variable will be overshadowed
+	function() {
+		i <- 42  # overshadows the parent scope variable
+		i <<- i + 1  # modifies current scope variable
+		i  # prints current scope variable
+	}
+}
+counter_one <- new_counter()
+counter_two <- new_counter()
+counter_one()  # [1] 43
+counter_one()  # [1] 43
+counter_two()  # [1] 43
+
+# modified example from 02.10 Scoping Rules: R Scoping Rules
+y <- 10
+f <- function(x) {
+	y <- 2  # overshadows the global scope variable
+	y^2 + g(x)  # uses current scope variable (value 2)
+}
+g <- function(x) {
+	y <<- y + 42  # modifies gloval scope variable (42 added to y, which is 10 at the beginning)
+	x * y # uses global scope variable (value 42), because g is defined in global environment
+}
+f(3) # 2^2 + 3*52 = 160
+y # 42 (changed by call to g)
+f(3) # 2^2 + 3*94 = 286
+y # 94 (changed by call to g)
+
+
+# vim: set ts=2 sw=2 noet:
+
+
