Data commit

Task/Scope-modifiers/R/scope-modifiers-1.r

@@ -0,0 +1,7 @@
+X <- "global x"
+f <- function() {
+  x <- "local x"
+  print(x) #"local x"
+}
+f()                          #prints "local x"
+print(x)                     #prints "global x"

Task/Scope-modifiers/R/scope-modifiers-2.r

@@ -0,0 +1,5 @@
+d <- data.frame(a=c(2,4,6), b = c(5,7,9))
+attach(d)
+b - a        #success
+detach(d)
+b - a        #produces error

Task/Scope-modifiers/R/scope-modifiers-3.r

@@ -0,0 +1,37 @@
+x <- "global x"
+print(x)                                   #"global x"
+
+local({ ## local({...}) is a shortcut for evalq({...}, envir=new.env())
+        ## and is also equivalent to (function() {...})()
+
+  x <- "outer local x"
+  print(x)                                 #"outer local x"
+  x <<- "modified global x"
+  print(x)                                 #"outer local x" still
+  y <<- "created global y"
+  print(y)                                 #"created global y"
+  local({
+
+    ## Note, <<- is _not_ a global assignment operator. If an
+    ## enclosing scope defines the variable, that enclosing scope gets
+    ## the assignment. This happens in the order of evalution; a local
+    ## variable may be defined later on in the same scope.
+
+    x <- "inner local x"
+    print(x)                               #"inner local x"
+    x <<- "modified outer local x"
+    print(x)                               #"inner local x"
+    y <<- "modified global y"
+    print(y)                               #"modified global y"
+    y <- "local y"
+    print(y)                               #"local y"
+
+    ##this is the only way to reliably do a global assignment:
+    assign("x", "twice modified global x", globalenv())
+    print(evalq(x, globalenv()))           #"twice modified global x"
+  })
+
+  print(x)                                 #"modified outer local x"
+})
+print(x)                                   #"twice modified global x"
+print(y)                                   #"modified global y"

Task/Scope-modifiers/R/scope-modifiers-4.r

@@ -0,0 +1,11 @@
+x <- "global x"
+f <- function() {
+  cat("Lexically enclosed x: ", x,"\n")
+  cat("Lexically enclosed x: ", evalq(x, parent.env(sys.frame())),"\n")
+  cat("Dynamically enclosed x: ", evalq(x, parent.frame()),"\n")
+}
+
+local({
+  x <- "local x"
+  f()
+})

Task/Scope-modifiers/R/scope-modifiers-5.r

@@ -0,0 +1,12 @@
+d <- data.frame(a=c(2,4,6), b = c(5,7,9))
+also <- c(1, 0, 2)
+with(d, mean(b - a + also)) #returns 4
+
+## with() is built in, but you might have implemented it like this:
+
+with.impl <- function(env, expr) {
+  env <- as.environment(env)
+  parent.env(env) <- parent.frame()
+  eval(substitute(expr), envir=env)
+}
+with.impl(d, mean(b - a + also))