tasks a-s

Task/Runtime-evaluation-In-an-environment/R/runtime-evaluation-in-an-environment.r

@@ -0,0 +1,15 @@
+evalWithAB <- function(expr, var, a, b) {
+  env <- new.env()           # provide a separate env, so that the choosen
+  assign(var, a, envir=env)  # var name do not collide with symbols inside
+                             # this function (e.g. it could be even "env")
+  atA <- eval(parse(text=expr), env)
+                             # and then evaluate the expression inside this
+                             # ad hoc env-ironment
+  assign(var, b, envir=env)
+  atB <- eval(parse(text=expr), env)
+  return(atB - atA)
+}
+
+print(evalWithAB("2*x+1", "x", 5, 3))
+print(evalWithAB("2*y+1", "y", 5, 3))
+print(evalWithAB("2*y+1", "x", 5, 3)) # error: object "y" not found