Added some tips on memory and speedy use of names via environments (due to Joseph Adler)

Author: ashander

CodeTricks.md

@@ -14,10 +14,10 @@ Matlab
  * Use the matlab profiler
 
 R
--
- * R profiler: Rprof (functionalize your code first)
- * sapply instead of for loops
+-----
 
+* R profiler: Rprof (functionalize your code first)
+* sapply instead of for loops
 
 
 Did you know, in R:
@@ -36,10 +36,34 @@ and  `a^2 ` is slower than `a*a`.
 
 Radford Neal [pointed this out](http://radfordneal.wordpress.com/2010/08/19/speeding-up-parentheses-and-lots-more-in-r/) in  R-2.11.1, try testing in R 2.14.1 first.  
 
-Also, although many matrix operations can be performed on `data.frame`, they are _much_ faster when performed on `matrix` data types[^dfmat].
+## Names and speed
+
+Although many matrix operations can be performed on `data.frame`, they are _much_ faster when performed on `matrix` data types[^dfmat].
 
 [^dfmat]: need a small example of this
 
+Accesssing arrays via index is faster than by name, but using names can make for more readable code. 
+To achieve fast access and still use names, one can construct an environment that uses a hash table for lookup
+
+```R
+labeled.environment <- function(n) {
+  e <- new.env(hash=TRUE,size=n)
+  from <- "0123456789"
+  to <- "ABCDEFGHIJ"
+  for (i in 1:n) {
+    assign(x=chartr(from,to,i), value=i, envir=e)
+  }
+  e
+}
+e.20 <- labeled.environment(20)
+
+## Access via
+e.20[["B"]]
+get("BH", env=e.20)
+```
+
+This tip is from  [Joseph Adler](http://broadcast.oreilly.com/2010/03/lookup-performance-in-r.html)
+
 
 ## Compiling code
 
@@ -48,3 +72,14 @@ As of `R` 2.13, the `compiler` package lets you compile code into byte for faste
 `compiled.function <- cmpfun(orig.function)`
 
 A demonstration [here](http://dirk.eddelbuettel.com/blog/2011/04/12/) shows that it speeds some basic functions up about 4X.
+
+## Memory 
+
+* preallocate memory 
+
+###  manage memory
+
+* free up space and display stats with `gc()`
+* check memory use by `x` using `object.size(x)`
+* use `memory.profile()`
+* use `mem.limits()` to get/set memory limits