Alice in Wond-R-land,loops, eval(parse()), & the next level in R

Looping in R is an important skill to develop

Loops tell R to do the same thing many times until R reaches the end. Each loop, the index is

updated. Here is a loop of length 2.

for (k in 1:2){

k=2

k=1

Examples of loops

Alice meets the White KnightKnight: Let me sing you a song to comfort you… The name of the song is called "Haddocks’ Eyes."

Alice: Oh, that's the name of the song, is it?

Knight: No, you don't understand, That’s what the name is called. The name really is "The Aged Aged Man."

Alice: Then I ought to have said "That's what the song is called"?

Knight: No, you oughtn't: that's quite another thing! The song is called "Ways and Means": but that's only what it's called, you know!

Alice: Well, what is the song, then?

Knight: I was coming to that. The song really is "A-sitting On A Gate": and the tune's my own invention.

Distinction between the object itself and what it is called

• Cheese is derived from milk. Most people like to eat it.

• "Cheese" is derived from a word in Old English. It has six letters.

Distinction between the object itself and what it is called

• USArrests is a data.frame with 50 rows and 4 columns

• “USArrests” is a string with 9 characters.

Distinction between the object itself and what it is called

• How do we take “USArrests”, the character string, and tell R to treat as USArrests, the data.frame?

• More generally: how do we tell R to treat any character strings (created using, e.g., paste) and evaluate it as though it were typed in by the user?

Eval & Parse

• parse(text=“FUN”): parse tells R to treat “FUN” as if it were typed in - FUN - by the user I.e., to change it from what the object is called to what it is.

• eval(parse()): eval tells R to evaluate the parsed statement. I.e., its like pressing <enter> after typing into R.

Why are they useful?• Often, you need to build commands

dynamically, e.g., within a for() loop. These commands may be to automate object assignment, alter titles, place statistics in graphs, analyzing different data subsets, and so forth.

• Along with system() and function(), for() loops and eval(parse()) are two of the most important new abilities you’ll gain in becoming an R wizard.

Today, you’re graduating to the next level of R!