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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.
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Alice in Wond-R-land,loops, eval(parse()), & the next level in R
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
Alice meets the White Knight Knight: 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.
