[Cs254f11] Simple mapping problem

[Lee Spector]

Jesse,

You've got several near misses there, and there are several ways to do what you want. My first impulse is:

(map #(+ % 5) 
     (map first m))

=> (90 97 71 85 55 90 71 42 108 39 118)

Reading from the inside out, it maps first down m to produce a list of all the pitches (the first elements of the notes in your melody). Then it maps a function down that list of pitches. That function is specified anonymously, using the #() syntax, and it is the function that returns the result of calling + on its argument (%) and 5.

In place of #(+ % 5) you could write, if you like, (fn [x] (+ x 5)).

Of course, you could use a variable there in place of the 5. 

If you don't like the nesting you could do something like:

(let [pitches (map first m)]
  (map #(+ % 5) pitches))

And there are many other options. Here's a particularly weird one just for the heck of it:

(->> (map first m)
     (map #(+ % 5)))

 -Lee



On Nov 17, 2011, at 12:05 PM, Jesse French wrote:

> Hi all,
> 
> I'm trying to figure out how to map some simple functions along a vector, and having trouble due to my lack of understanding of how map works.
> 
> (def m
> [[85 1]
> [92 5/16]
> [66 15/16]
> [80 1/8]
> [50 1/8]
> [85 1/4]
> [66 13/16]
> [37 1/16]
> [103 1/8]
> [34 1/8]
> [113 7/16]]
> 
> My goal is to map a '+ x' down the first value of this vector.
> 
> Even 
> (map first (map inc m))
> doesn't work, when I would like to be doing something like 
> (map first (map (+  m x))
> or
> (map (+  (map first m) x))
> 
> 
> (def p [100 15 29 7 63 59 44 72])
> (map inc p)
> This works, so I'm assuming my problem is with the syntax of how I'm nesting them.
> 
> 
> Any thoughts?
> 
> Jesse
> 
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--
Lee Spector, Professor of Computer Science
Cognitive Science, Hampshire College
893 West Street, Amherst, MA 01002-3359
lspector at hampshire.edu, http://hampshire.edu/lspector/
Phone: 413-559-5352, Fax: 413-559-5438