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Andrews Map: (Examples)

 [1/2] from: tim::johnsons-web::com at: 18-Feb-2003 7:48

Hi All: Just started playing with Andrew's Map function. Does any one have any examples of using it - in addition to those included? Some simpler ones would be good to start with - like test: [[1 2 3][4 5 6][7 8 9][10 11 12]] t1: Map/only test func[blk][blk: next blk] Maybe I'll do a code corner on this. Thanks -- Tim Johnson <[tim--johnsons-web--com]>

 [2/2] from: joel:neely:fedex at: 18-Feb-2003 17:03

Hi, Tim, We've kicked this one around before; see and the related thread for more discussion. However, I'll give a couple of quick examples below. Tim Johnson wrote:
> Does any one have any examples of using it [map] ... >
Here's a (simplified) version of MAP that I've been using for quite a while, along with another handy utility: map: function [ f [any-function!] b [block!] /all ][ result item ][ result: make block! length? b foreach val b [ item: f val if any [found? item all] [ append/only result item ] ] result ] ident: func [f [any-function!] x [any-type!]] [ if f x [x] ] First, a couple of simple illustrations:
>> map func [x] [x * x] [0 1 2 3 4 5]
== [0 1 4 9 16 25] MAP abstracts out the notion of "do this to all of those and give me all of the results" so that we don't have to hand-code the loop boilerplate over and over.
>> map func [x] [ident :even? x] [0 1 2 3 4 5]
== [0 2 4] IDENT evaluates to its second argument only if that value satisfies the first argument. My version of MAP doesn't include NONE in the results (unless you set the /ALL refinement) so this essentially can be read as "give me back only the values that satisfy this test". In a teaching situation, just to give one more specific application example, we could have a function that translates from a numerical (percentage) score to a letter grade: pct2grade: func [p [number!] /local lowbounds letters] [ lowbounds: [95 85 75 60 0] letters: ["A" "B" "C" "D" "F"] for i 1 length? lowbounds 1 [ if lowbounds/:i <= p [ return letters/:i ] ] last letters ] (In real life, I'd use an object, but I'm trying to keep to brief examples here.) We can also have a function that takes a total possible number of points and the number of points earned by a student, returning a percentage score (with a lower bound of zero!) raw2pct: func [t [number!] s [number!]] [ max 0 0.1 * to-integer 0.5 + 1000.0 * s / t ] We can stick a block of raw scores onto one (nested) expression: map :pct2grade map func [x][raw2pct 175 x] [ 115 180 33 159 168 144 ] and get the grades for the class == ["D" "A" "F" "B" "A" "C"] The real point of MAP to my way of thinking is that it continues the progression we started with structured programming; we can use a WHILE or FOR expression to directly state that we want something repeatedly evaluated under specific circumstances, without having to think about bolting together some more primitive concepts. Likewise, MAP allows us to express directly that we want to apply a function to a collection of values and get all of the results, without having to bolt together the looping and appending by hand every time. HTH! -jn- -- ---------------------------------------------------------------------- Joel Neely joelDOTneelyATfedexDOTcom 901-263-4446 Atlanta, Ga. - Scientists at the Centers for Disease Control today confirmed that hoof-and-mouth disease cannot be spread by Microsoft's Outlook email application, believed to be the first time the program has ever failed to propagate a major virus.