digitalmars.D.learn - map/filter/reduce: use functions or delegates or both?
- Falk Henrich (38/38) Mar 15 2007 Hi!
- Daniel Keep (5/14) Mar 15 2007 I implemented these a while back using rather a lot of templates :) You...
- Denton Cockburn (3/3) Mar 21 2007 I don't see the template functions being instantiated.
- Denton Cockburn (2/2) Mar 21 2007 oh yeah, map would need to be instantiated as well.
- Daniel Keep (12/20) Mar 21 2007 -- Daniel
Hi! I just started learning D. I really like D's combination of high- and low-level features. Hence, I started coding map/filter/reduce as known from functional languages for dynamic array types. Using anonymous delegates one can compute the sum of the squares of an integer array like int[] b = reduce( (int x, int y) { return x + y; }, 0, map ( (int x) { return x*x; }, a)); given a definition of map as (my newbee code) To[] map(From, To)(To delegate(From) f, From[] a) { To[] b; b.length = a.length; for(int i = 0; i < a.length; i++) { b[i] = f(a[i]); } return(b); } and reduce as (my newbee code) B reduce(A,B) ( B delegate(A,B) f, B e, A[] a) { foreach(A x; a) { e = f(x,e); } return(e); } Now I tried to improve the readability of the functionals by defining int plus(int x, int y) { return x + y; } int square(int x) { return x * x; } in order to write int[] b = reduce(plus, 0, map(square, a)); But this won't work since plus and square are functions and delegates. Therefore, I replaced "delegate" by "function" in the definitions above. But this breaks the application using anonymous delegates. Neither is it possible to have a delegate map together with a function map in the same module. Is there any solution to this dilemma? Thanks for advice. Falk
Mar 15 2007
Falk Henrich Wrote:Hi! ... Is there any solution to this dilemma? Thanks for advice. FalkI implemented these a while back using rather a lot of templates :) You can see the implementation here: http://www.prowiki.org/wiki4d/wiki.cgi?DanielKeep/functools Basically I solved the problem by using neither delegates or functions. Instead, I just used a template type called "tOp". Hope this helps :) -- Daniel
Mar 15 2007
I don't see the template functions being instantiated. e.g. int[] b = reduce!(int, int)(plus, 0, map(square, a)); but I just started learning D too.
Mar 21 2007
oh yeah, map would need to be instantiated as well. int[] b = reduce!(int, int)(plus, 0, map!(int, int)(square, a));
Mar 21 2007
Denton Cockburn wrote:oh yeah, map would need to be instantiated as well. int[] b = reduce!(int, int)(plus, 0, map!(int, int)(square, a));D has implicit function template instantiation.T first(T)(T[] arr) { return arr[0]; } auto one = first([1,2,3]);-- Daniel -- int getRandomNumber() { return 4; // chosen by fair dice roll. // guaranteed to be random. } http://xkcd.com/ v2sw5+8Yhw5ln4+5pr6OFPma8u6+7Lw4Tm6+7l6+7D i28a2Xs3MSr2e4/6+7t4TNSMb6HTOp5en5g6RAHCP http://hackerkey.com/
Mar 21 2007