Erlang (programming language)/Tutorials/Folding: Difference between revisions

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imported>Eric Evers
(New page: Fun with folding Fold is a powerful tool, when you get to know it. lists:foldr(F,S,L) takes three arguments: F is some folding function, S is some starting value, and L is some List that...)
 
imported>Eric Evers
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Line 6: Line 6:
   lists:foldr( fun(_C,A)->A+1 end, 0, [1,2,3]).
   lists:foldr( fun(_C,A)->A+1 end, 0, [1,2,3]).
   3
   3
We could reverse a list with fold if we like.
  lists:foldr(fun(C,A)->A++[C] end, [], [a,b,c]).
  [c,b,a]
Or to get fancy we could try a finite difference.
  lists:foldr(
    fun(C,{P,A})->{C,[P-C]++A} end,
    {0,[]}, 
    [1,4,9,16]).   
  {1,[3,5,7,-16]}

Revision as of 12:45, 13 September 2008

Fun with folding

Fold is a powerful tool, when you get to know it. lists:foldr(F,S,L) takes three arguments: F is some folding function, S is some starting value, and L is some List that needs folding. Let us do some simple folding. The following fold calculates the lenght of a list. Here the current list item, _C is ignored, and the accumulator, A, counts the number of elements in the list.

 lists:foldr( fun(_C,A)->A+1 end, 0, [1,2,3]).
 3

We could reverse a list with fold if we like.

 lists:foldr(fun(C,A)->A++[C] end, [], [a,b,c]). 
 [c,b,a]

Or to get fancy we could try a finite difference.

 lists:foldr(
   fun(C,{P,A})->{C,[P-C]++A} end, 
   {0,[]},   
   [1,4,9,16]).    
 {1,[3,5,7,-16]}