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 No edit summary |
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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]}