Erlang (programming language)/Tutorials/Folding: Difference between revisions
Jump to navigation
Jump to search
imported>Eric Evers No edit summary |
imported>Eric Evers No edit summary |
||
| Line 4: | Line 4: | ||
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. | 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]). | 1> lists:foldr( fun(_C,A)->A+1 end, 0, [1,2,3]). | ||
3 | 3 | ||
We could reverse a list with fold if we like. | We could reverse a list with fold if we like. | ||
lists:foldr(fun(C,A)->A++[C] end, [], [a,b,c]). | 2> lists:foldr(fun(C,A)->A++[C] end, [], [a,b,c]). | ||
[c,b,a] | [c,b,a] | ||
Or to get fancy we could try a finite difference. | Or to get fancy we could try a finite difference. | ||
lists:foldr( | 3> lists:foldr( | ||
fun(C,{P,A})->{C,[P-C]++A} end, | fun(C,{P,A})->{C,[P-C]++A} end, | ||
{0,[]}, | {0,[]}, | ||
[1,4,9,16]). | [1,4,9,16]). | ||
{1,[3,5,7,-16]} | {1,[3,5,7,-16]} | ||
In example 3, we used a tuple to remember the previous value so we could use it for next difference. Fold is | |||
a function with a limited attention span. We might like to delete the last item because is has a non-intuitive relation to the source list. | |||
Revision as of 12:52, 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.
1> lists:foldr( fun(_C,A)->A+1 end, 0, [1,2,3]). 3
We could reverse a list with fold if we like.
2> lists:foldr(fun(C,A)->A++[C] end, [], [a,b,c]). [c,b,a]
Or to get fancy we could try a finite difference.
3> lists:foldr(
fun(C,{P,A})->{C,[P-C]++A} end,
{0,[]},
[1,4,9,16]).
{1,[3,5,7,-16]}
In example 3, we used a tuple to remember the previous value so we could use it for next difference. Fold is a function with a limited attention span. We might like to delete the last item because is has a non-intuitive relation to the source list.