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Matrix multiplication: Difference between revisions
Added multiplication in Erlang
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(Added multiplication in Erlang) |
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end
</lang>
=={{header|Erlang}}==
<lang erlang>
%% Multiplies two matrices. Usage example:
%% $ matrix:multiply([[1,2,3],[4,5,6]], [[4,4],[0,0],[1,4]])
%% If the dimentions are incompatible, an error is thrown.
%%
%% The erl shell may encode the lists output as strings. In order to prevent such
%% behaviour, BEFORE running matrix:multiply, run shell:strings(false) to disable
%% auto-encoding. When finished, run shell:strings(true) to reset the defaults.
-module(matrix).
-export([multiply/2]).
transpose([[]|_]) ->
[];
transpose(B) ->
[lists:map(fun hd/1, B) | transpose(lists:map(fun tl/1, B))].
red(Pair, Sum) ->
X = element(1, Pair), %gets X
Y = element(2, Pair), %gets Y
X * Y + Sum.
%% Mathematical dot product. A x B = d
%% A, B = 1-dimension vector
%% d = scalar
dot_product(A, B) ->
lists:foldl(fun red/2, 0, lists:zip(A, B)).
%% Exposed function. Expected result is C = A x B.
multiply(A, B) ->
%% First transposes B, to facilitate the calculations (It's easier to fetch
%% row than column wise).
multiply_internal(A, transpose(B)).
%% This function does the actual multiplication, but expects the second matrix
%% to be transposed.
multiply_internal([Head | Rest], B) ->
% multiply each row by Y
Element = multiply_row_by_col(Head, B),
% concatenate the result of this multiplication with the next ones
[Element | multiply_internal(Rest, B)];
multiply_internal([], B) ->
% concatenating and empty list to the end of a list, changes nothing.
[].
multiply_row_by_col(Row, [Col_Head | Col_Rest]) ->
Scalar = dot_product(Row, Col_Head),
[Scalar | multiply_row_by_col(Row, Col_Rest)];
multiply_row_by_col(Row, []) ->
[].
</lang>
{{out}}
<pre>
[[7,16],[22,40]]
</pre>
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