Another update from ingydotnet^djgoku

Task/Matrix-multiplication/Erlang/matrix-multiplication.erl

@@ -0,0 +1,57 @@
+%% 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, []) ->
+    [].