root/OptimizingMATLABCode/blockAvgEx/logicalIndex.m @ 10
| 10 | anderm8 | function logicalIndex
|
|
%% Logical Indexing
|
|||
% Copyright 2015 The MathWorks, Inc.
|
|||
%
|
|||
% This is an example of how to do some logical indexing --
|
|||
% how you can vectorize code that might use an nested for loop
|
|||
% and an if statement.
|
|||
%% Finding elements using logical matrices
|
|||
% " While most indices are numeric, indicating a certain row or column
|
|||
% number, logical indices are positional. That is, it is the position of
|
|||
% each 1 in the logical matrix that determines which array element is
|
|||
% being referred to "
|
|||
N = 4000;
|
|||
A = magic(N);
|
|||
A2 = magic(N);
|
|||
A3 = magic(N);
|
|||
thresh = 1e6;
|
|||
%% Using an if and a nested for loop
|
|||
% In this example we want to find all values that are over myRef.
|
|||
% This is how we do that in a nested for loop with an if-statement:
|
|||
tic
|
|||
ix = 1;
|
|||
vals = zeros(numel(A),1);
|
|||
for jj = 1:N
|
|||
for ii = 1:N
|
|||
if A(ii,jj) > thresh
|
|||
vals(ix) = A(ii,jj);
|
|||
ix = ix + 1;
|
|||
end
|
|||
end
|
|||
end
|
|||
vals(ix:end) = [];
|
|||
toc
|
|||
%% Using the find Function
|
|||
% "The find function determines the indices of array elements that meet
|
|||
% a given logical condition. It returns the indices in the form of linear
|
|||
% indexing " !
|
|||
% myIndex = find(A2 > thresh);
|
|||
% display(myIndex(1:3))
|
|||
%%
|
|||
tic
|
|||
vals2 = A2(find(A2 > thresh));
|
|||
toc
|
|||
%% Using logical indexing
|
|||
tic
|
|||
vals3 = A3(A3 > thresh);
|
|||
toc
|
|||
%% Assert same answer
|
|||
assert(isequal(vals,vals2,vals3),'Values aren''t equal!');
|
|||
% Copyright 2015 The MathWorks, Inc.
|