root/OptimizingMATLABCode/blockAvgEx/logicalIndex.m @ 11
| 10 | anderm8 | function logicalIndex
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%% Logical Indexing
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% Copyright 2015 The MathWorks, Inc.
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%
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% This is an example of how to do some logical indexing --
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% how you can vectorize code that might use an nested for loop
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% and an if statement.
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%% Finding elements using logical matrices
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% " While most indices are numeric, indicating a certain row or column
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% number, logical indices are positional. That is, it is the position of
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% each 1 in the logical matrix that determines which array element is
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% being referred to "
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N = 4000;
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A = magic(N);
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A2 = magic(N);
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A3 = magic(N);
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thresh = 1e6;
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%% Using an if and a nested for loop
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% In this example we want to find all values that are over myRef.
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% This is how we do that in a nested for loop with an if-statement:
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tic
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ix = 1;
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vals = zeros(numel(A),1);
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for jj = 1:N
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for ii = 1:N
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if A(ii,jj) > thresh
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vals(ix) = A(ii,jj);
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ix = ix + 1;
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end
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end
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end
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vals(ix:end) = [];
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toc
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%% Using the find Function
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% "The find function determines the indices of array elements that meet
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% a given logical condition. It returns the indices in the form of linear
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% indexing " !
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% myIndex = find(A2 > thresh);
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% display(myIndex(1:3))
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%%
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tic
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vals2 = A2(find(A2 > thresh));
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toc
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%% Using logical indexing
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tic
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vals3 = A3(A3 > thresh);
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toc
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%% Assert same answer
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assert(isequal(vals,vals2,vals3),'Values aren''t equal!');
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% Copyright 2015 The MathWorks, Inc.
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