What a hell is wrong with logical indexing??? Really???
in [Matlab]
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From: Sergei Koulayev on 6 May 2010 10:22 People, check this out. I always thought I could treat 0-1 resulting from the logical operations as numbers. But apparently not. Why? It is completely unintuitive and dangerous. Can somebody give me a meaning of the following example? a = rand(100,1); i = a<0.5; i = i.^2; i(i==0) = 2; unique(i) ans = 1 2 i = a<0.5; i(i==0) = 2; unique(i) ans = 1
From: Sean on 6 May 2010 10:31 "Sergei Koulayev" <sergei.koulayev(a)mathworks.com> wrote in message <hrujad$fmn$1(a)fred.mathworks.com>... > People, > > check this out. I always thought I could treat 0-1 resulting from the logical operations as numbers. But apparently not. Why? It is completely unintuitive and dangerous. Can somebody give me a meaning of the following example? > > a = rand(100,1); > i = a<0.5; > i = i.^2; > i(i==0) = 2; > unique(i) > > ans = > > 1 > 2 > > i = a<0.5; > i(i==0) = 2; > unique(i) > > ans = > > 1 Try this: %%% a = rand(100,1); i = a<0.5; i = i.^2; i(i==0) = 2; class(i) unique(i) %%% a = rand(100,1); i = a<0.5; i(i==0) = 2; class(i) unique(i) %% Explanation 1st case: Squaring the i-vector turns it into a double so that values can be anything. 2nd case: The vector is logical so saying anything other than zero is true and it turns it to 1.
From: Steven Lord on 6 May 2010 10:44 "Sergei Koulayev" <sergei.koulayev(a)mathworks.com> wrote in message news:hrujad$fmn$1(a)fred.mathworks.com... > People, > > check this out. I always thought I could treat 0-1 resulting from the > logical operations as numbers. Of course. > But apparently not. Why? It is completely unintuitive and dangerous. Can > somebody give me a meaning of the following example? > > a = rand(100,1); > i = a<0.5; > i = i.^2; The above line converts i from being a _logical_ array into a _double_ array. [You can confirm this by looking at WHOS or by asking for its CLASS.] When you do so ... > i(i==0) = 2; this replaces all 0's in the double array with the double value 2. Therefore the array has two unique values, as your unique call indicated. > unique(i) > > ans = > > 1 > 2 > > i = a<0.5; > i(i==0) = 2; When you perform the above line, you're assigning the double value 2 into a _logical_ array. Therefore, the double value 2 needs to be converted into a logical value, and what is it converted into? >> x = logical(2) x = 1 The LOGICAL function converts 0 into false, errors if given a NaN or a complex value, and converts all other values (1, 2, pi, -73, Inf, 1.234567, etc.) into true. Therefore, when you perform the above operation the logical array i contains only true values, and so ... > unique(i) > > ans = > > 1 there is only one value in your logical array, true. This is the correct behavior. -- Steve Lord slord(a)mathworks.com comp.soft-sys.matlab (CSSM) FAQ: http://matlabwiki.mathworks.com/MATLAB_FAQ
From: Matt Fig on 6 May 2010 10:48 Nothing seems out of sorts to me. In the first case you are assigning a double to a double. In the second case you are assigning a double to a logical, so MATLAB preserves the logical. Sort of like this: A = 3; A(2) = uint8(9); A(3) = 'r'; whos A Now as for the warning in your case, yet none in the above example, I question that.
From: Sergei Koulayev on 6 May 2010 11:42
Hi Steve, thanks for the reply. I don't know about others, but I always cared about logical arrays as simply arrays that contain just 0 and 1's. It is quite disturbing that logical arrays can behave differently from arrays consisting of 0 and 1's (double). You explanation is about how things work in Matlab, and its good to know. But why Matlab should be so obsessed with preserving "logicalness" of the array - that I don't understand. Given than most users probably treat those arrays just like I do. Really, what's this thing about "logicalness"? Its clearly not needed for the indexing purposes. For example, when I write X(ix), why should I care of ix is double or logical, as long as it contains 0 and 1's? Sergei > When you perform the above line, you're assigning the double value 2 into a > _logical_ array. Therefore, the double value 2 needs to be converted into a > logical value, and what is it converted into? > > >> x = logical(2) > > x = > > 1 > > The LOGICAL function converts 0 into false, errors if given a NaN or a > complex value, and converts all other values (1, 2, pi, -73, Inf, 1.234567, > etc.) into true. Therefore, when you perform the above operation the > logical array i contains only true values, and so ... > > > unique(i) > > > > ans = > > > > 1 > > there is only one value in your logical array, true. This is the correct > behavior. > > -- > Steve Lord > slord(a)mathworks.com > comp.soft-sys.matlab (CSSM) FAQ: http://matlabwiki.mathworks.com/MATLAB_FAQ > |