# proper lvalues and whatnot Classic List Threaded 4 messages Open this post in threaded view
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## proper lvalues and whatnot

 One of the neat things about octave is its incorporation of C-like syntax, with cool operators like ++ and += and so on. I am wondering why it is not the case that assignment expressions are not likewise lvalues? Is there some reason why octave> (v = zeros(1,2))++ should not give  v = [1 1] ?? Also, I am wondering why it is that while subscripted vectors/matrices are lvalues, that octave> v = [1 1]; octave> v([1 1 2])++ does not give  v = [3 2]; ?? OK, so maybe I am a sick puppy.  But this is what I am really after. > [i,j,nz] = find(M); > (y=zeros(max(i),1))(i) += nz.*x(j) and have  y = M*x I have a lot of sparse matrices I want to play with and this sort of syntax would rock. Is there a conceptual reason why I should not try to modify octave to do this (i.e. how does this break the .m language as a language)? Is there a technical reason why I should not try to modify octave to do this (i.e. requires a total interpreter rewrite, I'd be in over my head)? -r
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## proper lvalues and whatnot

 On 18-Aug-2000, Ross A. Lippert <[hidden email]> wrote: | One of the neat things about octave is its incorporation of | C-like syntax, with cool operators like ++ and += and so on. | | I am wondering why it is not the case that assignment expressions | are not likewise lvalues? | | Is there some reason why | octave> (v = zeros(1,2))++ | should not give |  v = [1 1] | ?? Hmm.  Because the result of an assingment is not an lvalue, even in C? | Also, I am wondering why it is that while subscripted vectors/matrices | are lvalues, that | octave> v = [1 1]; | octave> v([1 1 2])++ | does not give |  v = [3 2]; | ?? Now I think this is a valid thing to want to do, and it corresponds to something that does work in C:   #include   int   main (void)   {     int x;     x = 0;     x++;     printf ("%d\n", x);     return 0;   } Octave also accepts this syntax, but maybe it doesn't do exactly what you expect.  For example (using the current sources, which are close to 2.1.31),   octave:1> v = 1:10   v =      1   2   3   4   5   6   7   8   9  10   octave:2> v([3, 4])++   ans =     3  4   octave:3> v   v =      1   2   4   5   5   6   7   8   9  10 That seems reasonable, doesn't it? But your example was   octave:4> v = [1, 1]   v =     1  1   octave:5> v([1, 1, 2])++   ans =     1  1  1   octave:6> v   v =     2  2 which doesn't do exactly what you seem to expect.  Does the actual Octave behavior make sense though?  I think it does, if you think about it in the following way:   expr++  <==>  expr += 1  <==>  expr = expr + 1 then what you have is   v = [1, 1]   v([1, 1, 2]) = v([1, 1, 2]) + 1                = [1, 1, 1] + 1                = [2, 2, 2] and now the indexed assignment works in order, assigning LHS(i) = RHS(i) for i = 1 to N.  You can see this behavior clearly by performing the operation v([1,1,1]) = [1, 2, 3]. FWIW, I'm not sure that this should be considered guaranteed behavior or just a quirk of the current implementation. | OK, so maybe I am a sick puppy.  But this is what I am really after. | | > [i,j,nz] = find(M); | > (y=zeros(max(i),1))(i) += nz.*x(j) | and have |  y = M*x I guess I'm dense, but I don't understand what the purpose of this is, or how it is equivalent to y = M*x. | I have a lot of sparse matrices I want to play with and this sort of | syntax would rock. Are you using an actual sparse matrix data type or trying to emulate them using full storage (but trying to avoid operations on zeros)?  If the latter, wouldn't it make more sense to work on a sparse matrix data type instead? | Is there a conceptual reason why I should not try to modify octave to | do this (i.e. how does this break the .m language as a language)? | | Is there a technical reason why I should not try to modify octave to | do this (i.e. requires a total interpreter rewrite, I'd be in over my | head)? Seems to me that it is somewhat difficult to pin down exactly what these operations should do, and then arrange for everyone to agree. jwe