C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  function that returns the determinant of a matrix          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message rikstick Guest Posted: Mon Dec 20, 2004 12:48 am    Post subject: function that returns the determinant of a matrix Does anyone know of a function that will return the determinant of a double scripted matrix with dimensions of 3x3 or greater? I would really appreciate the source code if you know of such a function. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top NJS Guest Posted: Tue Dec 21, 2004 10:31 am    Post subject: Re: function that returns the determinant of a matrix Type in "determinant cofactor expansion" into google, and you'll get links to a technique for solving determinants for arbitrary NxN matrices. There are probably numerically better ways of doing this. Personally, I would just use a linear algebra package; try searching on "LAPACK". - Niek Sanders rikstick wrote: Quote: Does anyone know of a function that will return the determinant of a double scripted matrix with dimensions of 3x3 or greater? I would really appreciate the source code if you know of such a function. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Branimir Maksimovic Guest Posted: Tue Dec 21, 2004 10:33 am    Post subject: Re: function that returns the determinant of a matrix rikstick wrote: Quote: Does anyone know of a function that will return the determinant of a double scripted matrix with dimensions of 3x3 or greater? I would really appreciate the source code if you know of such a function. One of my first steps in c++ back in 1994 was writing matrix class and calculating determinant, inverse matrix and such things. Is this some homework question ? Just look at the neariest math book and write that thing as you already know what is double scripted matrix. Greetings, Bane. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Francis Glassborow Guest Posted: Wed Dec 22, 2004 2:54 am    Post subject: Re: function that returns the determinant of a matrix In article <1103561473.539641.158930 (AT) z14g2000cwz (DOT) googlegroups.com>, NJS <njs8030 (AT) yahoo (DOT) com> writes Quote: Type in "determinant cofactor expansion" into google, and you'll get links to a technique for solving determinants for arbitrary NxN matrices. There are probably numerically better ways of doing this. Personally, I would just use a linear algebra package; try searching on "LAPACK". Many years ago I came across the case of a student at an Australian university doing an engineering degree who kept exceeding his resource limits. He was very puzzled because he had tested his code for evaluating an n by n determinant and it worked fine. However, whenever he tried to apply it to his engineering problem he exceeded his resource allowance and had to go to his supervisor for more. The third time this happened his supervisor asked to see the code. He had written a simple recursive expansion using co-factors. Tested it with 3x3, 4x4 and 5x5 test cases. He then tried to apply it to the real problem that was a 24x24. I seriously doubt that there is any machine in existence today that could evaluate a 24x24 determinant by cofactor expansion. C++ is not a miracle language. Solving problems requires expertise in the problem domain as well as in C++. The OP will need to study a bit more math before cutting code. -- Francis Glassborow ACCU Author of 'You Can Do It!' see http://www.spellen.org/youcandoit For project ideas and contributions: http://www.spellen.org/youcandoit/projects [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top NJS Guest Posted: Thu Dec 23, 2004 9:13 pm    Post subject: Re: function that returns the determinant of a matrix You're right about cofactor expansion being the wrong tool for the job. I knew using cofactors would be expensive, but it didn't occur to me that the algorithm is factorial. According to Maple and my friendly linear algebra textbook, a more reasonable approach is using gaussian elimination (through LU decomposition), which leads to a polynomial time algorithm. - Niek Francis Glassborow wrote: [snip] Quote: He had written a simple recursive expansion using co-factors. Tested it with 3x3, 4x4 and 5x5 test cases. He then tried to apply it to the real problem that was a 24x24. I seriously doubt that there is any machine in existence today that could evaluate a 24x24 determinant by cofactor expansion. [snip] [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. 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