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MATLAB building matrix

  1. Mar 30, 2009 #1
    i want to build a matrix that can solve cross product multiplication of 2 vectors, building a matrix A so that

    detA=PxS

    A=

    [tex]\hat{i}[/tex] [tex]\hat{j}[/tex] [tex]\hat{k}[/tex]
    P1 P2 P3
    S1 S2 S3


    how do i build a code so that my det will give me a vector answer?
     
  2. jcsd
  3. Mar 30, 2009 #2
    Why wouldn't you just use MATLAB's built in cross-product?

    Doing what you're asking (in MATLAB) is a somewhat complicated problem. It's easy to do this by hand, but MATLAB is a bit different. Think about how you would do this by hand - what are [tex]\hat{i}[/tex], [tex]\hat{j}[/tex], and [tex]\hat{k}[/tex]?

    Since each of these is a vector (three components in 3-space), MATLAB won't let you create a matrix containing these vectors - you'll get a dimension mismatch error. What you might be able to do is define symbols (do you have the symbolic math toolbox?) for i, j, and k, and use these in your matrix. This will give you an answer, but it won't be very useful, because you won't be able to operate on your result.

    -Kerry
     
  4. Mar 30, 2009 #3
    thanks, its one of the homework problems i have, using the cross function is much simpler but i am meant to doo it this way.

    another problem i need to do is make a table containing
    names and ages

    name1 48
    name2 33
    name4 90
    ...
    namen 22

    totally randomly, at least 15 names, and then perform a number of tasks on them, is there a way i can tell matlab to make the random table or do i need to plug in all the names and ages, can i make a nx2 matrix where the 1st colomn is made up of words not numbers? also can i tell matlab to make name(n) where n runs from 1->15
     
  5. Mar 30, 2009 #4
    Look into the sprintf function - that should do what you need. You can use it in conjuction with randn.

    -Kerry
     
  6. Mar 30, 2009 #5
    thanks a lot, btw i managed the det using
    syms i j k
     
  7. Mar 30, 2009 #6
    still havent managed to build the names matrix
    name1 48
    name2 33
    name4 90
    ...
    namen 22

    could you please help, or if possible write the code i need, i didnt really understand the ssprintf function.

    even if i try making the matrix it doesnt work, ie
    a=['name1' 48; 'name2'.......] i am only managing to make matrices of numbers
     
  8. Mar 30, 2009 #7
    Can you show me how you're trying to use the sprintf function? I don't understand what problems you're having if you don't show your work. Tell me what you don't understand, what errors your getting, what behavior you want vs. what behavior you're getting.

    -Kerry
     
  9. Mar 30, 2009 #8
    i just dont know what it actually does, what to type to use it, the help fiile didnt really mae sense to me, i am pretty much a matlab novice, only had a couple of lessons so far
     
  10. Mar 30, 2009 #9
    Scroll to the bottom of the help entry - there are examples there. Try them out. If you still have questions, I will be happy to help. You can cite specific parts of the examples that you don't understand. MATLAB does a very good job with their documentation, and I don't want to just repeat it here. I can give you my own examples, and my own explanation of what it is doing, but if you don't tell me what it is that you don't understand, my explanation will be very similar to what you will find in the MATLAB documentation.

    -Kerry
     
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