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Matlab operator question

  1. Jan 25, 2012 #1
    1. The problem statement, all variables and given/known data
    I have a problem with vectors x and y, where x=[-3,0,0,2,5,8] and y=[-5,-2,0,3,4,10]
    The problem asks me to determine z=y<~x
    I've searched my book and cannot determine what this operator is telling me to do. Am I supposed to determine the values of y that are not less than x? I enter this exaclty as stated into matlab and it returns a vector z=[1,1,1,0,0,0] If someone could help me out here, i'd appreciate it.

    2. Relevant equations


    3. The attempt at a solution
     
  2. jcsd
  3. Jan 26, 2012 #2

    Mark44

    Staff: Mentor

    ~x is the complement of vector x, which evaluates to a new vector with all the signs switched.

    z = y < ~ x causes each element of z to be set to 0 or 1 if an element of y is less than the corresponding element in ~x.

    From you answer, I'm guessing that the problem is actually z = y <= ~x.
     
  4. Jan 26, 2012 #3
    That's not what the ~ operator does in MATLAB. The ~ is for logical negation, which only applies to logical arrays. Any true element becomes a false element and vice versa. If x is not a logical array, I believe it is first converted into one before the negation is applied. Thus "~x" will have a 0 for every non-zero element in x, and a 1 for every element of x equal to zero.

    The idea of using the < operator on a logical array seems improper to me.
     
  5. Jan 26, 2012 #4

    Mark44

    Staff: Mentor

    Your explanation sounds reasonable to me.
    Not to me. You should be able to compare (<, <=, ==, >=, >) two logical values or two arrays of logical values.

    In the OP's question, [-5, -2, 0, 3, 4, 10] is being compared to ~x = ~[-3, 0, 0, 2, 5, 8]. Each nonzero entry in ~x is replaced by 1, and each zero entry is replaced by 0, I believe. Then the two arrays are compared, element by element, with the result array having a 1 where y(i) < ~x(i), and a 0 otherwise. That's my best guess, at any rate.

    I still have a concern about the 3rd element in the answer that the OP posted. I still believe that in the original problem, the comparison is <=, not <.
     
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