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matlablogical-operatorsinverse

Can anyone explain to me what is going on in this line of MatLAB code

y = rand(20,3); 
aa= unidrnd(2,20,3) - 1;
val = ( aa & y<1.366e-04) | (~aa & y<8.298e-04);
aa(val) = ~aa(val);

I have this code. Can any one explain to me what is happening here. I have tried to understand it step by step (debugging) but I cannot understand the purpose of using inverse '~' in line 4 and also using 'val' as indices.

Solution 
  • y = rand(20,3);

    Creates a matrix of uniformly distributed random numbers, y.

    aa= unidrnd(2,20,3) - 1;

    Creates a matrix of uniformly distributed random integers, that goes from 1 to 2, and then subtract one. Thus, aa is a matrix of 0s and 1s.

    val = ( aa & y<1.366e-04) | (~aa & y<8.298e-04);

    This line checks all the values where aa is 1AND y<1.366e-04 OR aa is 0 AND y<8.298e-04. Note that this barely happens, being y uniformly distributed numbers from 0 to 1, being them this smalls is unlikely. 

    aa(val) = ~aa(val);

    Take all those cases computed before, and make aa change from 0 to 1 or from 1 to 0 if it happened in that index.