Make very small numbers in complex matrix zero

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Math Works 2021-10-5

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1467071-make-very-small-numbers-in-complex-matrix-zero

评论： Math Works 2021-10-6

format shortG
a11 = 0;
a12 = exp(-j*0.5*pi);
a21 = exp(-j*0.5*pi);
a22 = 0;
S = [a11, a12; a21, a22]
S = 
            0 +          0i   6.1232e-17 -          1i
   6.1232e-17 -          1i            0 +          0i
A = ((1+a11)*(1-a22)+a12*a21)/(2*a21);
B = ((1+a11)*(1+a22)-a12*a21)/(2*a21);
C = ((1-a11)*(1-a22)-a12*a21)/(2*a21);
D = ((1-a11)*(1+a22)+a12*a21)/(2*a21);
X = [A, B; C, D]
X = 
   6.1232e-17 - 3.7494e-33i            0 +          1i
            0 +          1i   6.1232e-17 - 3.7494e-33i
X_cleanup = X;
X_cleanup(abs(X_cleanup)<0.0001) = 0
X_cleanup = 
            0 +          0i            0 +          1i
            0 +          1i            0 +          0i

Is there a way to automatically ask matlab to output matrix like what's in the X_cleanup instead of X?

Is there anyting under 'format'?

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Mathieu NOE 2021-10-5

hi

why not simply make the clean up a function that you can call anytime ?

Math Works 2021-10-5

It's just a bit annoying having to do that.

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Answer 1

Matt J 2021-10-5

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https://ww2.mathworks.cn/matlabcentral/answers/1467071-make-very-small-numbers-in-complex-matrix-zero#answer_801831

编辑：Matt J 2021-10-5

在 MATLAB Online 中打开

Is there a way to automatically ask matlab to output matrix like what's in the X_cleanup instead of X?

No, but note that if you expect your input data to be integer-valued, you can avoid the issue by generating them in integer form directly:

format shortG
a11 = 0;
a12 = -1i;
a21 = -1i;
a22 = 0;
S = [a11, a12; a21, a22]
S = 
            0 +          0i            0 -          1i
            0 -          1i            0 +          0i
A = ((1+a11)*(1-a22)+a12*a21)/(2*a21);
B = ((1+a11)*(1+a22)-a12*a21)/(2*a21);
C = ((1-a11)*(1-a22)-a12*a21)/(2*a21);
D = ((1-a11)*(1+a22)+a12*a21)/(2*a21);
X = [A, B; C, D]
X = 
            0 +          0i            0 +          1i
            0 +          1i            0 +          0i

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Matt J 2021-10-5

Well like I said, you can't make Matlab truncate results to zero on autopilot.

I think you would regret it even if it were implementable. In the situation you describe, Matlab would have no way of knowing if a computation was the final one or just an intermediate step in a future computation you are planning, so basically truncation would occur at every step in your computational pipeline and accumulate unpredictably.

Math Works 2021-10-6