Incorrect matrix substitution with ML2016b

2016 11 8
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(Matlab2016b-student) I had to substitute an (4x1)-Matrix with a (4x4)-Matrix. The result should have been a failure, instead it gave me a (4x4)-Matrix as output. How is this possible, when it is mathematical incorrect? In version Matlab2015b it gave the "correct" output that it's a failure because of the different matrix dimensions.

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James Tursa
James Tursa 2016-11-8
编辑：James Tursa 2016-11-8

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You don't show your code, but my guess is you did one of the element-wise operations that now has scalar expansion built in. E.g.,
(4x4) + (4x1)
The above will fail in earlier versions of MATLAB, but in R2016a it is automatically done as if you had invoked the bsxfun function with the @plus operation. See this link under "Implicit Expansion":
https://www.mathworks.com/help/matlab/release-notes.html

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Natalija Brkic
Natalija Brkic 2016-11-8
clear; clc;
A = [2 -3 5 1; 1 0 0 -1; 0 -3 -1 2; 3 2 0 -1];
f = [-3; 1; -15; 5];
x = A\f
x = inv(A) * f
fp = A*x
p = (f - A) * x % the result of (f-A) is a (4x4)-Matrix (what should be wrong)
James Tursa
James Tursa 2016-11-9
So for this particular problem, attempting to do (f-A) is of course wrong and earlier versions of MATLAB would complain, but the latest version doesn't. Is that your point?

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