Could anyone please explain this matlab code...?

2014 3 20
2 个回答
回答已采纳
更新时间：2016 12 6
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0 个投票

function g = NeumannBoundCond(f)
[nrow,ncol] = size(f);
g = f;
g([1 nrow],[1 ncol]) = g([3 nrow-2],[3 ncol-2]);
g([1 nrow],2:end-1) = g([3 nrow-2],2:end-1);
g(2:end-1,[1 ncol]) = g(2:end-1,[3 ncol-2]);

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Roger Stafford
Roger Stafford 2014-3-20

1 个投票

The best way to explain your code is to see it in action. Let f be the following:
f =
1 2 3 4 5 6 7
8 9 10 11 12 13 14
15 16 17 18 19 20 21
22 23 24 25 26 27 28
29 30 31 32 33 34 35
36 37 38 39 40 41 42
43 44 45 46 47 48 49
50 51 52 53 54 55 56
Then g will be this:
g =
17 16 17 18 19 20 19
10 9 10 11 12 13 12
17 16 17 18 19 20 19
24 23 24 25 26 27 26
31 30 31 32 33 34 33
38 37 38 39 40 41 40
45 44 45 46 47 48 47
38 37 38 39 40 41 40
As you see, only the outer boundary of the f matrix has been changed. Its values are copies of the rectangle of values two places in from the boundary. The effect is, I suppose, some kind of Neumann-like condition on the outer normal derivative.
Note that this code has one more line than it really needed. It doesn't have to make a special case of the corners. It could have been written equivalently as this:
g([1 nrow],:) = g([3 nrow-2],:);
g(:,[1 ncol]) = g(:,[3 ncol-2]);

更多回答（1 个）

Walter Roberson
Walter Roberson 2014-3-20

1 个投票

In general, the form
array([A B], C)
refers to selecting the elements of the array in which the row is A or B, and the column is any of the values given by C. So (A,C(1)), (A,C(2)), (A,C(3))... (B,C(1)), (B,C(2)), ...
With A, B, C, D all scalar, array([A B], [C D]) would designate four locations, (A,C), (A,D), (B,C), (B,D)

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