# Real quick one, passing through a matrix backwards or just flip it.

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Scragmore 2011 年 10 月 27 日
During a for or while loop I would like to start at the back of the matrix and cycle to the beginning. Or do I just flip it using a(end:-1:1,:) or flipud(a). I understand flipud but can you briefly explain the syntax for a(end:-1:1,:).

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### 採用された回答

Fangjun Jiang 2011 年 10 月 27 日
a=rand(8,9);
b=a(5:end,5:end)
"end" here is the length of that dimension.
1:end is the same as [1,2,3,...,end]
end:-1:1 is the opposite. It is the same as [end, end-1, end-2, end-3, ... 1].

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### その他の回答 (1 件)

Walter Roberson 2011 年 10 月 27 日
Cycling the loop backwards
for K=size(A,1):-1:1
can be quite efficient, and seldom requires more memory than working forwards (indeed, moving forwards can be less efficient if you have not preallocated a matrix.)

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