Basic question about initializing variables

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Ahmed Hossam
Ahmed Hossam 2017 年 4 月 20 日
編集済み: Ahmed Hossam 2017 年 4 月 20 日
What's the difference between
a = [ 1 2 3 ];
and
a = zeros(1,3);
a = [ 1 2 3 ];
Performance? Logic? I'm just interested to know. Thanks for your help!

採用された回答

Jan
Jan 2017 年 4 月 20 日
In
a = zeros(1,3);
a = [ 1 2 3 ];
Matlab reserves memory for 3 doubles and fills then with zeros in the first line. In the second line this memory is freed and a new vector is created. This needs about the double work compared to creating [1, 2, 3] directly and in consequence is a waste of time only.
This is not a pre-allocation. A pre-allocation would mean, that the reserved memory is re-used later:
a = zeros(1, 3);
a(1) = 1;
a(2) = 2;
a(3) = 3;
or
a = zeros(1, 3);
a(:) = 1:3;
The "(:)" is essential here, because it tells Matlab to overwrite the contents of the formerly existing memory.
  6 件のコメント
Ahmed Hossam
Ahmed Hossam 2017 年 4 月 20 日
Thank you for your help
Ahmed Hossam
Ahmed Hossam 2017 年 4 月 20 日
編集済み: Ahmed Hossam 2017 年 4 月 20 日
@ Stephen Cobeldick:
Ok, I understand both topics now.
First topic:
Just deklare and initialize a variable in one line! (beautiful feature!!)
Second topic:
Don't extend the memory of a variable in a loop!

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