What is the difference between [R,~]=size(M) and r0=Size(M,1)

Question

Soul Free 2018-9-1

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/417119-what-is-the-difference-between-r-size-m-and-r0-size-m-1

评论： Walter Roberson 2018-12-22

I know the second one returns the length of the 1st dimension which equals to number of rows and the first one ignores the columns as an output, but is there any difference if M has 2 dimensions?

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

ahmed nebli 2018-9-1

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/417119-what-is-the-difference-between-r-size-m-and-r0-size-m-1#answer_335026

the output of the first one is 2 numbers, each number representing the size of the dimension, and the output of the second one will be just one number which is the number of rows.

3 个评论
显示 1更早的评论隐藏 1更早的评论

ahmed nebli 2018-9-1

no, i just tested it its the same! there is no diffrence

Walter Roberson 2018-12-22

在 MATLAB Online 中打开

When you call size() passing in only an array, then the output depends upon the number of outputs you request.

If you request only one output, then the output is a vector of the dimensions, with at least two elements being returned, and more if needed, ending with the last dimension that is not 1.

If you request more than one output, then the value for each output except the last is the length of the corresponding dimension (e.g., third output corresponds to third dimension); but the last output will be the product of all of the remaining sizes. The rule is that the product of all of the returned elements will equal the number of elements in the array.

The case

[W,~]=size(A)

follows that rule about multiple outputs. It is equivalent to doing

 [W, AnInternalVariableNameThatIsNotUsed] = size(A)
 clear AnInternalVariableNameThatIsNotUsed

so W will store the length of the first dimension, and AnInternalVariableNameThatIsNotUsed would store the product of the length of all of the other dimensions, and then you would throw away AnInternalVariableNameThatIsNotUsed, leaving you with W storing the length of the first dimension.

When you call size() passing in an array and also a positive integer, then MATLAB extracts only that particular dimension and returns it. In the case of size(A,1) that would be only the first dimension.

So what difference is there between

[W,~] = size(A);

compared to

W = size(A,1);

The answer is that in both cases, W will end up storing the same value, but that the first of those will take longer to execute, since it has to extract the dimensions and multiply them out and then throw away that calculated value.

请先登录，再进行评论。