Why is 1/ones(3,1) OK while 1/ones(1,3) is not OK?

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Yuki Koyama
Yuki Koyama 2019-9-19
编辑： David Goodmanson 2019-9-20
When I tried the code
1/ones(1,3)
matlab said this is invalid because the dimension is not matched.
Then I tried the code
1/ones(3,1)
this returns the result
ans =
1 0 0
I don't know why the ans is like this.
  1 个评论
Stephen23
Stephen23 2019-9-19
编辑：Stephen23 2019-9-19
"I don't know why the ans is like this."
Did you read the mrdivide documentation, to know what you are actually calculating?:
https://www.mathworks.com/help/matlab/ref/mrdivide.html

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David Goodmanson
David Goodmanson 2019-9-19
编辑：David Goodmanson 2019-9-20
Hi Yuki,
In the expression x = 1/ones(3,1) multiplying both sides on the right by ones(3,1) gives
x*ones(3,1) = 1.
The row vector x = [1 0 0] fills the bill, since
[1 0 0] x [1
1 = 1.
1]
Note that the answer is not unique, since
x = [0 1 0], x = [1/3 1/3 1/3] etc. all work just as well.
As to why ones(1,3) errors out, in that case you would have
x*ones(1,3) = 1.
If you review the rule for sizes of rows and columns in matrix multiplication, i.e. (mxn) x (nxq) = mxq, you will find that the eqn above does not work.

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