Testing for identical numbers in matrix columns

3 次查看（过去 30 天）
Konstantin
Konstantin 2011-3-3
评论： MICHAEL MONT-ETON 2020-7-22
Hello!
I'm not a stupid, I just want to learn to use MATLAB effectively.
I have a matrix. In my work it is 64x64, for example let's see this one
A =
1 2 3 4
2 3 3 4
6 7 2 4
I want to now, in which columns all numbers are equal. I know how to do it for one column
A = [1,2,3,4; 2,3,3,4; 6,7,2,4];
REZ(1) = all((A(1,1)==A(:,1)));
Of course I can do this operation in cycle, but i think there is method to do it in one line.
If I run
>> X=(1:4);
>> REZ_2(X)=((A(1,X)==(A(:,X))));
It doesn't work, with error
??? Error using ==> eq
Matrix dimensions must agree.
How to solve this problem elegant?
  2 个评论
Andrew Newell
Andrew Newell 2011-3-3
Konstantin, it's not a stupid question, and it might even pick up a vote or two if you change the title to a more descriptive one (for example, "Which matrix columns have all numbers equal?").

请先登录，再进行评论。

请先登录，再回答此问题。

采纳的回答

Matt Tearle
Matt Tearle 2011-3-3
all(~diff(A))
Like Andrew's idea, but a bit neater(?). The not (~) converts the differences to logicals (0 difference -> true). Then all sees if all the differences in a column are 0.
Just for fun, here's another possibility:
all(bsxfun(@eq,A,A(1,:)))
(This actually has one advantage, in that it would work even if A has only one row!)
  2 个评论
Andrew Newell
Andrew Newell 2011-3-3
I prefer the ALL version because it's not very interesting to say that each element is equal to itself.

请先登录，再进行评论。

更多回答（2 个）

Jos (10584)
Jos (10584) 2011-3-3
Nothing shorter than:
~std(A)
  4 个评论
显示 2更早的评论
Oisín Moran
Oisín Moran 2018-1-17
编辑：Oisín Moran 2018-1-17
I know this was 7 years ago but for anyone coming across this now it is not guaranteed to work. For example:
~std(0.9*ones(6,1))
Returns
ans =
logical
1
Whereas
~std(0.9*ones(7,1))
Returns
ans =
logical
0
Jan
Jan 2018-1-17
@Oisín Moran: You are right:
std(0.9*ones(7,1))
>> 1.1992e-16
A rounding problem. This is not reliable also:
std(A) < 10*eps
The limit must reflect the size of the inputs and the summation to get mean(a), which is required for std is not stable.

请先登录，再进行评论。

Andrew Newell
Andrew Newell 2011-3-3
Here is one way:
find(~sum(abs(diff(A))))
The idea is that diff takes the differences between adjacent terms in a column, and if all the elements are the same then sum(abs(diff(A)))=0 (the abs is needed in case positive and negative differences cancel out). Then ~sum(abs(diff(A)))=1 only for columns in which all the elements are equal.
  2 个评论
MICHAEL MONT-ETON
MICHAEL MONT-ETON 2020-7-22
Thanks for this solution. I plan on using it in my research into inverse solutions for the groundwater equation- a variant of the Laplace equation. It will be used to determine if a particular row or column in the input array is homogeneous. I will credit Mr.Newell in the code, and in publication.

请先登录，再进行评论。

类别

在 Help Center 和 File Exchange 中查找有关 Function Creation 的更多信息

标签

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by