MATLAB one-liners

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Andrew Newell
Andrew Newell 2011-3-3
One of the joys of using MATLAB is that it has a stock of matrix functions like diff, sort, all, and so on, that can be combined in all sorts of interesting ways. For example, in a recent question, the challenge was to find a compact code to determine which columns of a matrix A have all elements equal. Matt Tearle came up with this nifty answer:
all(~diff(A))
What are your favorite one-line MATLAB expressions?

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采纳的回答

Walter Roberson
Walter Roberson 2011-3-3
cell2mat(arrayfun(@(K) accumarray(C, F(:,K), [], @mean), 1:size(F,2), 'Uniform', 0))
In response to a cssm question:
I have a feature matrix, F(m, n) and a cluster vector, C(m, 1).
Now I want to get the mean of feature in F according to C. Make
it simple as below:
F = [2 5; 3 7; 8 4]
C = [2; 1; 2]
output should be [3 7; (2+8)/2 (5+4)/2]
=[3 7; 5 4.5]
  2 个评论
Matt Fig
Matt Fig 2011-3-3
ARRAYFUN, and ACUMARRAY? About all that is missing is to somehow work BSXFUN in there too!
Andrew Newell
Andrew Newell 2011-3-28
Even though BSXFUN is missing, I decided to accept this one. Very clever, @Walter!

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更多回答（9 个）

Matt Tearle
Matt Tearle 2011-3-3
I'm a huge fan of logical indexing. Expressions like
mean(frogs(wombats > 42))
rock my world.
  3 个评论
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Sean de Wolski
Sean de Wolski 2011-3-3
I use the function 'keep' from the FEX which performs the inverse of clear.
I also use the variables 'in', 'out' a lot. So the other day I typed:
'keep out'
which kind of made my day.
Andrew Newell
Andrew Newell 2011-3-3
I like it!

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Sean de Wolski
Sean de Wolski 2011-3-3
Here's another from a thread today:
Given a connected components analysis (bwconncomp) and some criteria for objects to meet: remove objects that don't meet that criteria from your binary image:
I(cell2mat(CC.PixelIdxList(~idx)')) = false;
  1 个评论
Sean de Wolski
Sean de Wolski 2011-3-3
idx can usually be a one line expression from cellfun making this a super-awesome-long-one-liner.

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Matt Fig
Matt Fig 2011-3-3
Here is a good one. After already writing a solution to this Question, I stumbled upon this:
groups = mat2cell(A,diff([0;find(diff(A) ~= 1);length(A)]),1);
Pretty Slick.
  6 个评论
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Matt Tearle
Matt Tearle 2011-3-3
Matt Fig: "LI is tops, but not always necessary or useful"
An unbeliever! Persecute! Kill the heretic!
Walter Roberson
Walter Roberson 2011-3-3
Ah, I found my copy, and it was _not_ a 1 liner. I had used
b=diff(a); %find differences
idx = find([b 2]>1); %find their indexes
cel = mat2cell(a, 1, [idx(1) diff(idx)]); %break up the matrix

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Matt Tearle
Matt Tearle 2011-3-3
Inspired by something I'm working on right now & your comment to my previous answer...
If you have an n-by-1 structure array people with a field suck (which contains a scalar for each struct element), and you want to find the average:
mean([people.suck])
Extract multiple elements, concatenate, apply function. All in one line.
  1 个评论
Andrew Newell
Andrew Newell 2011-3-3
I like compact expressions with social commentary.

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Andrew Newell
Andrew Newell 2011-3-3
Turn a structure array S into a cell array with the names of the fields in the first column:
C = horzcat(fieldnames(S), squeeze(struct2cell(S)))
Andrew Newell
Andrew Newell 2011-3-6
Here is a real beauty from a comment by Tim Davis on a guest blog for Loren Shure. Suppose you have three 2D vectors p1, p2 and p3, and you want to know if they are collinear. The shortest solution also happens to be numerically the most reliable:
rank ([p2-p1 ; p3-p1]) < 2
It is also easily generalized to more dimensions!
Oleg Komarov
Oleg Komarov 2011-3-3
eval('fliplr(['''' 33 33 33 33 33 76 105 118 69 32 109 39 73 ''''])')
MuAhahauHAh!!!
  3 个评论
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Oleg Komarov
Oleg Komarov 2011-3-3
You're the evilest!
the cyclist
the cyclist 2011-3-28
I used to be an admin on a chess server where "qu" could be used as a shorthand to quit out of the interface. A common prank was to tell newbies that "qu" could be used to display the "quote of the day".

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Andrew Newell
Andrew Newell 2011-3-4
Here is an interesting way of calculating n rows of Pascal's triangle:
round(expm(diag(1:n-1,-1)))
Now if we could just remove the zeros and center it on the same line!
  3 个评论
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Doug Eastman
Doug Eastman 2011-3-29
It's not pretty but just for fun, here's one way to do it:
trimmedTriangle = cell2mat(cellfun(@(x) x(1:size(num2str(expm(diag(1:n-1,-1))),2)),cellfun(@(x,y)[x y],cellfun(@(x) repmat(' ',1,x),num2cell(round(linspace(size(num2str(expm(diag(1:n-1,-1))),2)/2,0,n))'),'UniformOutput',false),regexprep(mat2cell(num2str(expm(diag(1:n-1,-1))),ones(n,1)),' 0',' '),'UniformOutput',false),'UniformOutput',false))
Andrew Newell
Andrew Newell 2011-3-29
Wow! Three nested cellfuns! This deserves a separate answer.

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Drew Weymouth
Drew Weymouth 2011-3-4
Read in an image and convert it to a grayscale, double matrix of data range 0..1
im= rgb2gray(double(imread('filename.jpg'))/255);
  1 个评论
Walter Roberson
Walter Roberson 2011-3-4
or more generally, rgb2gray(im2double(imread('filename.jpg')))
Your code would fail for images that happened to be already double or happened to be uint16.

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