How do I count the number of matching pairs in 2 vectors and use that as colors in a contour plot?

Question

Jennifer 2011-3-30

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评论： Hassan 2018-3-15

采纳的回答： Walter Roberson

在 MATLAB Online 中打开

I have 2 vectors A and B. Both are column vectors and both contain 1000 values.

For example:

A=[1,2,5,1,6,2,8,2,9];
B=[3,4,2,3,7,4,5,4,8];

I wish to find out how many times that (A,B) match so that I can create a contour plot of (A,B) which is colour coded in accordance to the number of matching occurrences:

e.g. (1,3) = 2 occurrences (2,4) = 3 occurrences (5,2) = 1 Occurrence ....etc

I know that I need to create a for loop and I was hoping the unique command would work but as yet I have been unable to generate a working code.

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Answer 1

Walter Roberson 2011-3-30

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在 MATLAB Online 中打开

accumarray([A(:), B(:)])

Provided that the values are positive integers. Otherwise,

[bA, mA, nA] = unique(A);
[bB, mB, nB] = unique(B);
accumarray([nA(:), nB(:)])

Then entry (I,J) counts the match bA(I) to bB(J)

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Jennifer 2011-3-31

Brilliant - thank you.

This works with the example I gave above, however with my actual vectors (more than 1500 values) I get the following error message:

??? Error using ==> accumarray

First input SUBS must contain positive integer subscripts.

I have tried rounding the values and also a few other tricks and can't seem to get it to work. It is rather frustrating.

Do you know what might be causing the problem? I can send you the vectors if needed.

Any advice welcome and thanks in advance

Teja Muppirala 2011-3-31

What Walter wrote should work. You have to feed in nA and nB into accumarray. Are you sure you wrote it correctly?

[bA, mA, nA] = unique(A);

[bB, mB, nB] = unique(B);

accumarray([nA(:), nB(:)],1)

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Answer 2

Todd Flanagan 2011-3-31

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https://ww2.mathworks.cn/matlabcentral/answers/4363-how-do-i-count-the-number-of-matching-pairs-in-2-vectors-and-use-that-as-colors-in-a-contour-plot#answer_6196

在 MATLAB Online 中打开

A = [1,2,5,1,6,2,8,2,9];
B = [3,4,2,3,7,4,5,4,8];

c = accumarray([A(:), B(:)])

c =

   0     2     0     0     0     0     0
   0     0     3     0     0     0     0
   0     0     0     0     0     0     0
   0     0     0     0     0     0     0
   1     0     0     0     0     0     0
   0     0     0     0     0     1     0
   0     0     0     0     0     0     0
   0     0     0     1     0     0     0
   0     0     0     0     0     0     1

looking at unique rows

pairs = unique([A(:), B(:)], 'rows')
pairs =
   3
   4
   2
   7
   5
   8

To find the count for row 1

>> c(1,3)

ans =

2

If your rows don't contain positive integers, you can still do the problem by converting it to indeces using unique.

Unique can create a vector of the unique elements and also a vector of indeces to reconstruct the original vector based on these values. For example:

>> [bA, mA, nA] = unique(A)

bA =
   1     2     5     6     8     9
mA =
   4     8     3     5     7     9
nA =
   1     2     3     1     4     2     5     2     6

bA(nA) gives you back the original vector but lets you operate in terms of something (indeces) that ensure you are using positive integers.

>> bA(nA)
ans =
    1     2     5     1     6     2     8     2     9

Now to use accumarray:

[bA, mA, nA] = unique(A);
[bB, mB, nB] = unique(B);
accumarray([nA(:), nB(:)],1)
ans =
     0     2     0     0     0     0
     0     0     3     0     0     0
     1     0     0     0     0     0
     0     0     0     0     1     0
     0     0     0     1     0     0
     0     0     0     0     0     1

To find the rows again,

your pairs are now in terms of indeces:

>> pairs = unique([nA(:) nB(:)], 'rows')

pairs =

So, c(1,2) = 2

and the original pair is

>> [bA(1) bB(2)]

ans =

1 3

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Carlos Goncalves Moreira 2018-1-11

Hi Jennifer,

Did you find a working code to plot these results?

Thanks a lot

Hassan 2018-3-15

hi, you can use digraph

G=digraph(accumarray([nA(:), nB(:)],1));

Best, HM

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Answer 3

Hassan 2018-3-15

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Hi all, The suggested method accumarray([nA(:), nB(:)],1) is good to have counts of pairs in 2D representation. What if I interested in a square matrix, I mean full representation of length n by n?

let's say I have two vectors x=1:10, and y=1:8. accumarray([nA(:), nB(:)],1) will result in 10 by 8 matrix. How can I make the result 10 by 10 with two lines of zeros?

Thanks a lot, HM