# Making Pairs of Indices in Matrix- Pairwise Function

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Andrea Lucia Alfonso on 16 Jun 2020
Commented: Matt J on 16 Jun 2020
Hello,
I want to make a pairwise function in matlab simialar to how it is done in python. Or is there one already in matlab?
Overall, I am trying to return a list of pairs of indices in my matrix and then take the distance between each pair.
Any input would be greatly appreciated.
Thank you!

Matt J on 16 Jun 2020
Edited: Matt J on 16 Jun 2020
It depends what Matlab data-type you consider to be the equivalent of a Python tuple. I tend to think of cell arrays as the equivalent of a tuple because cells can store sequences of mixed types, but matrix form is generally the form in Matlab that will support most mathematical operations you might want to do. Below, I generate a set of pairs in both matrix and cell array form.
n=4;
pairMatrix=[0:n-1;1:n].';
pairCell=num2cell(pairMatrix,2)
Overall, I am trying to return a list of pairs of indices in my matrix and then take the distance between each pair.
You mean, the Euclidean distances when each pair is viewed as a 2D coordinate? If you have the Statistics Toolbox, then this can be done using pdist2,
>> pdist2(pairMatrix,pairMatrix)
ans =
0 1.4142 2.8284 4.2426
1.4142 0 1.4142 2.8284
2.8284 1.4142 0 1.4142
4.2426 2.8284 1.4142 0

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Matt J on 16 Jun 2020
Here's one way to do that:
row = double([0,0,1,0,0,1,0,0,0,0,1]);
loc=find(row); %locations of 1's
D=diff(loc); %differences
j=0;
for i=loc(1):loc(end)
if row(i)==1,
j=j+1;
else
row(i)=D(j);
end
end
row,
Here's another, without loops:
row = double([0,0,1,0,0,1,0,0,0,0,1]);
loc=find(row); %locations of 1's
D=diff(loc); %differences
c=cumsum(row); %cumulative sum along row
mask=c>=1 & c<=numel(D) & ~row; %binary mask of the 0's that are sandwiched by 1's
Andrea Lucia Alfonso on 16 Jun 2020
Thank you so much! This definitley helped!
Matt J on 16 Jun 2020
You're quite welcome, but please Accept-click the answer to signify that you are satisfied with the solution.

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