CODEWITHSUNDEEP
matlabvector
jjepsuomi
jjepsuomi

Reputation: 4373

How to find all index pairs of unequal elements in vector (Matlab)

Lets say I have the following vector in Matlab:

V = [4, 5, 5, 7];

How can I list (in a n-by-2 matrix for example) all the index pairs corresponding to unequal elements in the vector. For example for this particular vector the index pairs would be:

index pairs (1, 2) and (1, 3) corresponding to element pair (4,5)

index pair (1, 4) corresponding to element pair (4,7)

index pairs (2, 4)  and (3, 4) corresponding to element pair (5,7)

The reason I need this is because I have a cost-function which takes a vector such as V as input and produces a cost-value.

I want to see how does the random swapping of two differing elements in the vector affect the cost value (using this for steepest descent hill climbing).

The order of the index pairs doesn't also matter. For my purposes (1,2) is the same as (2,1).

For example if my cost-function was evalCost(), then I could have V = [4, 5, 5, 7] and

evalCost(V) = 14

whereas for W = [4, 7, 5, 5] the cost could be:

evalCost(W) = 10

How to get the list of "swapping" pair indexes in Matlab. Hope my question is clear =)

Upvotes: 2

Views: 855

Answers (2)

Luis Mendo
Luis Mendo

Reputation: 112689

Use bsxfun and then the two-ouput version of find to get the pairs. triu is applied to the output of bsxfun to consider only one of the two possible orders.

[ii jj] = find(triu(bsxfun(@ne, V, V.')));
pairs = [ii jj];

Upvotes: 1

Daniel
Daniel

Reputation: 36710

I don't understand the cost function part, but the first part is simple:

[a,b]=unique(V)
C = combnk(b,2)

C contains the indices, and V(C) the values:

C = combnk(b,2)

C =

     1     2
     1     4
     2     4

V(C)

ans =

     4     5
     4     7
     5     7

Upvotes: 3

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