Best way to deal with index-data pairs with multiple data values per index
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Andrew
le 22 Juin 2015
Réponse apportée : Walter Roberson
le 22 Juin 2015
Hello,
I have a large vector of index-data pairs that may contain multiple data values for any given index. The way this is represented in the data is that the index vector may contain repeated indices with the corresponding location in the data vector containing the multiple data values. For example
ind = [ 1, 2, 4, 2, 5, 6, 4, 4, 9];
data = [32, 14, 36, 45, 3, 1, 78, 90, 69];
In this case index 2 has data values of 14 and 45.
Now, what I want to do is to assign these data values into a matrix using the index values, and if multiple data values exist for a given index I want to take the maximum. For example, if ind contains indices for a 3x3 matrix I want the following (working off of the previous example)
mat = zeros(3);
mat = input_data(ind,data,mat)
OUTPUT:
mat = [32,90,0;
45, 3,0;
0, 1,69]
where, as you can see, the value for index 2 is 45 since 45 is greater than 36.
What is the best way to do this quickly? My vectors are very long so I don't want to use a loop because it will decrease performance too much. I suspect I will probably need to use the unique command (and possibly sort) somehow but I can't figure out how to implement it.
Any help is greatly appreciated.
Thanks,
Andrew
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Azzi Abdelmalek
le 22 Juin 2015
ind = [1,2,4,2,5,6,4,4,9];
data = [32,14,36,45,3,1,78,90,69]
out=reshape(accumarray(ind',data',[],@(x) max(x)),3,3)
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Plus de réponses (2)
Guillaume
le 22 Juin 2015
accumarray is perfect for this:
ind = [ 1, 2, 4, 2, 5, 6, 4, 4, 9];
data = [32, 14, 36, 45, 3, 1, 78, 90, 69];
m = accumarray(ind', data', [], @max)
You can reshape m afterward (or possibly pass the size to accumarray)
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Walter Roberson
le 22 Juin 2015
mat = zeros(3);
maxdata = accumarray(ind(:), data(:), [numel(mat), 1], @max);
mat = reshape(maxdata, size(mat));
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