Sum of rows with whose row numbers are specified by another array

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Saeid 2019-5-31

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https://ww2.mathworks.cn/matlabcentral/answers/464917-sum-of-rows-with-whose-row-numbers-are-specified-by-another-array

评论： Saeid 2019-6-10

采纳的回答： Stephen23

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I have an array of the form:

TTX =
   9     2     7
  10    10     1
   2    10     9
  10     5    10
   7     9     7
   1     2     8
   3     5     8
   6    10     4
  10     8     7
  10    10     2

I want to add the elements of rows 1:3, then add the elements of the rows 4:5, then 6:end.

How can I do that?

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Stephen23 2019-5-31

@Saeid: you just editted your question, removed the original examples, and asked something different from what you originally asked. This discourages people from helping you.

I already answered your original question, but now my answer makes no sense because you removed all of the relevant information.

In future please use comments for adding new information.

Saeid 2019-6-1

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采纳的回答

Stephen23 2019-5-31

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https://ww2.mathworks.cn/matlabcentral/answers/464917-sum-of-rows-with-whose-row-numbers-are-specified-by-another-array#answer_377327

编辑：Stephen23 2019-5-31

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From R2015b you can use splitapply:

>> TTX = [...
   9     2     7
  10    10     1
   2    10     9
  10     5    10
   7     9     7
   1     2     8
   3     5     8
   6    10     4
  10     8     7
  10    10     2];
>> [U,~,X] = unique(TTX(:,1));
>> Y = splitapply(@(x)sum(x,1),TTX(:,2:4),X);
>> M = [U,Y]
M =
  23    20    20
  19    10    14
  18    21    16
   8    20    13

EDIT: this matches the original question's output explanation and example.

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Stephen23 2019-5-31

编辑：Stephen23 2019-5-31

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@Saeid: read about repelem, use it to generate an index vector. Apply that indexing vector to the column indices of your array:

>> R = [3,5,2];
>> X = repelem(1:numel(R),R);
>> M = randi(9,5,3)
M =
     2     6     7
     4     1     7
     9     8     4
     8     9     6
     9     7     2
>> M(:,X)
ans =
     2     2     2     6     6     6     6     6     7     7
     4     4     4     1     1     1     1     1     7     7
     9     9     9     8     8     8     8     8     4     4
     8     8     8     9     9     9     9     9     6     6
     9     9     9     7     7     7     7     7     2     2