Summing specific values in arrays

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Max
Max 2015-10-9
评论： Stephen23 2015-10-10
If i have something for example say
2,3
4,5
2,5
1,5
2,1
1,1
how would i sum up all the values on the second column for values of the first column. For example for all the numbers in the first column that have a value 2, I want to some up all the values the second column corresponding to 2. i.e
2,3
2,5
2,1
would be the only things considered and the answer would be (3+5+1) which is 9. These are all the values to the right of 2. I hope this makes sense, and any help would be appreciated.

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采纳的回答

Thorsten
Thorsten 2015-10-9
编辑：Thorsten 2015-10-9
If you don't need all values you can use the one-liner
s = sum(X(X(:,1)==2, 2));
X(:,1)==2 results in a logical array that has 1's at all the rows that are equal to 2, else zero. If you use this as an index to X(.,2), the values in the 2nd column are selected for those rows in column 1 that equal 2. These values are summed, and you got your desired result.

更多回答（2 个）

Stalin Samuel
Stalin Samuel 2015-10-9
a = [2,3;4,5;2,5;1,5;2,1;1,1]
prompt = 'Enter any no fom first column ';
x = input(prompt)
r = find(a(1:end,1)==x)
rslt = sum(a(r,2))
  1 个评论
Max
Max 2015-10-9
Sorry I'm quite new to matlab I dont really understand the answer.

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Stephen23
Stephen23 2015-10-9
编辑：Stephen23 2015-10-9
This is exactly what accumarray is for:
>> X = [2,3;4,5;2,5;1,5;2,1;1,1]
X =
2 3
4 5
2 5
1 5
2 1
1 1
>> accumarray(X(:,1),X(:,2))
ans =
6
9
0
5
where the first row corresponds to the 1's, the second row the 2's, etc. So it performs all of the sums at once, without any extra code at all! And lets compare to your example, the 2's = 3+5+1 = 9, so it gives the result that you looking for.
Note that the first input to accumarray must be integer indices, so if these are negative or floating point values then you can use unique's third output to generate a vector of integer indices to use for this input.
  2 个评论
Max
Max 2015-10-9
How would I display the answer as just 9 and not in column form :/?
Stephen23
Stephen23 2015-10-10
>> Y = accumarray(X(:,1),X(:,2));
>> Y(2)
ans = 9

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