Summing specific values in arrays

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.

Answer 1

[Thorsten]

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.

Answer 2

[Stalin Samuel]

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))

[Max]

Sorry I'm quite new to matlab I dont really understand the answer.

Answer 3

[Stephen23]

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.

[Max]

How would I display the answer as just 9 and not in column form :/?

[Stephen23]

>> Y = accumarray(X(:,1),X(:,2));
>> Y(2)
ans =   9