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From: Namo Namo on 15 Jul 2010 22:40
I have two vectors -- one contains the values, and the other is the corresponding index. For example

A = [0.1 0.3 0.2 0.5 .... ]
B = [1 3 4 2 1 ]

The have the same length. I need to select the elements in A with the same index in B, and sum them. For each index in B, the numbers of the corresponding elements in A are not the same. So I cannot just sort and reshape and sum, etc.

I can do a for loop

S = zeros(length(B),1);
for k =1:length(B)
S(k) = sum( A(B==k) );
end

But for large A and B, this takes too long. Any advice is welcome.
From: Roger Stafford on 15 Jul 2010 22:51
"Namo Namo" <wynamo(a)yahoo.com> wrote in message <i1ogq8$9j7$1(a)fred.mathworks.com>...
> I have two vectors -- one contains the values, and the other is the corresponding index. For example
>
> A = [0.1 0.3 0.2 0.5 .... ]
> B = [1 3 4 2 1 ]
>
> The have the same length. I need to select the elements in A with the same index in B, and sum them. For each index in B, the numbers of the corresponding elements in A are not the same. So I cannot just sort and reshape and sum, etc.
>
> I can do a for loop
>
> S = zeros(length(B),1);
> for k =1:length(B)
> S(k) = sum( A(B==k) );
> end
>
> But for large A and B, this takes too long. Any advice is welcome.
- - - - - - - -
S = sum(A(B));

Note: A and B don't have to be the same length for this to work. However the indices in B must all lie within the range 1:length(A).

Roger Stafford
From: Walter Roberson on 15 Jul 2010 23:04
Namo Namo wrote:
> I have two vectors -- one contains the values, and the other is the
> corresponding index. For example
>
> A = [0.1 0.3 0.2 0.5 .... ]
> B = [1 3 4 2 1 ]
>
> The have the same length. I need to select the elements in A with the
> same index in B, and sum them.

Consider using accumarray()
From: Namo Namo on 15 Jul 2010 23:09
I just tried this with the simple example

>> A = [0.1 0.3 0.2 0.5 0.2 0.9];
>> B = [1 3 4 2 1 4 ];
>> sum(A(B))
ans =
1.7000

This is not what I want. I may have made a typo in my previous post. What I want is to sum all A elements corresponding to 1 in B, 2 in B, etc. So here since there are 1 to 4 in B, I expect to get 4 numbers

sum(A(B==1)) ... sum(A(B==4))

for example, A(B==4) = [0.2 0.9], and the sum is 1.1

> - - - - - - - -
> S = sum(A(B));
>
> Note: A and B don't have to be the same length for this to work. However the indices in B must all lie within the range 1:length(A).
>
> Roger Stafford
From: Roger Stafford on 15 Jul 2010 23:09
"Roger Stafford" <ellieandrogerxyzzy(a)mindspring.com.invalid> wrote in message <i1ohen$jid$1(a)fred.mathworks.com>...
> "Namo Namo" <wynamo(a)yahoo.com> wrote in message <i1ogq8$9j7$1(a)fred.mathworks.com>...
> > I have two vectors -- one contains the values, and the other is the corresponding index. For example
> >
> > A = [0.1 0.3 0.2 0.5 .... ]
> > B = [1 3 4 2 1 ]
> >
> > The have the same length. I need to select the elements in A with the same index in B, and sum them. For each index in B, the numbers of the corresponding elements in A are not the same. So I cannot just sort and reshape and sum, etc.
> >
> > I can do a for loop
> >
> > S = zeros(length(B),1);
> > for k =1:length(B)
> > S(k) = sum( A(B==k) );
> > end
> >
> > But for large A and B, this takes too long. Any advice is welcome.
> - - - - - - - -
> S = sum(A(B));
>
> Note: A and B don't have to be the same length for this to work. However the indices in B must all lie within the range 1:length(A).
>
> Roger Stafford
- - - - - - -
I didn't read your code carefully enough, Namo. Forget what I sent earlier. The following might do the job:

S = accumarray(B,A(B));

Roger Stafford
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