need suggestion on data structure for this data

[...]
My current implementation is a for-loop that goes through all A's and
checks if the B value is the query, and if yes, return the
corresponding A and C. But this is very inefficient.
Could anyone please suggest me a better data structure for doing this,
better be a built-in one?

My data is organized like this, where A always goes from 1 to some
number N:
A 1 2 3 ...
B 53,26 42,18 15,86 ...
C 59 43 31 ...
>
>
I often need to look up in this way:
Given B=18, what is the corresponding value of A and C? (Answer is 2
and 43).
My current implementation is a for-loop that goes through all A's and
checks if the B value is the query, and if yes, return the
corresponding A and C. But this is very inefficient.
Could anyone please suggest me a better data structure for doing this,
better be a built-in one?
Observe that B always appears in pairs, and when I look up, I may use
either value within the pair.

bahoo wrote:>
Are the values of B unique? I assume they are, otherwise even your
brute-force algorithm seems to have problems.
>
The usual way to address something like this is to create an index. In
.NET, the Dictionary<generic class would work well for this. Using
the values from B as your index, and the corresponding column number (A)
as your value.
>
You would have to scan through all of the data once, adding two new
entries to the Dictionary for each column (one for each value of B).
Then given a value for B to look up, you would simply use that value as
the index for the Dictionary<>, which would return your A value, which
you could then use to also get the C value.
>
There are lots of other ways to implement an index; don't feel you have
to use Dictionary<>. It just happens to be relatively convenient. An
alternative might be to maintain an array sorted based on the B values,
and containing a data structure that includes the A value. Then a
binary search would quickly find the right array element based on the B
value, which would then yield the A value.
>
Any index will, of course, improve performance at the cost of memory
usage. You don't say how many columns of data you might have, but if
it's small, your brute-force iterative method might well be just fine.
>
Pete

bahoo wrote:>
Are the values of B unique? I assume they are, otherwise even your
brute-force algorithm seems to have problems.
>
The usual way to address something like this is to create an index. In
.NET, the Dictionary<generic class would work well for this. Using
the values from B as your index, and the corresponding column number (A)
as your value.
>
You would have to scan through all of the data once, adding two new
entries to the Dictionary for each column (one for each value of B).
Then given a value for B to look up, you would simply use that value as
the index for the Dictionary<>, which would return your A value, which
you could then use to also get the C value.
>
There are lots of other ways to implement an index; don't feel you have
to use Dictionary<>. It just happens to be relatively convenient. An
alternative might be to maintain an array sorted based on the B values,
and containing a data structure that includes the A value. Then a
binary search would quickly find the right array element based on the B
value, which would then yield the A value.
>
Any index will, of course, improve performance at the cost of memory
usage. You don't say how many columns of data you might have, but if
it's small, your brute-force iterative method might well be just fine.
>
Pete

Thanks Pete, but sorry that I didn't mention the B values are NOT
unique: different A's can have the same B value. When I do look up, I
wish to retrieve all these A's and C's that correspond to the query
B.
What I did in my brute-force approach is to scan through all the B's,
and every time I find a match, I record the A's and C's in a list.
If "Dictionary" doesn't apply here, what would you suggest next?

[...]
What I did in my brute-force approach is to scan through all the B's,
and every time I find a match, I record the A's and C's in a list.
If "Dictionary" doesn't apply here, what would you suggest next?

You can use a Dictionary<int, List<...>where you put the items in the
lists.