efficient lookup for vector<>

[kimw]

I would like to implement a function similar to the math function from excel, that is, given a vector and a value in the vector, return the relative position of that value in that container.

However, my code

vector<double> vec;
double x = 25.0;
int pos = 0;

for (int i = 0; i!=vec.size(); ++i) {
     if (x == vec[i])
     pos = i;
}

return pos;

is not as efficient as i wanted to be. What are more efficient ways to implement this simple function?

Thank you.

[Lerner]

You could consider the find() or find_first_of() algorhithms or you could consider a map or multimap structure instead of a vector.

[kimw]

thank you. i'm new to map and could you fill me some details for me

map<double, int> counter;
vector<double> vec;
double x = 25.0;        // value to search for
int pos = 0;
int i = 0;
while (i < vec.size()) {
      ++counter[!x];
      i += 1;
}
pos = ++counter[!x];

am i on the right track with this? thanks

[iamthwee]

When you say not efficient, how bad are we talking. How many values are in your little vector?

How long does it take to find the one you want?

[Lerner]

Depending on your needs, map/multimap may not be what you want. If you want a brief overview here's a reference, though I'd recommend a good STL reference book if you don't already have one and want to start working with STL materials.

http://www.cppreference.com/cppmap/index.html

I suspect find() or find_first_of() is likely to be your best bet if the vector is unsorted. If the vector is sorted, then you may be able to use a more efficient search mechanism.

[kimw]

thank you all. i guess i'll stick to find() then.
my vectors usually have 10k+ elements and i have many of such vectors. my goal is just to maximize efficiency as much as possible...

[Ancient Dragon]

sort the vector first, then you can use a binary search algorithm