Custom binary search in vector - c++

Question

Suppose I have a vector<int> myVec. Let there be n elements in it. I know that these elements are in sorted order(ascending) and also that they are unique. Let n = 10 and myVec be {2, 4, 6, 8, 10, 12, 14, 16, 18, 20}. I'm given l and r such that 0<=l<=r<=n-1. Now i want to search an element val in the subvector that is defined by the bounds l and rsuch that
if val is found return val
if val is not found then return (if possible) a value in the subvector which is just smaller than val.
Return false(or -1 maybe) if either of the above is not possible.
In the above case if if l = 3 and r = 5. The subvector is {8, 10, 12}. If val = 8 return 8. If val = 7 return false (or -1). If val = 9 return 8.
How do I implement this. I want order comparable to binary search. Also, is it possible to use std::binary_search() present under algorithm header file.

Answer 1

something like this?
int search(int l, int r, int value) {
if (l > vec.Size() || r > vec.Size() || l > r) return -1;
for (int i = r; i >= l; --i) {
int v = vector[i];
if (v <= value) return v;
}
return -1;
}
or does it need to be binary?
int BinarySearch(int l, int r, int value) {
return PrivateBinarySearch(l, r, (l+r)/2, value);
}
int PrivateBinarySearch(int l, int r, int index, int value) {
if (vector[index] == value) return value;
else if (vector[index] > value) {
if (index == l) return -1;
else if (index == r) return -1;
else return PrivateBinarySearch(l, index, (index-1+l)/2, value);
}
else { // vector[index] < value
if (index == l) return vector[index];
else if (index == r) return vector[index];
else return PrivateBinarySearch(index, r, (index+1+r)/2, value);
}
Hope this helps

Answer 2

This should work for you and is pretty extensible and flexible:
template<typename T>
typename vector<T>::const_iterator
find_or_under (typename vector<T>::const_iterator start, typename vector<T>::const_iterator end,
const T& val)
{
auto el = std::lower_bound(start, end, val);
//if not found, propagate
if (el == end)
return el;
//if it's equal, just return the iterator
if ((*el) == val)
return el;
//if there is no value of an equal or smaller size, return the end
if (el == start)
return end;
//otherwise, return the previous element
return el-1;
}
//Functor representing the search
struct CustomSearch
{
//Create a searcher from a subrange
CustomSearch (const vector<int> &v, size_t l, size_t r)
{
start = std::lower_bound(std::begin(v), std::end(v), l);
end = find_or_under(start, std::end(v), r) + 1;
}
//Calling the searcher
//Returns this->end on not found
auto operator() (int val)
{
return find_or_under(start, end, val);
}
vector<int>::const_iterator start;
vector<int>::const_iterator end;
};
int main() {
vector<int> v = {2, 4, 6, 8, 10, 12, 14, 16, 18, 20};
CustomSearch searcher {v, 3, 8};
cout << *searcher(6);
}

Answer 3

Using traditional binary search with minor modification:
#include <iostream>
#include <vector>
int search(const std::vector<int> &vec, int l, int r, int val)
{
int pivot, xl = l, xr = r, mid;
do {
/* Not exact match, check if the closest lower match is in the
* subvector. */
if (xl > xr) {
return xr >= l ? vec[xr]: -1;
}
mid = (xl + xr) / 2;
pivot = vec[mid];
if (val < pivot) {
xr = mid - 1;
} else if (val > pivot) {
xl = mid + 1;
} else if (val == pivot) {
return val;
}
} while (true);
}
int main()
{
std::vector<int> myVec(10);
myVec[0] = 2;
myVec[1] = 4;
myVec[2] = 6;
myVec[3] = 8;
myVec[4] = 10;
myVec[5] = 12;
myVec[6] = 14;
myVec[7] = 16;
myVec[8] = 18;
myVec[9] = 20;
int l = 3, r = 5;
std::cout << "search(3, 5, 8) = " << search(myVec, 3, 5, 8) << std::endl;
std::cout << "search(3, 5, 7) = " << search(myVec, 3, 5, 7) << std::endl;
std::cout << "search(3, 5, 9) = " << search(myVec, 3, 5, 9) << std::endl;
return 0;
}
enter code here