Binary search on strings instead of numbers - c++

Question

I am reading up on binary search on an array of numbers and I believe I know how it works and how to implement it. Now I need to know how to do binary search on an array of strings ? I know binary search requires the array to be sorted already. suppose I have an array of strings already sorted how do I implement binary search on it ? I know if it was an array of numbers I would go to the middle index of the array and determin if the required search no is on the left or the right and do that recursively. How would i do that for strings?

Answer 1

As long as the concept of "it is equal to" and "it is less than" are defined for the type you are working on, you can implement the algorithm. It does not matter if the values are numbers, letters, or custom objects. The following example demonstrates this concept:
template<typename Iterator>
Iterator search(Iterator initial, Iterator final, const typename Iterator::value_type& value) {
if(value < *initial) {
// bail out immediately
return final;
}
while(initial != final) {
auto mid = std::next(initial, std::distance(initial, final) / 2);
if(*mid == value) {
return mid;
} else if(*mid < value) {
initial = std::next(mid);
} else {
final = std::prev(mid);
}
}
return final;
}
As long as the operations *mid == value and *mid < value are defined, I can search in a container of any type (another requirement is that I must be able to randomly access my iterator).
This is by no means a full answer, and there are many more details involved, but hopefully you get the idea.
Full sample program:
#include <vector>
#include <list>
#include <iostream>
template<typename Iterator>
Iterator search(Iterator initial, Iterator final, const typename Iterator::value_type& value) {
if(value < *initial) {
// bail out immediately
return final;
}
while(initial != final) {
auto mid = std::next(initial, std::distance(initial, final) / 2);
if(*mid == value) {
return mid;
} else if(*mid < value) {
initial = std::next(mid);
} else {
final = std::prev(mid);
}
}
return final;
}
int main() {
{
std::vector<int> v {1, 2, 3, 4, 5};
auto it = search(v.begin(), v.end(), 3);
if(it == v.end()) {
std::cout << "Not Found!" << std::endl;
} else {
std::cout << "Found in position: " << std::distance(v.begin(), it)
<< " (value is : " << *it << ")" << std::endl;
}
}
{
std::vector<char> v {'a', 'b', 'c', 'd', 'e'};
auto it = search(v.begin(), v.end(), 'd');
if(it == v.end()) {
std::cout << "Not Found!" << std::endl;
} else {
std::cout << "Found in position: " << std::distance(v.begin(), it)
<< " (value is : " << *it << ")" << std::endl;
}
}
{
std::list<float> v {-1, 0, 1, 2, 3, 4};
auto it = search(v.begin(), v.end(), 0);
if(it == v.end()) {
std::cout << "Not Found!" << std::endl;
} else {
std::cout << "Found in position: " << std::distance(v.begin(), it)
<< " (value is : " << *it << ")" << std::endl;
}
}
{
std::vector<char> v {'a', 'b', 'c', 'd', 'e'};
auto it = search(v.begin(), v.end(), 'f');
if(it == v.end()) {
std::cout << "Not Found!" << std::endl;
} else {
std::cout << "Found in position: " << std::distance(v.begin(), it)
<< " (value is : " << *it << ")" << std::endl;
}
}
}
Sample Run:
Found in position: 2 (value is : 3)
Found in position: 3 (value is : d)
Found in position: 1 (value is : 0)
Not Found!

Answer 2

A naive approach would be to assign a unique value to each letter (if you're doing English, that's easy; only 26 values), and compare the values of the first letter of each string. If the letters are the same, then you compare the second letter, and so on.

Answer 3

If you use array of std::string, it just the same since you have all compare operators.
so you just need to replace the type of the array and you can do the search as is for numbers.

Answer 4

Exact same way. If you use std::string, it already has operator== and operator < which are all you need for binary search. If you just have character pointers, you can use strcmp, where a negative value is less than and 0 is equal.