c-resources/CPlusPlus20ForProgrammers-m.../examples/ch12/fig12_09/fig12_09.cpp

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// fig12_09.cpp
// binarySearch function with a precondition requiring a sorted vector.
#include <algorithm>
#include <iostream>
#include <vector>
template<class T>
int binarySearch(const std::vector<T>& items, const T& key)
[[pre: items.size() > 0]]
[[pre audit: std::is_sorted(items.begin(), items.end())]] {
size_t low{0}; // low index of elements to search
size_t high{items.size() - 1}; // high index
size_t middle{(low + high + 1) / 2}; // middle element
int loc{-1}; // key's index; -1 if not found
do { // loop to search for element
// if the element is found at the middle
if (key == items[middle]) {
loc = middle; // loc is the current middle
}
else if (key < items[middle]) { // middle is too high
high = middle - 1; // eliminate the higher half
}
else { // middle element is too low
low = middle + 1; // eliminate the lower half
}
middle = (low + high + 1) / 2; // recalculate the middle
} while ((low <= high) && (loc == -1));
return loc; // return location of key
}
int main() {
// sorted vector v1 satisfies binarySearch's sorted vector precondition
std::vector v1{10, 20, 30, 40, 50, 60, 70, 80, 90};
int result1{binarySearch(v1, 70)};
std::cout << "70 was " << (result1 != -1 ? "" : "not ") << "found in v1\n";
// unsorted vector v2 violates binarySearch's sorted vector precondition
std::vector v2{60, 70, 80, 90, 10, 20, 30, 40, 50};
int result2{binarySearch(v2, 60)};
std::cout << "60 was " << (result2 != -1 ? "" : "not ") << "found in v2\n";
}
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