C++ Binary Search Algorithm not working - c++

Question

So I have a vector of ints named bList that has information already in it. I have it sorted before running the binary search.
//I have already inserted random ints into the vector
//Sort it
bubbleSort();
//Empty Line for formatting
cout << "\n";
//Print out sorted array.
print();
cout << "It will now search for a value using binary search\n";
int val = binSearch(54354);
cout<<val;
My bubble sort algorithm does work.
I have it returning an int which is the location of the searched value in the list.
//Its one argument is the value you are searching for.
int binSearch(int isbn) {
int lower = 0;
int upper = 19;//Vector size is 20.
int middle = (lower + upper) / 2;
while (lower < upper) {
middle = (lower + upper) / 2;
int midVal = bList[middle];
if (midVal == isbn) {
return middle;
break;
} else if (isbn > midVal) {
lower = midVal + 1;
} else if (isbn < midVal) {
upper - midVal - 1;
}
}
}
But for some reason, when I run it, it just keeps running and doesn't return anything.

Answer 1

Here the bug is:
// ...
} else if (isbn > midVal) {
lower = midVal + 1;
} else if (isbn < midVal) {
upper - midVal - 1;
}
You may want
lower = middle + 1;
and
upper = middle - 1;
instead.
You also need to explicitly return something when the required number cannot be found.

Answer 2

You still have a slight logic problem with your while condition:
int binary_search(int i, const std::vector<int>& vec) // you really should pass in the vector, if not convert it to use iterators
{
int result = -1; // default return value if not found
int lower = 0;
int upper = vec.size() - 1;
while (lower <= upper) // this will let the search run when lower == upper (meaning the result is one of the ends)
{
int middle = (lower + upper) / 2;
int val = vec[middle];
if (val == i)
{
result = middle;
break;
}
else if (i > val)
{
lower = middle + 1; // you were setting it to the value instead of the index
}
else if (i < val)
{
upper = middle - 1; // same here
}
}
return result; // moved your return down here to always return something (avoids a compiler error)
}
Alternatively, you could switch it to use iterators instead:
template<class RandomIterator>
RandomIterator binary_search(int i, RandomIterator start, RandomIterator end)
{
RandomIterator result = end;
while (start <= end) // this will let the search run when start == end (meaning the result is one of the ends)
{
RandomIterator middle = start + ((end - start) / 2);
if (*middle == i)
{
result = middle;
break;
}
else if (i > *middle)
{
start = middle + 1;
}
else if (i < *middle)
{
end = middle - 1;
}
}
return result;
}