Binary Search For an Array in C++

Question

My Binary search keeps returning zero I'm not sure why. Maybe I'm passing my arguments in incorrectly? Or maybe I'm not returning it right? I'm not quite sure what is wrong, if anyone could explain how I can return the values I want instead of zero thanks. my array is filled with words I didn't include that part of my code because it is being filled in by a file. I'm not entering the first word so I shouldnt be getting zero. I've tried multiple words.

     using namespace std;
     #include <iostream>
     #include <fstream>
     #include <string>


     //function prototypes
     void selectionSort(string[], int);
     int binarySearch(string[], int, string);

     int main()
  {
int open;
int name[8];
ifstream inputFile;
string filename;
int i = 0;
int size = 1024;
int newSize;
string exit;
string words[1024];
string word;
string newArray[1024];
string search;
int results;
string createdFile;
int startScan, minIndex, minValue;
//newArray[0]="This";


cout << "Please Enter the Filename: ";
cin >> filename;
inputFile.open(filename.c_str());
if (!inputFile) {
    cout << "Your File could not be opened!";
}
else if (inputFile) {

    while (i<1024) {
        inputFile >> word;
        //cout << i;
        if (word.length()<2) {              //gets rid of single character words
        i++;
        }
        else if(inputFile.eof()) {
        //cout<<newSize<<endl;
        newSize=i;
        //cout<< "DONE!"<<newSize;
        break;
        }
        else{
        newArray[i] = word;
        i++;
        //cout<<newArray[i]<<" " ;
        //newSize=1;



        //cout<<newSize;
        }

        //cout << newArray;
        //i++;
    }




}
inputFile.close();
//Take values stored in array and put them in another array with i number of elements

    string finalArray[i];
    size=i;
    for(int j = 0; j<i;j++)
        {
        finalArray[j]=newArray[j];

        //cout<<finalArray[j];
        }

cout << "Your document has been sorted. " << endl;
selectionSort(finalArray, size);
cout<< "Out putting Your array"<<endl;
for (int z = 0; z<size; z++) {
    cout << finalArray[z] << " ";
}

    cout << "Please insert a word to search!: " << endl;
    cin >> search;
    results = binarySearch(finalArray, size, search);
    cout << results << endl;
    //results = binarySearch(finalArray, size, search);
    //cout << results;
    cout << "To exit press 0, or enter another word..." << endl;



}//end of main


   void selectionSort(string array[], int size) {
int startScan, minIndex;
string minValue;

for (startScan = 0; startScan < (size - 1); startScan++)
{
    minIndex = startScan;
    minValue = array[startScan];
    for (int index = startScan + 1; index <size; index++) {
        if (array[index]<minValue) {
            minValue = array[index];
            minIndex = index;
        }
    }
    array[minIndex] = array[startScan];
    array[startScan] = minValue;

}
   }


   int binarySearch(string words[], int numElems, string word) {

int first = 0,                 //first array element
    last = numElems - 1,            //last array element
    middle,                        //mid point of search
    position = -1;                 //position of search value
bool found = false;            //flag

while (!found && first == last)
{
    middle = (first + last) / 2; //this finds the mid point
    if (words[middle] == word) {
        found = true;
        position = middle;
    }
    else if (words[middle]> word) // if it's in the lower half
    {
        last = middle - 1;
    }
    else {
        first = middle + 1;                 //if it's in the upper half
    }
    return position;

     }
   }//end binarySearch

Answer 1

The problem is that you have return position; inside the while loop, but not in any of the if/else blocks, so it returns after the first iteration even when it hasn't yet found the search string. You should move that to after the loop, or do it in the if block that sets position when it finds a matching element.

Another problem is that the while condition is true. You want to keep looping while first is not equal to last . Actually, that's not the right condition, either, because if the array only has one element, first and last will always be equal, and you'll stop before you test whether it matches. So the correct test is while (first <= last) .

int binarySearch(string words[], int numElems, string word) 
{
    int first = 0,                 //first array element
    last = numElems - 1,            //last array element
    middle;                        //mid point of search

    while (first <= last)
    {
        middle = (first + last) / 2; //this finds the mid point
        if (words[middle] == word) {
            return middle;
        }
        else if (words[middle]> word) // if it's in the lower half
        {
            last = middle - 1;
        }
        else {
            first = middle + 1;                 //if it's in the upper half
        }
    }
    return -1;  // not found
}