Recursive Binary Search using array. HELP!

[lugiamaster]

OK so I'm trying to write a program that performs recursive binary search. this program suppose to ask the user to enter a number to look for, it will then call binarySearch and display the result. if it does find the number, it's suppose to give me the position in which it was found, if it doesn't find it, it shows that it wasn't found.
an example run would be:
1. number to Find: 1 in position 1 //(It is not position 0!)
2. number to Find: 19 in position 10
3. number to Find: 5 in position 3
4. number to Find: 17 in position 9
5. number to Find: 21 fails
6. number to Find: 0 fails

i'd appreciate if you guys could help me add/edit stuff in my program

when i run it it gives me segmentation fault

#include <iostream>
#include <cmath>
using namespace std;

// the function
int binarySearch(int L[],int x, int first, int last)
{
  //int middle = (first + last) / 2;

  if (last >= first)  //first > last)
    {
      //return -1;
      int middle = (first + last) / 2;

      if (x == L[middle])
        return middle;
      else if (x < L[middle])
        return binarySearch(L, x, first, middle - 1);
      else //(x > L[middle])
        return binarySearch(L, x, middle + 1, last);
    }
  return -(first + 1);    // failed to find key
}

int main()
{
  /* int myList[size] = n;
  int myfirst = 0;
  int mylast = n - 1;
  int findthis;*/

  int myList[] = {1, 3, 5, 7, 9, 11, 13, 15, 17, 19};

  int size;
  //int myList[size];
  int n = myList[size];
  int myfirst = 0;
  int mylast = n - 1;
  int findthis;

  cout << "Please enter a number to find: " << endl;
  cin >> findthis;

  int resultloc = binarySearch(myList, findthis, myfirst, mylast);
  //Right here it suppose to display the result based on the function, if   //number enter was found it displays that it was found and it's position, if it //wasn't found it tells us "number not found" i don't know the syntax for it   //though

  return 0;
}

[Duthomhas]

You are confusing indices into your data and the integer data itself.

You are also confusing some different variables. You have listed 'size' and 'n', both of which represent the same thing: the length of your data. But, 'size' has no initial value, and 'n' is given a random value from your data.

Line 8: Good job moving this, but you didn't actually have to.

Line 10: Remember, 'last' should always be greater than or equal to 'first', and they are both inclusive -- the way you are using it. (I would have written it so that 'last' is exclusive.)

Line 13: You need to rethink your calculation. It might help if you get out a piece of paper and draw yourself an example, with indices (like 9 through 16), then calculate the middle index for it.

Line 22: Why not just -1?

Hope this helps.

[lugiamaster]

OK so this is what i got now, it works except when i enter a bigger number than 19, it tells me that the number that is bigger than 19 is found in position 11, but there's no position 11, it only goes to position 10. can you help me fix this small problem, i don't know where it is?

#include <iostream>
#include <cmath>
using namespace std;

// the function
int binarySearch(int L[],int x, int first, int last)
{
  if (last >= first)  //first > last)
    {
      //return -1;
      int middle = (first + last) / 2;

      if (x == L[middle])
        return middle;
      else if (x < L[middle])
        return binarySearch(L, x, first, middle - 1);
      else //if(x > L[middle])
        return binarySearch(L, x, middle + 1, last);
    }
  else
    return -1;//(first + 1);    // failed to find key
}


int main()
{
  /* int myList[size] = n;
  int myfirst = 0;
  int mylast = n - 1;
  int findthis;*/

  int myList[] = {1, 3, 5, 7, 9, 11, 13, 15, 17, 19};

  //int size;
  //int myList[size];
  //int n = myList[10];
  int myfirst = 0;
  int mylast = 10;//n - 1;
  int findthis;
  //int i = 0;

  cout << "Please enter a number to find: " << endl;
  cin >> findthis;


  int resultloc = binarySearch(myList, findthis, myfirst, mylast);
  if ( resultloc == -1 )
    {
      cout << "number not found" <<endl;
    }
  else
    {
      cout <<" number is found in position " << resultloc + 1 << endl;//binarySearch(myList, findthis, myfirst, mylast) <<endl;
    }

  /*if(resultloc == middle)
    {
      cout << findthis << " was found in position " << middle << endl;
      }*/

  return 0;
}