logical error with binary search program

[hardknock]

Hello

Let me say this, the site has been very helpful especially when a person is willing to try the code themselves instead of just asking for answers. I feel that it helps one learn much quicker with me being new to C++...with that being said, the below code is giving me some erroneous answer during the execution.

The user enters a number from the Array list or a number that is not on the array list and the program is supposed to tell whether the number exist in the array and where it is located at what index.

The strange thing is that if you notice the 4th index, which contains the number 3302, if you run the program and enter the search value of 3302, the program gives out the appropriate answer...

...says 3302 found at index=4.

If you run the program and enter a number which isn't in the list, it says "not found", which is also CORRECT.

However, the issue arises when you enter any other number into the search value say, 1681 or 7140, both of which are in the array list but the program skips over them as if they are not there and says "Not Found".

I have run the debugger and from what I have investigated, somehow, my function definition is not giving me the correct answer. It is saying that my "if(first <= last)" condition is not true and returning a -1; however it also does that when I enter 3302 yet returns accurately as it is supposed to.

Can anyone enlighten me on what is going on.

#include<iostream>
using namespace std;

//declare function
int binSearch (int A[], int size, int tarVal);


void main()
{
	const int Array_Size=10;
	int myArray[Array_Size]={1681, 2106, 2309, 3184, 3302, 4190, 4498, 6512, 7140, 9963};

	cout << "The "<< Array_Size << " array elements are: " << endl;
	for (int i=0; i<Array_Size; i++)
		{
			cout << myArray[i] << " ";
		}
	
	
			cout << endl << "Enter the target value: ";
			int tarVal;
			cin >> tarVal;
		
	
	int index = binSearch(myArray, Array_Size, tarVal);

	if (index > 0)
	{
		cout << "Target value= " << tarVal << "  found at array index " << index << endl;
	}
	else
		cout << "Target Value =  " << tarVal << " not found " << endl;
		
}
int binSearch (int A[], int size, int tarVal)
	{
		int first=0, last=size-1, mid;

		if(first <= last)		
			{
				mid = (first + last)/2;
			}
			if (tarVal < A[mid])
			{
				last = mid-1;
			}
			else if (tarVal > A[mid])
			{
				first = mid+1;
			}
			else return mid;
			{
				return -1;
			}
		
					
	}

[prophetjohn]

First of all, your binary search function is not a loop. You need to keep performing these actions until either mid == tarVal or first >= last.

Second of all, try and follow the program logically. If you plug in 3302 for tarVal, mid is evaluated to 9/2, which is equal to 4. 3302 is not greater than or less than A[4] because it is equal. The final else statement is what executes and it says return mid;.

Now look at what happens when tarVal is anything else. Let's try and walk 1681 through the function. mid still evaluates to 4. if (tarVal < A[mid]) evaluates to true because 1681 is less than 3302. Since that if statement evaluated to true, the else if and else are both skipped. The only statement left is return -1; so that's what it does.

However I think you think the the return -1; is associated with that final else, since you wrapped it in curly braces, but that's not the case. What belongs to that final else is return mid;. The funtion will always return either mid = (first + last)/2 or -1

[hardknock]

nvm, i just realized that where i have the "IF" statement, it should have been a while statement which would allow it to run until the condition is no longer true....