binary search algorithms using iterative and recursive - c++

Question

I am looking for an element x in a sorted array. It compares xx or the array range equals to zero I am getting segmentation fault where I went wrong I couldn't find my code is below
I am compiling in gcc complier.
#include <iostream>
using namespace std;
// iterative
int bsearch(int a[], int sz, int x)
{
int low = 0;
int high = sz -1;
while(low <= high) {
int mid = (low+high)/2;
if(x < a[mid])
high = mid - 1;
else if(x > a[mid])
low = mid + 1;
else
return a[mid];
}
return -1;
}
// recursive
int bsearch_recursive(int a[], int low, int high, int x)
{
if(low > high) return -1;
int mid = (low + high)/2;
if(x < a[mid])
bsearch_recursive(a, low, mid-1, x);
else if(x > a[mid])
bsearch_recursive(a, mid+1, high, x);
else
return a[mid];
}
void print(int n)
{
if(n == -1) {
cout << "not found" << endl;
return;
}
cout << "found" << endl;
}
int main()
{
int a[]={3, 7, 9, 16, 23, 34, 67, 87, 92};
int arraySize = sizeof(a)/sizeof(int);
int result;
result = bsearch(a, arraySize, 7);
print(result);
result = bsearch(a, arraySize, 92);
print(result);
result = bsearch(a, arraySize, 77);
print(result);
result = bsearch_recursive(a, 0, arraySize-1, 7);
print(result);
result = bsearch_recursive(a, 0, arraySize-1, 92);
print(result);
result = bsearch_recursive(a, 0, arraySize-1, 77);
print(result);
return 0;
}

Answer 1

Your recursive search needs to have a return value on each path, otherwise its results are undefined.
A recursive function works exactly like other functions - if it claims to be returning a value, it must do that. It doesn't just automatically return the result of the terminating recursive call.
int bsearch_recursive(int a[], int low, int high, int x)
{
if(low > high) return -1;
int mid = (low + high)/2;
if(x < a[mid])
return bsearch_recursive(a, low, mid-1, x);
else if(x > a[mid])
return bsearch_recursive(a, mid+1, high, x);
else
return a[mid];
}
Your compiler should have warned you about this - if it didn't, switch on more warnings.
If it did and you didn't care, start listening to warnings.

Answer 2

Below function has problem:
int bsearch_recursive(int a[], int low, int high, int x)
When you call this function recursively, you should return the value as shown below
int mid = (low + high)/2;
if(x < a[mid])
return bsearch_recursive(a, low, mid-1, x); // added return
else if(x > a[mid])
return bsearch_recursive(a, mid+1, high, x); // added return
else
return a[mid];
If you don't return from some code paths of a function that returns, the code behavious is undefined.
As side notes
if you intend to use this code for very large arrays, (low + high) may overflow, so use
int mid = low + (high - low)/2;
To make sure your compiler warns you about this compile with -Wall
option.
Returning -1 in case of error is not a good idea if array may contain both positive and negative numbers. You can return array index if found and -1 if error or device some other not found mechanism.