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gasto

Binary search implementation

13 posts in this topic

I did not consult any reference for this implementation, so I guess there are bugs lurking everywhere.

 

Constructive criticisms welcome. And yes, it is C-ish.

 
int BinarySearch(int data[], unsigned int elements, int key)
{
    if(elements==0)
    {
        cerr << "Binary search not able to perform on 0 element array";
        exit(-1);
    }
    unsigned int half_offset = elements<<31? (elements-1)/2 : elements/2;   // truncation will happen anyway.
    unsigned int current_index = elements<<31? (elements-1)/2 : elements/2; // Non portable bitwise ops.
    
        
    while(half_offset!=0)
    {
        if(data[current_index]==key)
            return current_index;
        else if(data[current_index]<key)
        {
            current_index += half_offset<<31? (half_offset+1)/2 : half_offset/2;
        }
        else if(data[current_index]>key)
        {
            current_index -= half_offset<<31? (half_offset+1)/2 : half_offset/2;
        }
        half_offset /= 2; // Truncated to floor
    }
    if(data[current_index]==key)
                return current_index;
}
Edited by gasto
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Is that code snippet complete? You should be getting an error that not all paths return a value. You end the function on an if-statement. I.e., when execution drops through the IF, there's no return value.

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// Non portable bitwise ops.

Moreover, if you were to test for even or odd numbers (though, as pointed out, it isn't necessary), wouldn't it be easier (and more portable) to AND with 1 like so: (x & 1)? I think that communicates clearer intent.

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Corrected according to feedback:

 
unsigned int BinarySearch(int data[], unsigned int elements, int key)
{
    if(elements==0)
    {
        cerr << "Binary search not able to perform on 0 element array";
        exit(-1);
    }
    unsigned int half_offset = elements/2 /*upper half smaller when even elements*/;
    unsigned int current_index = elements/2; 
    
        
    while(half_offset!=0)
    {
        if(data[current_index]==key)
            return current_index;
        else if(data[current_index]<key)
        {
            current_index += (half_offset+1)/2;
        }
        else if(data[current_index]>key)
        {
            current_index -= (half_offset+1)/2;
        }
        half_offset /= 2; // Truncated to floor
    }
    if(data[current_index]==key)
                return current_index;
    else
        cerr << "Finished array search without any matching results";
}
Edited by gasto
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    if(data[current_index]==key)
                return current_index;
    else
        cerr << "Finished array search without any matching results";
    return <fail-value>; // perhaps -1 as suggested above
}

As mentioned before, you need to add a return <value> before the closing brace.

Edited by Buckeye
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you should replace your exit call with a return to be honest, you dont want to terminate the application when this function fails. Instead just return -1 because its very unlikely you are going to find your element at max uint. The calling code knows how big the array is anyway so if max uint is a valid return value the calling code can determine this.

Edited by NightCreature83
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How do I choose a failure value in an unsigned int?

Maybe you can return a value higher than the array size (elements+1) and when you call that function always check if the return value is less than elements. It might work if you know you won't have gigant arrays as input. It doesn't sound very intuitive maybe, but if you can't change the return type of the function you'll have setup a convention.

 

Anyway, it's not THAT werid since some functions that work over collections do a similar thing. Look at std::find, if the value can't be found it returns "last", which is a parameter you must supply to that function instead of a constant error value, then when you call std::find you always do something like:

returnedValue = std::find(array.begin(),array.end(),value);
if ( returnedValue  != array.end() )
{
    // something
}
Edited by DiegoSLTS
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