Selection Sort .. Help

Question

1.00/5 (2 votes)

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Hi ..
I'm Trying To Implement The Algorithmic Steps of Selection Sort.

1 -> Find The Min Index
2 -> Exchange It With The Top & Increment.

The Output Is Not Correct .. Please Have A L^_^K. >!?

C++

void in(int arr[], int size)
{
    for(int i = 0; i < size; i++)
    {
        cout << "Element # " << i+1 << ": ";
        cin >> arr[i];
    }
}
void out(int arr[], int size)
{
    for(int j = 0; j < size; j++)
    {
        cout << arr[j] << "||";
    }
}
int find_min(int arr[], int size)
{
    int index = 0;
    for(int k = 0; k < size; k++)
    {
        if( arr[index] > arr[k])
        {
            index = k;
        }
    }
    return index;
}
void exchange(int &x, int &y)
{
    int temp = x;
    x = y;
    y = temp;
}
void selection_sort(int arr[], int size)
{
    int loc;
    for(int i = 0; i < size; i++)
    {
        loc = find_min(arr, size - i);
        exchange(arr[loc], arr[i]);
    }
} 

int main()
{
    const int size = 5;
    int my_array[size];
    in(my_array, size);
    out(my_array, size);
    cout << "After Selection Sort.." << "\n\n";
    selection_sort(my_array, size);
    cout << "\n\t" << "p r i n t i n g.." << endl;
    out(my_array, size);

    return 0;
}

= = > Update(V2)

C++

int find_min(int ar[], int size)
{
    int index = 0;
	bool found = false;
	for(int i = 1; i < size; i++)
    {
		if(ar[index] > ar[i])
        {
			index = i;
			found = true;
        }
    }

	if (found)
	{
		return index;
	}
	else
		return -1;
	
}

void selection_sort(int arr[], int size)
{
    int loc;
	for(int count = 0; count < size; count++)
    {
		loc = find_min(arr, size - count);
		if (loc >= 0)
		{
			exchange(arr[count], arr[loc]);
		}
    }
}

Posted 19-Sep-13 4:18am

Usman Hunjra

Updated 21-Sep-13 9:58am

v4

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nv3 · Accepted Answer · 2013-09-19T04:45:00

Solution 1

The main error is that in step i you should find the minimum of the elements i ... N-1 and switch places with arr[i]. So shouldn't it be

C++

for(int i = 0; i < size; i++)
{
    loc = find_min(arr+i, size - i);
    exchange(arr[loc], arr[i]);
}

A more cosmetic problem is that you do one superfluous compare in your find_min function:

C++

int find_min(int arr[], int size)
{
    int index = 0;
    for(int k = 0; k < size; k++)
    {
        if( arr[index] > arr[k])

In the first iteration you are comparing arr[0] with arr[0]. So why don't you start the loop with k = 1:

C++

int find_min(int arr[], int size)
{
    int index = 0;
    for(int k = 1; k < size; k++)
    {
        if( arr[index] > arr[k])

Posted 19-Sep-13 4:45am

nv3

Comments

Usman Hunjra 19-Sep-13 11:35am

Thank You nv3.
But Sir.. The Result Is Still not correct ..
FurtherMore i can't understand these statements;
for(int i = 0; i < size; i++)
{
loc = find_min(arr+i, size - i); // why add +i in arr.. ??????
exchange(arr[loc], arr[i]);
}

nv3 19-Sep-13 13:19pm

arr+i does not add i to the array elements, but evaluates to a pointer to the i-th element of the array. (Remember: The name of an array is at the same time a pointer to the first element). And that's what you want, to find the smallest of the elements i ... N-1.

Usman Hunjra 19-Sep-13 15:13pm

yes thank you ..
But still the output is not correct .. !!?!???

nv3 19-Sep-13 15:18pm

Your welcome. Then your next step should be to run your program with a debugger and observe what happens in each iteration of your loop. It is important that you learn how to that, so better get familiar with the debugger on your system right away.

Usman Hunjra 20-Sep-13 11:05am

yeah i did .. but i can't figure out the mistake ..
Any Further Help .. ??!

nv3 20-Sep-13 11:31am

So what have you tried exactly. Did the program compile and link? If you ran it in the debbugger, on which line did the problem occur?

Usman Hunjra 21-Sep-13 14:17pm

Yes The Program Compiles like a charm ..
Assuming array of 5 integers as: 9, 8, 6, 7, 1
Okay after using Debugger,, The Problem is in the find_min function. At the very first call to the function find_min in the function selection_sort it works fine but at the second call the value returned by the find_min function is "ZERO Again" that is the problem, Sir, I really need your help here.
Actually i don't know how to fix that .. however i try to solve it by using bool operator and if-else but still unable to correct the output ..
Please Help ..
Thanks For Your Consideration ..

nv3 21-Sep-13 17:45pm

You did not apply the change I suggested. Even in your version 2 you still have the line:
loc = find_min(arr, size - count);
That won't work. It must be
loc = find_min(arr+count, size - count);
Consider: In each loop iteration you want to find the minimum of the *still unsorted part* of the array and assign that to arr[count].

Furthermore, the remaining part:
if (loc >= 0)
{
exchange(arr[count], arr[loc]);
}
is wrong and must be replaced with
exchange (arr[count], arr[loc+count]);

I think, it is the arr[loc+count] which you didn't do in your first attempt. The locaction index that find_min returns is always relative to the beginning of the part of the array that you specify as argument. So if you let find_min start at arr+count, you must also add count to its return value to get the absolut index.

Usman Hunjra 21-Sep-13 15:20pm

furthermore i've updated check v2 ..

Usman Hunjra 22-Sep-13 13:00pm

Thank You Sir ..
Thank You So Much ..
+5 Respect :)

nv3 22-Sep-13 14:55pm

You are welcome.