In C#, how do I rotate a list to the right by the specified number of places?

Question

5.00/5 (1 vote)

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I'm given this line of code in particular:

C#

public void Test8(List<int> items, int places)

The tasks for this problem is to come up with solution where I am able to rotate the List to the right by the specified number of places. A List rotation can be seen as circular, which means the elements that fall of the end would wrap around to the beginning, and vice versa.

How can I do this the correct way? Is there multiple ways to do this manually without using any sort fancy code?

Example:

Initial Array or List: 20, 30, 40, 50,60, 70
Rotated to the right by 3 places: 50, 60, 70, 20, 30, 40.

What I have tried:

C#

public void Test8(List<int> items, int places)
{
    int rotate = places;
    int[] results = new int[items.Count];

    for (int i = 0; i < items.Count; i++)
    {
        results[i] = items[(i + rotate) % items.Count];
    }
}

Posted 18-Jul-16 23:31pm

Member 12642406

Updated 19-Jul-16 4:13am

Leo Chapiro

v2

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3 solutions

Solution 2

You have basically 2 techniques
- Rotation with copy to a new list
- Rotation in place

Rotation with copy is the technique of solution 1.
To calc to new positionof an element, you have to apply the formula:
NewPosition= (OldPosition+ Rotation) % SizeOfList

Rotation in place
Only rotation of 1 and -1 are practical. You just have to repeat as many times as needed.
- pseudo code

C++

if (rotation== -1)
{
    tmp= list[0]
    for scan= 1; scan < list.count; scan++)
    {
        list[scan-1]= list[scan];
    }
    list[list.count-1]= tmp;
}
else
{
    tmp= list[list.count-1]
    for scan= list.count-1; scan >=0; scan--)
    {
        list[scan]= list[scan-1];
    }
    list[0]= tmp;
}

Posted 19-Jul-16 1:46am

Patrice T

Comments

[no name] 19-Jul-16 7:59am

Only a mind game:
There is a third option, "virtualRotation". I mean something like
listIndex= (userIndex + rotationValue) % list.Count;
The only problem I have, how do I Need to adapt IEnumerator that the list remains a list?

Patrice T 19-Jul-16 10:53am

Sure

Solution 3

A slightly more efficient version of Solution 1:

C#

public void Test8(List<int> items, int places)
{
    // Handle cases where the number of places is greater than or equal to the number of items:
    places %= items.Count;
    if (places == 0) return;
    
    int[] range = new int[places];
    items.CopyTo(items.Count - places, range, 0, places);
    items.RemoveRange(items.Count - places, places);
    items.InsertRange(0, range);
}

Posted 19-Jul-16 4:13am

Richard Deeming

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Leo Chapiro · Accepted Answer · 2016-07-18T23:58:00

Solution 1

Try this:

C#

public void Test8(List<int> items, int places)
{ 
    int nItem;

    for (int i = 0; i < places; i++)
    {
        nItem = items[items.Count - 1];     // get the last item of the list
        items.RemoveItem(items.Count - 1);  // remove this item from the end  ...
        items.InsertItem(0, nItem);         // ... and insert this (last) item in front of the list
    }
}

Posted 18-Jul-16 23:58pm

Leo Chapiro

Updated 19-Jul-16 0:02am

v4

Comments

Member 12642406 19-Jul-16 6:15am

I added the code you suggested, but the inputs are inconsistent all around which makes the test to fail in all 5 runs. Look at the screenshot I took. -> http://imgur.com/3qTOxch

Member 12642406 19-Jul-16 6:17am

Never mind, I got it too work. Was writing something wrong.

Richard Deeming 19-Jul-16 10:05am

That will work, but it won't be very efficient. :)

Each time you InsertItem(0, ...), every other item in the list has to be moved up one place.

I'd be inclined to use CopyTo, RemoveRange and InsertRange.