Programming Forums > Application Development > C#

elford · Mar 8th, 2006, 6:52 PM

I have a static method (quicksort) that looks to me like it should return a new set of data thats sorted, but should not change the org. data set.

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//dataCopy is a float[]
QuickSortStatic.quickSort(dataCopy);
......
public static float[] quickSort(float[] input)
        {
            data = input;
            recQuickSort(0, data.Length - 1); //does the qsort
            return data;
        }

I want to have a line

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float[] f = QuickSortStatic.quickSort(dataCopy);

I want f to be an array that is sorted, and dataCopy to be the org., unsorted data. Instead, both f and dataCopy come back as sorted. Somehow dataCopy is being mutated by quicksort. How can I maintain the org. array?

nnxion · Mar 9th, 2006, 4:45 AM

What do you think this does: data = input;?
You are not creating a copy anywhere.

elford · Mar 9th, 2006, 5:05 PM

Quick sort is is a different class completly. The var "data" is encapsulated in that class as a static var that all the other quick sort methods modify when doing the sort. I don't see why the var "input" would be modified after setting "data = input;" because it is not used anywhere else in the qsort class.

Where is my understanding breaking down?

nnxion · Mar 10th, 2006, 4:48 AM

I'm not sure what you mean exactly. Could you post me some more code.
You are basing your quicksort off of this example?

Dameon · Mar 10th, 2006, 5:54 PM

Although float is value-type, an Array is not. Internally, an array of floats is actually an Array object. As such, arrays are passed by reference. When you data to input, the two array references then point to the same Array, not unlike any other object. Ever wondered why

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float[] bob;

is null? Ever wondered why you use the new keyword to instantiate an array?

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float[] bob = new float[10];

May want to look into the Array.Copy method.

elford · Mar 11th, 2006, 12:50 PM

Well, my program runs sucessfully using a workaround with the Clone method, so for the time being, I need to leave it at that. There's still major other portions of it that need to get done, so I need to quit fine tuning what already works. :-)

nnxion · Mar 11th, 2006, 3:18 PM

Note that the clone method makes a shallow copy:

Quote:

Originally Posted by MSDN

A shallow copy of an Array copies only the elements of the Array, whether they are reference types or value types, but it does not copy the objects that the references refer to. The references in the new Array point to the same objects that the references in the original Array point to.

In contrast, a deep copy of an Array copies the elements and everything directly or indirectly referenced by the elements.

Instead of:
data = input;
Try:
Array.Copy(input, data, input.Length);

Mar 8th, 2006, 6:52 PM	#1
elford Programmer Join Date: Nov 2005 Posts: 35 Rep Power: 0	Parameter Mutation I have a static method (quicksort) that looks to me like it should return a new set of data thats sorted, but should not change the org. data set. (Toggle Plain Text) //dataCopy is a float[] QuickSortStatic.quickSort(dataCopy); ...... public static float[] quickSort(float[] input) { data = input; recQuickSort(0, data.Length - 1); //does the qsort return data; } //dataCopy is a float[] QuickSortStatic.quickSort(dataCopy); ...... public static float[] quickSort(float[] input) { data = input; recQuickSort(0, data.Length - 1); //does the qsort return data; } I want to have a line (Toggle Plain Text) float[] f = QuickSortStatic.quickSort(dataCopy); float[] f = QuickSortStatic.quickSort(dataCopy); I want f to be an array that is sorted, and dataCopy to be the org., unsorted data. Instead, both f and dataCopy come back as sorted. Somehow dataCopy is being mutated by quicksort. How can I maintain the org. array?

Mar 9th, 2006, 4:45 AM	#2
nnxion Programming Guru Join Date: Jun 2005 Location: elemental plane Posts: 1,429 Rep Power: 5	What do you think this does: data = input;? You are not creating a copy anywhere. __________________ "Employ your time in improving yourself by other men's writings, so that you shall gain easily what others have labored hard for." -- Socrates

Mar 10th, 2006, 4:48 AM	#4
nnxion Programming Guru Join Date: Jun 2005 Location: elemental plane Posts: 1,429 Rep Power: 5	I'm not sure what you mean exactly. Could you post me some more code. You are basing your quicksort off of this example? __________________ "Employ your time in improving yourself by other men's writings, so that you shall gain easily what others have labored hard for." -- Socrates

Mar 10th, 2006, 5:54 PM	#5
Dameon Troll Join Date: Apr 2005 Location: Texas Posts: 732 Rep Power: 4	Although float is value-type, an Array is not. Internally, an array of floats is actually an Array object. As such, arrays are passed by reference. When you data to input, the two array references then point to the same Array, not unlike any other object. Ever wondered why (Toggle Plain Text) float[] bob; float[] bob; is null? Ever wondered why you use the new keyword to instantiate an array? (Toggle Plain Text) float[] bob = new float[10]; float[] bob = new float[10]; May want to look into the Array.Copy method. __________________ MD5(sig) = bcef75433db02e9ad9bf81d6f7c5c270

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Mar 9th, 2006, 5:05 PM	#3
elford Programmer Join Date: Nov 2005 Posts: 35 Rep Power: 0	Quick sort is is a different class completly. The var "data" is encapsulated in that class as a static var that all the other quick sort methods modify when doing the sort. I don't see why the var "input" would be modified after setting "data = input;" because it is not used anywhere else in the qsort class. Where is my understanding breaking down?

Mar 11th, 2006, 12:50 PM	#6
elford Programmer Join Date: Nov 2005 Posts: 35 Rep Power: 0	Well, my program runs sucessfully using a workaround with the Clone method, so for the time being, I need to leave it at that. There's still major other portions of it that need to get done, so I need to quit fine tuning what already works. :-)

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Programming Forums > Application Development > C#

Parameter Mutation