Array copying

Posted on 2003-11-09
Medium Priority
Last Modified: 2010-04-01
I am working on a project using different sorting algorithms.  The original unsorted array must be used by each sorting algorithm.  My question is, how is the best way to pass the array into a function but have it return to the original array when the function is done?  Should I just pass a copy of the original?  A code example would be very helpful.  Thanks,

Question by:Ant_Dogg

Expert Comment

ID: 9711787
STL is simplest

#include <algorithm>

MyObj myArray[NUM_ELEMENTS];
// fill myArray with values.

MyObj cpyArray[NUM_ELEMENTS];

copy(myArray, myArray + NUM_ELEMENTS, cpyArray);

then cpyArray is a copy of myArray which you can pass to your sorting algorithm.

MyObj can be any class which has a copy constructor, any of the primitives like int, double, etc. will be fine.
LVL 11

Expert Comment

ID: 9711874
Where is this being assigned? You should take a look at two other questions here:


Both pertain to sorting a an array containing strings or integers and counting the number of comparisons and timing the results.  Sounds a lot like another student doing the same work. Take a look for discussion of approach and some code; you will want to actually write your own code for reasons of academic honesty.

Hope this helps, -bcl

Expert Comment

ID: 9711893
Declare your original unsorted input array as a constant parameter of your sorting function, which means the function will not change the input array. Create a temporary working array in your function to manipulate the array, so that the implementaion of your sorting is transperent to outside world.

int MySort(const int Data[],int nElement)
//      Data[0] = 1; // This is not allowed.

      int *m_Data = new int[nElement];

// your algorithms here

      delete [] m_Data;
      return 0;
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Expert Comment

ID: 9712034
I think the memcpy function should allocate the actual size of array

memcpy(m_Data,Data,nElement * sizeof(int));

Expert Comment

ID: 9712977
The best way is to use STL function sort.
template<class RandomAccessIterator>
   void sort(
      RandomAccessIterator _First,
      RandomAccessIterator _Last
It doesnot need a copy of original array.
LVL 11

Expert Comment

ID: 9714563

Using the STL is, of course, the right answer if the goal is solely to have the array end up in sorted order. The original poster mentions that the project is for comparing different sorting algorithms. With STL sort you get a sort with certain performance bounds but with any given STL implementation you get one sort (okay, you get one general sort and one list<>::sort). The question asked was how to present the same test data to multiple sorts. The choices are reading the file multiple times (loading each array from a persistent copy of the data) or read the file once into a "cached" copy of the contents and then copy the in-memory version of the array (or seed a pseudo-random number generator with a know value and generate the "random" contents of the array before each sort but this is really close to reading it from the file).

LVL 22

Accepted Solution

grg99 earned 240 total points
ID: 9715436
The easiest way is to put the array in a struct.  C can copy structs with the assignment operator, so it's as simple as:

typedef struct foo { int TheArray[1000] }  EasyCopyArrayType;

EasyCopyArrayType   Main, Copy;

for( i= .... )   Main.TheArray[i] = rand( 9999 );

Copy = Main;

SortByInsertion( Copy );

Copy = Main;

SortByInsertion( Copy );

... etc...


Author Comment

ID: 9716380
>>typedef struct foo { int TheArray[1000] }  EasyCopyArrayType;

That sounds like it will work for me.  But what is the foo there for if it isn't used when calling the structure? Would this work just the same?

typedef struct {int Array[1000]} EasyArrayCopy;
EasyArrayCopy Main, Copy;


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