Thread: Array manipulation Question

  1. #1
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    Array manipulation Question

    Ok so i'm writing a program to sort an array using heap sort. I have my main function that gets the information on what the array is, then using a function to sort it. The function is defined as: void maxheapify(int A[], int i, int len)

    So I've figured out that the array doesn't really get modified when I pass the array into the function. But I can't just do A=maxheapify(A, i, len) either because in maxheapify A is an int*... And if I turn the A in main to an int* then I get a whole crap load of errors....

    So anyone have any suggestions? I can post my whole code if need be.

  2. #2
    Cat without Hat CornedBee's Avatar
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    I see no reason why that shouldn't modify the array contents. You must have an error in the implementation.
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
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  3. #3
    Hurry Slowly vart's Avatar
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    So I've figured out that the array doesn't really get modified when I pass the array into the function
    And why not?
    All problems in computer science can be solved by another level of indirection,
    except for the problem of too many layers of indirection.
    David J. Wheeler

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    I printed out the array before and after and it comes out the same.... (not sorted)

  5. #5
    Hurry Slowly vart's Avatar
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    Quote Originally Posted by JJJIrish05 View Post
    I printed out the array before and after and it comes out the same.... (not sorted)
    And what about showing the code?
    All problems in computer science can be solved by another level of indirection,
    except for the problem of too many layers of indirection.
    David J. Wheeler

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    Code:
    #include <iostream>
    #include <iomanip>
    #include <cstdlib>
    
    using namespace std;
    
    
    int operationsCounter=0;
    
    void maxheapify(int A[], int i, int len)
    {
    	int l=2*i;
    	int r=2*i+1;
    	int largest, exchange;
    	if ((l<=len) && (A[l]>A[i]))
    	    {
    		largest=l;
    		operationsCounter++;
    	    }
    	else
    	    {
    		largest=i;
    		operationsCounter++;
    	    }
    	operationsCounter++;
    	if ((r<=len) && (A[r]>A[largest]))
    	    {
    		largest=r;
    		operationsCounter++;
    	    }
    	operationsCounter++;
    	if (largest!=i)
    	{
    		exchange=A[largest];
    		A[largest]=A[i];
    		A[i]=exchange;
    		maxheapify(A, largest, len);
    		operationsCounter+=4;
    	}
    	operationsCounter++;
            //return (A);
    }
    
    int main()
    {
      int exchange, len, option, fakelen;
      cout << "How big of an array do you want to sort? " << endl;
      cin >> len;
      int A[len];
      cout << "To have " << len << " numbers generated randomly please enter 1, to enter the array yourself please enter 2: " << endl;
      cin >> option;
      if (option == 1)
        {
          for (int i=0; i<len; i++)
    	{
    	  A[i]=rand();
    	  operationsCounter++;
    	}
        }
      else if (option == 2)
        {
          cout << "Enter the array: " << endl;
          for (int i=0; i<len; i++)
    	{
    	  cin >> A[i];
    	  operationsCounter++;
    	}
        }
      else
        {
          cout << "You didn't enter 1 or 2.  What a fool. Restart the program if you must." << endl;
        }
      cout << "The array before sorting: ";
      for(int i=0; i<len; i++)
        cout << A[i] << " ";
      cout << endl;
      fakelen=len;
      for(int i=len; i>1; i--)
        {
          fakelen--;
          exchange=A[1];
          A[1]=A[i];
          A[i]=exchange;
          maxheapify(A, 1, fakelen);
        }
      cout << "The array after sorting: ";
      for(int i=0; i<len; i++)
        cout << A[i] << " ";
      cout << endl << "The approximate number of operations for an array of size " << len << " is: " << operationsCounter << endl;
      return 0;
    }

  7. #7
    Hurry Slowly vart's Avatar
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    Code:
    cin >> len;
      int A[len];
    How do you succeded to compile this code?
    All problems in computer science can be solved by another level of indirection,
    except for the problem of too many layers of indirection.
    David J. Wheeler

  8. #8
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    why? is that not how you declare arrays in c++? It compiles fine and takes in the numbers and prints them out....

  9. #9
    Hurry Slowly vart's Avatar
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    Quote Originally Posted by JJJIrish05 View Post
    why? is that not how you declare arrays in c++? It compiles fine and takes in the numbers and prints them out....
    No, it is not... It is C99, but rest is not C

    Code:
    1>test.cpp(49) : error C2057: expected constant expression
    1>test.cpp(49) : error C2466: cannot allocate an array of constant size 0
    1>test.cpp(49) : error C2133: 'A' : unknown size
    All problems in computer science can be solved by another level of indirection,
    except for the problem of too many layers of indirection.
    David J. Wheeler

  10. #10
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    compiles fine for me using gcc.... so then how am i supposed to declare it?

  11. #11
    Hurry Slowly vart's Avatar
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    or use std::vector
    or using new
    All problems in computer science can be solved by another level of indirection,
    except for the problem of too many layers of indirection.
    David J. Wheeler

  12. #12
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    why do i have to use one of those? couple sites i've looked at said the way I did is the way to do it.... why shouldn't it work and how exactly do i declare it? int[] A = new int A[len] or something like that?

  13. #13
    Hurry Slowly vart's Avatar
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    Also your algorithm is very strange. You never access the A[0]
    This part of main
    Code:
    exchange=A[1];
          A[1]=A[i];
          A[i]=exchange;
    also has no explanation for me
    All problems in computer science can be solved by another level of indirection,
    except for the problem of too many layers of indirection.
    David J. Wheeler

  14. #14
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    oh ya.... lol... 1 is supposed to be 0.... let me change that....

  15. #15
    Hurry Slowly vart's Avatar
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    Quote Originally Posted by JJJIrish05 View Post
    why do i have to use one of those? couple sites i've looked at said the way I did is the way to do it.... why shouldn't it work and how exactly do i declare it? int[] A = new int A[len] or something like that?
    Code:
    int* A = new int[len];
    couple sites i've looked at said the way I did is the way to do it
    And what are they? This is not C++, so maybe you looked fro some other langugage - C99 for example
    All problems in computer science can be solved by another level of indirection,
    except for the problem of too many layers of indirection.
    David J. Wheeler

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