Re: I need help with a function that swaps array halves.

From:
 cheetahclaws@yahoo.com
Newsgroups:
comp.lang.c++
Date:
Wed, 05 Sep 2007 18:02:42 -0700
Message-ID:
<1189040562.234940.155330@w3g2000hsg.googlegroups.com>
On Sep 5, 8:19 pm, "Jim Langston" <tazmas...@rocketmail.com> wrote:

<cheetahcl...@yahoo.com> wrote in message

news:1189036219.364561.317970@22g2000hsm.googlegroups.com...

I am trying to create a function that reverses the halves of an
integer array. For example. if the array contains 1,2,3,4 --- after it
has gone to the function I constructed, the array will look like
3,4,1,2. In the case the array contains an odd amount of elements e.g.
1,2,3,4,5, it will look like 4,5,3,1,2.

I have created such an array as followed:
==================================
 for(i=0;i<=((size/2)-1);i++)
{
if(size%2 != 0)
{
t = a[i];
a[i] = a[(size/2)+i+1];
a[(size/2)+i+1] = t;
}
else
{
t = a[i];
a[i] = a[(size/2)+i];
a[(size/2)+i] = t;
}
}
==================================

NOW, the problem is I want to create a function that does the same
thing, but I want to use a Nested For loop to do it as such without
using IF statements to handle what to do when size is odd or not.

I came up with the following code:
==================================
void reverseHalves(int a[], int size)
{
int i, t, j=0;

for(i=0;i<=((size/2)-1);i++)
{
t = a[i];
a[i] = a[(size/2)+i+1];
a[(size/2)+i+1] = t;

for(j=(size-(size/2));j<=(size/2)-1;j++)
{
t = a[j];
a[j] = a[(size/2)+j-1];
a[(size/2)+j-1] = t;
}
}
==================================

The problem is the second nested-for-loop version I have come up with
doesn't work. It only works for odd numbers. When I enter an even
number, I always end up getting a -24214214 or something like that for
one of the numbers.

It alllllmost works. I have been at this for about three hours and
almost at the end of my rope. Can someone PLEASE FOR THE LOVE OF GOD
GIVE ME SOME HELP BEFORE I BLOW MY BRAINS OUT?!?!!


This looks like homework, but you have shown a lot of effort. Look at this
code, although you'll have to fix one bug when the arrays are odd sized:

#include <iostream>

void reverseHalves(int a[], int size)
{
    int half = size / 2;
    bool odd = size % 2 == 1;

    int j = half;
    if ( odd )
        j++;

    for ( int i = 0; i < half; ++i )
    {
        std::swap( a[i], a[j++] );
    }

}

int main()
{
    int Array[] = { 1, 2, 3, 4, 5, 6 };
    reverseHalves( Array, sizeof( Array ) / sizeof( Array[0] ) );
    for ( int i = 0; i < sizeof( Array ) / sizeof( Array[0] ); ++i )
        std::cout << Array[i] << " ";
    std::cout << "\n";

    int Array2[] = { 1, 2, 3, 4, 5, 6, 7 };
    reverseHalves( Array2, sizeof( Array2 ) / sizeof( Array2[0] ) );
    for ( int i = 0; i < sizeof( Array2 ) / sizeof( Array2[0] ); ++i )
        std::cout << Array2[i] << " ";
    std::cout << "\n";

}

Output:
4 5 6 1 2 3
5 6 7 4 1 2 3

The output is close to what you want except for odd, where the question
becomes... do you want
4567123
or
5671234

Fix the code to express what you want.


Thanks for the help. I'm starting to put together a picture of
something here. I was just wondering, how does the swap function work?
If I wanted to exclude it from the For-Loop and use something like:

        t=a[i];
    a[i] = a[j];
    a[j] = t;
    j++;

where t is a temporary variable, would that be the same?

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