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raisenero · Guest

I'm looking to improve the way I'm rearranging a vector, switching the
contents of the first half with the second half (i.e. The first half of
the vector is now the second half, and the second half is now the first
half.)

My code works, but it just seems long and clunky to me, like I'm going
about doing it in a more complicated way than is necessary. I know
this is probably a very basic question, but I just don't have the
programming experience. Most of my coding always seems to come down to
"Keep typing until the program eventually does what I want.", so my
solutions to things like this don't seem as elegant as I think they
should be.

I thought maybe I could use the generic algorithm "copy" to solve the
problem, but I just didn't seem to be getting the results I expected so
this is the best I've done so far.

Thanks very much for any insight. I've included the source for the
smallest functioning program that illustrates what I've done.

#include <iostream>
#include <vector>

using namespace std;

int main()
{
vector<int> source;

// populate source with values
for(int i = 0; i != 10; ++i)
{
source.push_back(i);
}

// output the contents of the source vector
for(vector<int>::const_iterator csi = source.begin(); csi !=
source.end(); ++csi)
{
cout << *csi;
}

cout << endl;

// ** This section of code just seems like there ought
// to be an easier way. The code is switching the contents
// of the first half with the second half of the source vector

vector vector<int> temp2;
vector<int>::iterator si = source.begin();

// make si denote the middle of the source vector
for(int i = 0; i != 5; ++i)
{
si++;
}

// make temp hold the contents of the second half of the vector
// while temp2 holds the contenst of the first half.
temp.insert(temp.end(), si, source.end());
temp2.insert(temp2.end(), source.begin(), si);

// make source now hold the second half of the original vector
// followed by inserting the first half of the original vector
source = temp;
source.insert(source.end(), temp2.begin(), temp2.end());

// ** End Section that I think seems too complicated

for(vector<int>::const_iterator csi = source.begin(); csi !=
source.end(); ++csi)
{
cout << *csi;
}

return 0;
}

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Éric Malenfant · Guest

std::swap_ranges may be what you're looking for:

#include <iostream>
#include <iterator>
#include <algorithm>
#include <vector>

const std::vector<int>::size_type half_size = 5;
std::vector<int> tst;
std::fill_n(std::back_inserter(tst), half_size, 0);
std::fill_n(std::back_inserter(tst), half_size, 1);
std::cout << "Before swapping:n";
std::copy(tst.begin(), tst.end(),
std::ostream_iterator std::cout << std::endl;

std::swap_ranges(tst.begin(), tst.begin() + half_size,
tst.begin() + half_size);

std::cout << "After swapping:n";
std::copy(tst.begin(), tst.end(),
std::ostream_iterator std::cout << std::endl;

HTH,

Éric Malenfant

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velthuijsen@hotmail.com · Guest

You might want to look at <algorithm>
It has a function named rotate.

The function rotate which works as follows:
std::rotate(Source.begin(), Source.begin()+OffsetValue,
Source.end());

The only thing left is to sort out what to store in OffsetValue.
It is either Source.size()/2 or (Source.size()+1)/2
It depends on where you want to assign the middle value of an odd sized
array.
The first option assigns the middle value to the lower part (of the
result), the second option to the upper part (of the result).

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Stefan Vetsch · Guest

hi [insert your real name here]

how about that (i think the swapVectorChunks is the most interesting
part for you):

#include <vector>
#include <iostream>

template <class _T>
void swapVectorChunks (_T & rclVector)
{
const _T::size_type xiOldSize = rclVector.size();
const _T::size_type xiHalf = xiOldSize / 2;

rclVector.resize(rclVector.size() + xiHalf, _T::value_type());

copy(rclVector.begin(),
rclVector.begin() + xiHalf,
rclVector.begin() + xiOldSize);

rclVector.erase(rclVector.begin(), rclVector.begin() + xiHalf);
}

inline std::ostream & operator <<(std::ostream & rclOs,
const std::vector {
std::vector<int>::const_iterator xIx;

rclOs << "{";

for(xIx = crclVec.begin(); xIx != crclVec.end(); ++xIx)
{
if(xIx != crclVec.begin())
rclOs << ", ";

rclOs << *xIx;
}

rclOs << "}";

return(rclOs);
}

int main()
{
std::vector
for(int xIx = 0; xIx < 10; ++xIx)
xclTheVec.push_back(xIx);

std::cout << xclTheVec << std::endl;

swapVectorChunks(xclTheVec);

std::cout << xclTheVec << std::endl;

return(0);
}

regards stefan

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rwp · Guest

Hi!
Something like this may be what you want

vector<int>::iterator si = source.begin();
vector<int>::iterator se = source.end();
ptrdiff_t d = distance(si, se);
vector<int>::iterator sm = si;
advance(sm, d/2);

for (; sm != se; ++si,++sm)
{
swap(*si, *sm);
}

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Liton · Guest

I would have rather used the following:

swap_ranges(source.begin(), source.begin()+5, source.begin()+5);

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tony_in_da_uk@yahoo.co.uk · Guest

I'd suggest you simply call std::swap() on the elements you need to
exchange. E.g. Given a vector with elements [0] through [10], you'd
want to swap [0..4] with [6..11]:

typedef std::vector<T> Ts;
Ts ts;
ts.push_back( lots and lots );
Ts::size_type size = ts.size();
for (Ts::size_type i = 0; i <= size / 2; ++i)
std::swap(ts[i], ts[size - i]);

Cheers, Tony

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Allan W · Guest

[email]velthuijsen (AT) hotmail (DOT) com[/email] wrote:

Francis Glassborow · Guest

In article <1122984015.787256.271810 (AT) z14g2000cwz (DOT) googlegroups.com>,
raisenero <iveymi (AT) gmail (DOT) com> writes

Dietmar Kuehl · Guest

raisenero wrote:

Pete Becker · Guest

raisenero wrote: