how do I copy the previous line of the same vector to the current row?

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Hi,

I have a 2D vector and want only the last column to change each time. I want the previous row to be copied over (with 1 less element).

Basically if i have an 2D vector visualised as:

1 2 3 4
. . . . <---- I want this line to be 2 3 4 (?) where the ? is a value inputted by other means.

So if I had it working it would be like this:

1 2 3 4
2 3 4 20
3 4 20 5
4 20 5 6

so user inputted last value (done)
How do I copy the previous row's values (except the first value) over? I've tried copy ( ) and MyVector.insert( ) ... but the only way I can get working is using a for loop which takes several lines. Can I not use a function like insert when it involves the same vector being copied? Many thanks.
P.S. The first line is inputted before I try the insert.

Posted 7-Dec-12 4:06am

lostandconfused1234

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André Kraak 7-Dec-12 10:24am

You might want to consider using something different like a list, which should make what you want to do easier.

lostandconfused1234 7-Dec-12 10:27am

Yes, well I have it working using a while loop so:
while(n < NumColumns-1) *inserts the data*,
I'm just trying to optimise my code into the shortest number of lines possible. Somewhat interesting as to why I can't do:
while (file != eof) {
MyVector.insert ( MyVector[i], Myvector[i][1], MyVector[i].end() );
i++
}

lostandconfused1234 7-Dec-12 10:28am

forgot my manners - thank you though :D

Quirkafleeg 7-Dec-12 11:49am

A deque should be the preferred option to a list.

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Eugen Podsypalnikov · Accepted Answer · 2012-12-07T04:40:00

An std::list could pop'n'push at its both sides :) :

C++

typedef std::list<int> listYours;
typedef std::auto_ptr<listYours> ptrYours;

void ProduceShiftedListByValue(const ptrYours& listExist, ptrYours& listNew, int iValue)
{
  listNew.reset(listExist.get() ?
                new listYours(*listExist.get()) : // assumed std::list has a copy constructor :)
                NULL);
  if (listNew.get()) {
    if (listNew->size()) {
      listNew->pop_front();
    }
    listNew->push_back(iValue);
  }
}

typedef std::list<ptrYours> 2dYours;
// or: typedef std::list<listyours&> 2dYours;
// or: typedef std::list<listyours> 2dYours;