CODEWITHSUNDEEP
c++qtiteratorqmap
Mosi
Mosi

Reputation: 1258

iterator expected to have different behaviour when calling next()/previous() on items

I have created a simple map and an iterator on it. When i move the iterator to next items, it performs well. After forwarding the iterator, if I ask it to go back for previous item and get value() of the iterator, it is not really the previous item value and actually the value is not changed at all. It seems there is something wrong or maybe I'm using it in a wrong way! Where is the problem?

see the below code

#include "mainwindow.h"
#include <QApplication>
#include <QMap>
#include <qiterator.h>

int main(int argc, char *argv[])

{
    QApplication a(argc, argv);

QMap<double,int> map;
map.insert(4234,3);
map.insert(4200,2);
map.insert(4100,1);
map.insert(4000,0);

QMapIterator<double, int> i(map);
i.toFront();

i.next();
i.next();
int va1 = i.value();    // val1 is 1 as expected

i.previous();

int val2 = i.value(); // It is expected that val2 should be 0 but is still Surprisingly 1!!!!

return a.exec();
}

Upvotes: 4

Views: 128

Answers (1)

Kuba hasn&#39;t forgotten Monica
Kuba hasn&#39;t forgotten Monica

Reputation: 98425

This is by design and a behavior of the Java-style iterators. The iterators have two important pieces of state associated with them:

  1. Position.
  2. Direction.

In all cases, the iterator points to the item it most recently stepped over.

Using next() and previous() reverses the iterator's direction. After next(), the iterator steps right and points to the item to its left. After previous(), the iterator steps left and points to the item to its right.

Here's the annotated execution order. The - flag indicates the pointed-to value based on iterator's direction. The v symbol indicates the iterator position.

i.toFront();
-v
  4000 4100 4200 4234
  0    1    2    3

i.next();
  ----v
  4000 4100 4200 4234
  0    1    2    3

i.next();
       ----v
  4000 4100 4200 4234
  0    1    2    3

i.previous();
      v----
  4000 4100 4200 4234
  0    1    2    3

i.previous();
 v----
  4000 4100 4200 4234
  0    1    2    3

Test case:

#include <QtCore>
int main()
{
   QMap<double, int> map;
   map.insert(4234., 3);
   map.insert(4200., 2);
   map.insert(4100., 1);
   map.insert(4000., 0);

   QMapIterator<double, int> i(map);
   i.toFront();

   i.next();
   qDebug() << i.key() << i.value();
   i.next();
   qDebug() << i.key() << i.value();

   i.previous();
   qDebug() << i.key() << i.value();
   i.previous();
   qDebug() << i.key() << i.value();
}

Output:

4000 0
4100 1
4100 1
4000 0

If you didn't expect that behavior, perhaps C++-style iterators would be easier to apply.

Upvotes: 1

Related Questions