The source code of the main application can be found in [`jkqtplotter_simpletest_datastore_iterators.cpp`](https://github.com/jkriege2/JKQtPlotter/tree/master/examples/simpletest_datastore_iterators/jkqtplotter_simpletest_datastore_iterators.cpp).
This tutorial cites parts of this code to demonstrate different ways of working with JKQTPDatastore's iterator-interface.
In every code-segment below, we will use these two declarations from the code to access the internal datastore of the JKQTPlotter instance:
```.cpp
// 1. create a plotter window and get a pointer to the internal datastore (for convenience)
JKQTPlotter plot;
JKQTPDatastore* datastore=plot.getDatastore();
```
In the example [JKQTPlotterBasicJKQTPDatastore] we discussed how to copy data from external container into and explicitly access data in columns inside a JKQTPDatastore. This tutorial explains how to use the iterator interface of JKQTPDatastore to access the data, build columns and also interact with algorithms from the C++ standard template library (or other iterator-based libraries, like e.g. boost). Also have a look at the [statisticslibrary] and [JKQTPlotterBasicJKQTPDatastoreStatistics], as these also use the iterator-interface of JKQTPDatastore.
# Iterator-based Column Data Access To Existing Rows
In other tutorials we used e.g. `JKQTPDatastore::set()` to set values in data columns. Using this scheme, you can write code like shown below to draw a cose curve:
for (size_t i=0; i<datastore->getRows(XCol); i++) {
datastore->set(YCol, i, cos(datastore->get(XCol, i)));
}
```
Here we added two columns with 50 entries. `XCol` contains linearly spaced values between 0 and 2*pi and `YCol` contains 50 uninitialized values. Then we iterate an index `i` over all these items (`datastore->getRows(XCol)` returns the rows in a column, i.e. 50 in the example above) and used `JKQTPDatastore::set()` to store the calculated values in the two columns. The current x-values is read from `XCol` using `JKQTPDatastore::get()`. The resulting plot looks like this:
for (auto itX=datastore->begin(XCol); itX!=datastore->end(XCol); ++itX, ++itY) {
*itY=cos(*itX);
}
```
# Back-Inserter for Columns
Above we used two previously sized columns and accessed (read and writing) existing rows in them. But `JKQTPDatastore` also provides an iterator comparable to [`std::back_inserter`](https://de.cppreference.com/w/cpp/iterator/back_inserter), which allows to add rows at the end of an existing (here initially empty) column:
You can write this a bit more compact, if you use `JKQTPDatastore::addLinearColumn()` and the C++ STL-algorithm [`std::transform()`](https://de.cppreference.com/w/cpp/algorithm/transform):
Another example would be to replace all value `y<-0.5` with the value `1.0` using [`std::replace_if()`](https://de.cppreference.com/w/cpp/algorithm/replace):
# Special Properties of the JKQTPDatastore-Iterators
Note that the iterator classes of `JKQTPDatastore` (namely `JKQTPColumnIterator` and `JKQTPColumnConstIterator`) provide additional function to access the properties of the data-column row they point to:
-`JKQTPColumnIterator::isValid()` checks whether the iterator points to a valid row in a column. it is `false` e.g. for an iterator returned by `JKQTPDatastore::end()`
-`JKQTPColumnIterator::getPosition()` returns the row/position inside the column the iterator points to
-`JKQTPColumnIterator::getImagePosition()` / `JKQTPColumnIterator::getImagePositionX()` / `JKQTPColumnIterator::getImagePositionY()` return the x-/y-location of the pointed-to pixel in an image column
-`JKQTPColumnIterator::getImageColumns()` / `JKQTPColumnIterator::getImageRows()` return the width/height of the image represented by the image column (the pointed-to pixel is part of)
