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115 lines
7.4 KiB
Markdown
115 lines
7.4 KiB
Markdown
# Tutorial (JKQTPDatastore): Iterator-Based usage of JKQTPDatastore {#JKQTPlotterBasicJKQTPDatastoreIterators}
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[JKQTPlotterBasicJKQTPDatastore]: @ref JKQTPlotterBasicJKQTPDatastore "Basic Usage of JKQTPDatastore"
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[JKQTPlotterBasicJKQTPDatastoreIterators]: @ref JKQTPlotterBasicJKQTPDatastoreIterators "Iterator-Based usage of JKQTPDatastore"
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[JKQTPlotterBasicJKQTPDatastoreStatistics]: @ref JKQTPlotterBasicJKQTPDatastoreStatistics "Advanced 1-Dimensional Statistics with JKQTPDatastore"
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[JKQTPlotterBasicJKQTPDatastoreRegression]: @ref JKQTPlotterBasicJKQTPDatastoreRegression "Regression Analysis (with the Statistics Library)"
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[JKQTPlotterBasicJKQTPDatastoreStatisticsGroupedStat]: @ref JKQTPlotterBasicJKQTPDatastoreStatisticsGroupedStat "1-Dimensional Group Statistics with JKQTPDatastore"
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[JKQTPlotterBasicJKQTPDatastoreStatistics2D]: @ref JKQTPlotterBasicJKQTPDatastoreStatistics2D "Advanced 2-Dimensional Statistics with JKQTPDatastore"
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[statisticslibrary]: @ref jkqtptools_math_statistics "JKQTPlotter Statistics Library"
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This tutorial project (see `./examples/datastore_iterators/`) explains how to use the iterator-based interface to JKQTPDatastore.
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***Note*** that there are additional tutorial explaining other aspects of data mangement in JKQTPDatastore:
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- [JKQTPlotterBasicJKQTPDatastore]
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- [JKQTPlotterBasicJKQTPDatastoreIterators]
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- [JKQTPlotterBasicJKQTPDatastoreStatistics]
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- [JKQTPlotterBasicJKQTPDatastoreRegression]
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- [JKQTPlotterBasicJKQTPDatastoreStatisticsGroupedStat]
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- [JKQTPlotterBasicJKQTPDatastoreStatistics2D]
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[TOC]
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The source code of the main application can be found in [`datastore_iterators.cpp`](https://github.com/jkriege2/JKQtPlotter/tree/master/examples/datastore_iterators/datastore_iterators.cpp).
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This tutorial cites parts of this code to demonstrate different ways of working with JKQTPDatastore's iterator-interface.
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In every code-segment below, we will use these two declarations from the code to access the internal datastore of the JKQTPlotter instance:
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```.cpp
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// 1. create a plotter window and get a pointer to the internal datastore (for convenience)
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JKQTPlotter plot;
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JKQTPDatastore* datastore=plot.getDatastore();
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```
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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.
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# Iterator-based Column Data Access To Existing Rows
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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:
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```.cpp
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size_t XCol=datastore->addLinearColumn(50, 0, 4.0*M_PI, "cos curve: x-data");
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size_t YCol=datastore->addColumn(datastore->getRows(XCol), "cos curve: y-data");
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for (size_t i=0; i<datastore->getRows(XCol); i++) {
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datastore->set(YCol, i, cos(datastore->get(XCol, i)));
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}
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```
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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:
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The same loop can be written using iterators:
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```.cpp
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size_t XCol=datastore->addLinearColumn(50, 0, 4.0*M_PI, "cos curve: x-data");
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size_t YCol=datastore->addColumn(datastore->getRows(XCol), "cos curve: y-data");
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auto itY=datastore->begin(YCol);
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for (auto itX=datastore->begin(XCol); itX!=datastore->end(XCol); ++itX, ++itY) {
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*itY=cos(*itX);
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}
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```
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# Back-Inserter for Columns
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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:
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```.cpp
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size_t XCol=datastore->addColumn("cos curve: x-data");
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size_t YCol=datastore->addColumn("cos curve: y-data");
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auto biXCol=datastore->backInserter(XCol);
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auto biYCol=datastore->backInserter(YCol);
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for (double x=0; x<4.0*M_PI; x+=4.0*M_PI/50.0) {
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*++biXCol=x;
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*++biYCol=cos(x);
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}
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```
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# Using C++ STL algorithms
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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):
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```.cpp
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size_t XCol=datastore->addLinearColumn(50, 0, 4.0*M_PI, "cos curve: x-data");
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size_t YCol=datastore->addColumn("cos curve: y-data");
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std::transform(datastore->begin(XCol), datastore->end(XCol), datastore->backInserter(YCol), &cos);
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```
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Of course you can now also interface other algorithms, like e.g. [`std::sort()`](https://de.cppreference.com/w/cpp/algorithm/sort):
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```.cpp
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std::sort(datastore->begin(colY), datastore->end(colY));
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```
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With this line of code, the `YCol` is sorted in ascending order and the plot becomes:
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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):
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```.cpp
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std::replace_if(datastore->begin(YCol), datastore->end(YCol), [](double v) { return v<-0.5; }, 1.0);
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```
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Finally also the [erase-remove idiom](https://en.wikipedia.org/wiki/Erase%E2%80%93remove_idiom) (e.g. known from `std::vector`) is supported:
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```.cpp
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datastore->eraseFromColumn(std::remove_if(datastore->begin(YCol), datastore->end(YCol), [](double v) { return v<0; }), datastore->end(YCol));
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```
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# Special Properties of the JKQTPDatastore-Iterators
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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:
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- `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()`
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- `JKQTPColumnIterator::getPosition()` returns the row/position inside the column the iterator points to
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- `JKQTPColumnIterator::getImagePosition()` / `JKQTPColumnIterator::getImagePositionX()` / `JKQTPColumnIterator::getImagePositionY()` return the x-/y-location of the pointed-to pixel in an image column
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- `JKQTPColumnIterator::getImageColumns()` / `JKQTPColumnIterator::getImageRows()` return the width/height of the image represented by the image column (the pointed-to pixel is part of)
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