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299 lines
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<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
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<title>Data Elements</title>
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<td align="right" valign="center"><img src="logo32.png" align="right" width="64" height="32" border="0"></td></tr></table><h1 align=center>Data Elements</h1>
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<p>
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<p> We will use a C++ class called <tt>Element</tt> to provide storage and
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access for data elements.
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<p> (Extracts from <tt>element.h</tt>.)
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<p>
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<pre> private:
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</pre><pre> double m_value;
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<a href="qcolor.html">QColor</a> m_valueColor;
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int m_valuePattern;
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<a href="qstring.html">QString</a> m_label;
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<a href="qcolor.html">QColor</a> m_labelColor;
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double m_propoints[2 * MAX_PROPOINTS];
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</pre>
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<p> Each element has a value. Each value is displayed graphically with a
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particular color and fill pattern. Values may have a label associated
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with them; the label is drawn using the label color and for each type
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of chart has a (relative) position stored in the <tt>m_propoints</tt> array.
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<p>
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<pre> #include <<a href="qcolor-h.html">qcolor.h</a>>
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#include <<a href="qnamespace-h.html">qnamespace.h</a>>
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#include <<a href="qstring-h.html">qstring.h</a>>
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#include <<a href="qvaluevector-h.html">qvaluevector.h</a>>
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</pre>
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<p> Although the <tt>Element</tt> class is a purely internal data class, it
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<tt>#include</tt>s four Qt classes. Qt is often perceived as a purely GUI
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toolkit, but it provides many non-GUI classes to support most aspects
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of application programming. We use <a href="qcolor-h.html">qcolor.h</a> so that we can hold the
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paint color and text color in the <tt>Element</tt> class. The use of <a href="qnamespace-h.html">qnamespace.h</a> is slightly obscure. Most Qt classes are derived from the
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<a href="qt.html">Qt</a> superclass which contains various
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enumerations. The <tt>Element</tt> class does not derive from <a href="qt.html">Qt</a>, so we need to include <a href="qnamespace-h.html">qnamespace.h</a> to have access to
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the Qt enum names. An alternative approach would have been to have
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made <tt>Element</tt> a <a href="qt.html">Qt</a> subclass. We include <a href="qstring-h.html">qstring.h</a> to make use of Qt's Unicode strings. As a convenience we
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will <tt>typedef</tt> a vector container for <tt>Element</tt>s, which is why we
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pull in the <a href="qvaluevector-h.html">qvaluevector.h</a> header.
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<p> <pre> typedef QValueVector<Element> ElementVector;
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</pre>
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<p> Qt provides a number of containers, some value based like
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<a href="qvaluevector.html">QValueVector</a>, and others pointer based. (See <a href="collection.html">Collection Classes</a>.) Here we've just typedefed one container
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type; we will keep each data set of elements in one <tt>ElementVector</tt>.
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<p> <pre> const double EPSILON = 0.0000001; // Must be > INVALID.
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</pre>
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<p> Elements may only have positive values. Because we hold values as
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doubles we cannot readily compare them with zero. Instead we specify a
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value, <tt>EPSILON</tt>, which is close to zero, and consider any value
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greater than <tt>EPSILON</tt> to be positive and valid.
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<p> <pre> class Element
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{
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public:
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enum { INVALID = -1 };
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enum { NO_PROPORTION = -1 };
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enum { MAX_PROPOINTS = 3 }; // One proportional point per chart type
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</pre>
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<p> We define three public enums for <tt>Element</tt>s. <tt>INVALID</tt> is used by
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the isValid() function. It is useful because we are going to use a
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fixed size vector of <tt>Element</tt>s, and can mark unused <tt>Element</tt>s by
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giving them <tt>INVALID</tt> values. The <tt>NO_PROPORTION</tt> enum is used to
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signify that the user has not positioned the Element's label; any
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positive proportion value is taken to be the text element's position
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proportional to the canvas's size.
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<p> If we stored each label's actual x and y position, then every time the
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user resized the main window (and therefore the canvas), the text
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would retain its original (now incorrect) position. So instead of
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storing absolute (x, y) positions we store <em>proportional</em> positions,
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i.e. x/width and y/height. We can then multiply these positions by
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the current width and height respectively when we come to draw the
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text and the text will be positioned correctly regardless of any
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resizing. For example, if a label has an x position of 300 and the
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canvas is 400 pixels wide, the proportional x value is 300/400 = 0.75.
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<p> The <tt>MAX_PROPOINTS</tt> enum is problematic. We need to store the x and y
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proportions for the text label for every chart type. And we have
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chosen to store these proportions in a fixed-size array. Because of
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this we must specify the maximum number of proportion pairs needed.
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This value must be changed if we change the number of chart types,
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which means that the <tt>Element</tt> class is strongly coupled to the
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number of chart types provided by the <tt>ChartForm</tt> class. In a
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larger application we might have used a vector to store these points
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and dynamically resized it depending on how many chart types are
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available.
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<p> <pre> Element( double value = INVALID, QColor valueColor = Qt::gray,
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int valuePattern = Qt::SolidPattern,
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const <a href="qstring.html">QString</a>& label = <a href="qstring.html#QString-null">QString::null</a>,
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<a href="qcolor.html">QColor</a> labelColor = Qt::black ) {
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init( value, valueColor, valuePattern, label, labelColor );
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for ( int i = 0; i < MAX_PROPOINTS * 2; ++i )
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m_propoints[i] = NO_PROPORTION;
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}
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</pre>
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<p> The constructor provides default values for all members of the <tt>Element</tt> class. New elements always have label text with no position.
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We use an init() function because we also provide a set() function
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which works like the constructor apart from leaving the proportional
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positions alone.
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<p> <pre> bool isValid() const { return m_value > EPSILON; }
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</pre>
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<p> Since we are storing <tt>Element</tt>s in a fixed size vector we need to be
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able to check whether a particular element is valid (i.e. should be
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used in calculations and displayed) or not. This is easily achieved
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with the isValid() function.
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<p> (Extracts from <tt>element.cpp</tt>.)
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<p>
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<pre> double Element::proX( int index ) const
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{
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<a href="qapplication.html#Q_ASSERT">Q_ASSERT</a>(index >= 0 && index < MAX_PROPOINTS);
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return m_propoints[2 * index];
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}
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</pre>
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<p> Getters and setters are provided for all the members of <tt>Element</tt>.
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The proX() and proY() getters and the setProX() and setProY() setters
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take an index which identifies the type of chart the proportional
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position applies to. This means that the user can have labels
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positioned separately for the same data set for a vertical bar chart,
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a horizontal bar chart and for a pie chart. Note also that we use the
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<tt>Q_ASSERT</tt> macro to provide pre-condition tests on the chart type
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index; (see <a href="debug.html">Debugging</a>).
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<p> <h2> Reading and Writing Data Elements
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</h2>
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<a name="1"></a><p> (Extracts from <tt>element.h</tt>.)
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<p>
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<pre> QTextStream &operator<<( <a href="qtextstream.html">QTextStream</a>&, const Element& );
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QTextStream &operator>>( <a href="qtextstream.html">QTextStream</a>&, Element& );
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</pre>
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<p> To make our <tt>Element</tt> class more self-contained we provide overloads
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for the << and >> operators so that <tt>Element</tt>s may be written to
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and read from text streams. We could just as easily have used binary
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streams, but using text makes it possible for users to manipulate
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their data using a text editor and makes it easier to generate and
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filter the data using a scripting language.
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<p> (Extracts from <tt>element.cpp</tt>.)
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<p>
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<pre> #include "element.h"
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#include <<a href="qstringlist-h.html">qstringlist.h</a>>
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#include <<a href="qtextstream-h.html">qtextstream.h</a>>
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</pre>
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<p> Our implementation of the operators requires the inclusion of <a href="qtextstream-h.html">qtextstream.h</a> and <a href="qstringlist-h.html">qstringlist.h</a>.
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<p> <pre> const char FIELD_SEP = ':';
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const char PROPOINT_SEP = ';';
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const char XY_SEP = ',';
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</pre>
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<p> The format we are using to store the data is colon separated fields
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and newline separated records. The proportional points are semi-colon
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separated, with their x, y pairs being comma separated. The field
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order is value, value color, value pattern, label color, label points,
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label text. For example:
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<pre>
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20:#ff0000:14:#000000:0.767033,0.412946;0,0.75;0,0:Red :with colons:!
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70:#00ffff:2:#ffff00:0.450549,0.198661;0.198516,0.125954;0,0.198473:Cyan
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35:#0000ff:8:#555500:0.10989,0.299107;0.397032,0.562977;0,0.396947:Blue
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55:#ffff00:1:#000080:0.0989011,0.625;0.595547,0.312977;0,0.59542:Yellow
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80:#ff00ff:1:#000000:0.518681,0.694196;0.794063,0;0,0.793893:Magenta or Violet
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</pre>
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<p> There's no problem having whitespace and field separators in label
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text due to the way we read <tt>Element</tt> data.
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<p> <pre> QTextStream &operator<<( <a href="qtextstream.html">QTextStream</a> &s, const Element &element )
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{
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s << element.value() << FIELD_SEP
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<< element.valueColor().name() << FIELD_SEP
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<< element.valuePattern() << FIELD_SEP
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<< element.labelColor().name() << FIELD_SEP;
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for ( int i = 0; i < Element::MAX_PROPOINTS; ++i ) {
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s << element.proX( i ) << XY_SEP << element.proY( i );
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s << ( i == Element::MAX_PROPOINTS - 1 ? FIELD_SEP : PROPOINT_SEP );
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}
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s << element.label() << '\n';
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return s;
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}
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</pre>
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<p> Writing elements is straight-forward. Each member is written followed
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by a field separator. The points are written as comma separated (<tt>XY_SEP</tt>) x, y pairs, each pair separated by the <tt>PROPOINT_SEP</tt>
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separator. The final field is the label followed by a newline.
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<p> <pre> QTextStream &operator>>( <a href="qtextstream.html">QTextStream</a> &s, Element &element )
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{
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<a name="x2553"></a> <a href="qstring.html">QString</a> data = s.<a href="qtextstream.html#readLine">readLine</a>();
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element.setValue( Element::INVALID );
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int errors = 0;
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bool ok;
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<a name="x2552"></a> <a href="qstringlist.html">QStringList</a> fields = QStringList::<a href="qstringlist.html#split">split</a>( FIELD_SEP, data );
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<a name="x2555"></a> if ( fields.<a href="qvaluelist.html#count">count</a>() >= 4 ) {
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double value = fields[0].toDouble( &ok );
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if ( !ok )
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errors++;
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<a href="qcolor.html">QColor</a> valueColor = QColor( fields[1] );
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<a name="x2550"></a> if ( !valueColor.<a href="qcolor.html#isValid">isValid</a>() )
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errors++;
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int valuePattern = fields[2].toInt( &ok );
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if ( !ok )
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errors++;
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<a href="qcolor.html">QColor</a> labelColor = QColor( fields[3] );
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if ( !labelColor.<a href="qcolor.html#isValid">isValid</a>() )
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errors++;
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<a href="qstringlist.html">QStringList</a> propoints = QStringList::<a href="qstringlist.html#split">split</a>( PROPOINT_SEP, fields[4] );
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<a name="x2551"></a> <a href="qstring.html">QString</a> label = data.<a href="qstring.html#section">section</a>( FIELD_SEP, 5 );
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if ( !errors ) {
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element.set( value, valueColor, valuePattern, label, labelColor );
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int i = 0;
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<a name="x2554"></a> for ( QStringList::iterator point = propoints.<a href="qvaluelist.html#begin">begin</a>();
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<a name="x2556"></a> i < Element::MAX_PROPOINTS && point != propoints.<a href="qvaluelist.html#end">end</a>();
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++i, ++point ) {
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errors = 0;
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<a href="qstringlist.html">QStringList</a> xy = QStringList::<a href="qstringlist.html#split">split</a>( XY_SEP, *point );
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double x = xy[0].toDouble( &ok );
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if ( !ok || x <= 0.0 || x >= 1.0 )
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errors++;
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double y = xy[1].toDouble( &ok );
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if ( !ok || y <= 0.0 || y >= 1.0 )
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errors++;
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if ( errors )
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x = y = Element::NO_PROPORTION;
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element.setProX( i, x );
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element.setProY( i, y );
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}
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}
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}
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return s;
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}
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</pre>
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<p> To read an element we read one record (i.e. one line). We break the
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data into fields using <a href="qstringlist.html#split">QStringList::split</a>(). Because it is possible
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that a label will contain <tt>FIELD_SEP</tt> characters we use
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<a href="qstring.html#section">QString::section</a>() to extract all the text from the last field to the
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end of the line. If there are enough fields and the value, colors and
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pattern data is valid we use <tt>Element::set()</tt> to write this data into
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the element; otherwise we leave the element <tt>INVALID</tt>. We then
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iterate through the points. If the x and y proportions are valid and
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in range we set them for the element. If one or both proportions is
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invalid they will hold the value zero; this is not suitable so we
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change invalid (and out-of-range) proportional point values to <tt>NO_PROPORTION</tt>.
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<p> Our <tt>Element</tt> class is now sufficient to store, manipulate, read and
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write element data. We have also created an element vector typedef for
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storing a collection of elements.
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<p> We are now ready to create <tt>main.cpp</tt> and the user interface through
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which our users will create, edit and visualise their data sets.
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<p> <center><table cellpadding="4" cellspacing="2" border="0">
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<tr bgcolor="#f0f0f0">
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<td valign="top">For more information on Qt's data streaming facilities see <a href="datastreamformat.html">QDataStream Operators' Formats</a>, and see
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the source code for any of the Qt classes mentioned that are similar
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to what you want to store.
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</table></center>
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<p> <p align="right">
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<a href="tutorial2-02.html">« The 'Big Picture'</a> |
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<a href="tutorial2.html">Contents</a> |
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<a href="tutorial2-04.html">Mainly Easy »</a>
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</p>
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<p>
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<!-- eof -->
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<p><address><hr><div align=center>
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<table width=100% cellspacing=0 border=0><tr>
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<td>Copyright © 2007
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<a href="troll.html">Trolltech</a><td align=center><a href="trademarks.html">Trademarks</a>
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<td align=right><div align=right>Qt 3.3.8</div>
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</table></div></address></body>
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