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| 6 | <title>Data Elements</title>
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| 24 | <font color="#004faf">Main Classes</font></a>
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| 30 | <font color="#004faf">Functions</font></a>
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| 31 | </td>
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| 32 | <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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| 33 |
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| 34 |
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| 35 | <p>
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| 36 | <p> We will use a C++ class called <tt>Element</tt> to provide storage and
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| 37 | access for data elements.
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| 38 | <p> (Extracts from <tt>element.h</tt>.)
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| 39 | <p>
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| 40 |
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| 41 | <pre> private:
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| 42 | </pre><pre> double m_value;
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| 43 | <a href="qcolor.html">QColor</a> m_valueColor;
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| 44 | int m_valuePattern;
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| 45 | <a href="qstring.html">QString</a> m_label;
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| 46 | <a href="qcolor.html">QColor</a> m_labelColor;
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| 47 | double m_propoints[2 * MAX_PROPOINTS];
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| 48 | </pre>
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| 49 | <p> Each element has a value. Each value is displayed graphically with a
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| 50 | particular color and fill pattern. Values may have a label associated
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| 51 | with them; the label is drawn using the label color and for each type
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| 52 | of chart has a (relative) position stored in the <tt>m_propoints</tt> array.
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| 53 | <p>
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| 54 |
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| 55 | <pre> #include <<a href="qcolor-h.html">qcolor.h</a>>
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| 56 | #include <<a href="qnamespace-h.html">qnamespace.h</a>>
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| 57 | #include <<a href="qstring-h.html">qstring.h</a>>
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| 58 | #include <<a href="qvaluevector-h.html">qvaluevector.h</a>>
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| 59 | </pre>
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| 60 | <p> Although the <tt>Element</tt> class is a purely internal data class, it
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| 61 | <tt>#include</tt>s four Qt classes. Qt is often perceived as a purely GUI
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| 62 | toolkit, but it provides many non-GUI classes to support most aspects
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| 63 | of application programming. We use <a href="qcolor-h.html">qcolor.h</a> so that we can hold the
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| 64 | 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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| 65 | <a href="qt.html">Qt</a> superclass which contains various
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| 66 | 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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| 67 | the Qt enum names. An alternative approach would have been to have
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| 68 | 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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| 69 | will <tt>typedef</tt> a vector container for <tt>Element</tt>s, which is why we
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| 70 | pull in the <a href="qvaluevector-h.html">qvaluevector.h</a> header.
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| 71 | <p> <pre> typedef QValueVector<Element> ElementVector;
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| 72 | </pre>
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| 73 | <p> Qt provides a number of containers, some value based like
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| 74 | <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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| 75 | type; we will keep each data set of elements in one <tt>ElementVector</tt>.
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| 76 | <p> <pre> const double EPSILON = 0.0000001; // Must be > INVALID.
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| 77 | </pre>
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| 78 | <p> Elements may only have positive values. Because we hold values as
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| 79 | doubles we cannot readily compare them with zero. Instead we specify a
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| 80 | value, <tt>EPSILON</tt>, which is close to zero, and consider any value
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| 81 | greater than <tt>EPSILON</tt> to be positive and valid.
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| 82 | <p> <pre> class Element
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| 83 | {
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| 84 | public:
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| 85 | enum { INVALID = -1 };
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| 86 | enum { NO_PROPORTION = -1 };
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| 87 | enum { MAX_PROPOINTS = 3 }; // One proportional point per chart type
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| 88 | </pre>
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| 89 | <p> We define three public enums for <tt>Element</tt>s. <tt>INVALID</tt> is used by
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| 90 | the isValid() function. It is useful because we are going to use a
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| 91 | fixed size vector of <tt>Element</tt>s, and can mark unused <tt>Element</tt>s by
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| 92 | giving them <tt>INVALID</tt> values. The <tt>NO_PROPORTION</tt> enum is used to
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| 93 | signify that the user has not positioned the Element's label; any
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| 94 | positive proportion value is taken to be the text element's position
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| 95 | proportional to the canvas's size.
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| 96 | <p> If we stored each label's actual x and y position, then every time the
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| 97 | user resized the main window (and therefore the canvas), the text
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| 98 | would retain its original (now incorrect) position. So instead of
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| 99 | storing absolute (x, y) positions we store <em>proportional</em> positions,
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| 100 | i.e. x/width and y/height. We can then multiply these positions by
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| 101 | the current width and height respectively when we come to draw the
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| 102 | text and the text will be positioned correctly regardless of any
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| 103 | resizing. For example, if a label has an x position of 300 and the
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| 104 | canvas is 400 pixels wide, the proportional x value is 300/400 = 0.75.
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| 105 | <p> The <tt>MAX_PROPOINTS</tt> enum is problematic. We need to store the x and y
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| 106 | proportions for the text label for every chart type. And we have
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| 107 | chosen to store these proportions in a fixed-size array. Because of
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| 108 | this we must specify the maximum number of proportion pairs needed.
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| 109 | This value must be changed if we change the number of chart types,
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| 110 | which means that the <tt>Element</tt> class is strongly coupled to the
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| 111 | number of chart types provided by the <tt>ChartForm</tt> class. In a
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| 112 | larger application we might have used a vector to store these points
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| 113 | and dynamically resized it depending on how many chart types are
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| 114 | available.
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| 115 | <p> <pre> Element( double value = INVALID, QColor valueColor = Qt::gray,
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| 116 | int valuePattern = Qt::SolidPattern,
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| 117 | const <a href="qstring.html">QString</a>& label = <a href="qstring.html#QString-null">QString::null</a>,
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| 118 | <a href="qcolor.html">QColor</a> labelColor = Qt::black ) {
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| 119 | init( value, valueColor, valuePattern, label, labelColor );
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| 120 | for ( int i = 0; i < MAX_PROPOINTS * 2; ++i )
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| 121 | m_propoints[i] = NO_PROPORTION;
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| 122 | }
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| 123 | </pre>
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| 124 | <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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| 125 | We use an init() function because we also provide a set() function
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| 126 | which works like the constructor apart from leaving the proportional
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| 127 | positions alone.
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| 128 | <p> <pre> bool isValid() const { return m_value > EPSILON; }
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| 129 | </pre>
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| 130 | <p> Since we are storing <tt>Element</tt>s in a fixed size vector we need to be
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| 131 | able to check whether a particular element is valid (i.e. should be
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| 132 | used in calculations and displayed) or not. This is easily achieved
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| 133 | with the isValid() function.
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| 134 | <p> (Extracts from <tt>element.cpp</tt>.)
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| 135 | <p>
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| 136 |
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| 137 | <pre> double Element::proX( int index ) const
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| 138 | {
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| 139 | <a href="qapplication.html#Q_ASSERT">Q_ASSERT</a>(index >= 0 && index < MAX_PROPOINTS);
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| 140 | return m_propoints[2 * index];
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| 141 | }
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| 142 | </pre>
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| 143 | <p> Getters and setters are provided for all the members of <tt>Element</tt>.
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| 144 | The proX() and proY() getters and the setProX() and setProY() setters
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| 145 | take an index which identifies the type of chart the proportional
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| 146 | position applies to. This means that the user can have labels
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| 147 | positioned separately for the same data set for a vertical bar chart,
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| 148 | a horizontal bar chart and for a pie chart. Note also that we use the
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| 149 | <tt>Q_ASSERT</tt> macro to provide pre-condition tests on the chart type
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| 150 | index; (see <a href="debug.html">Debugging</a>).
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| 151 | <p> <h2> Reading and Writing Data Elements
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| 152 | </h2>
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| 153 | <a name="1"></a><p> (Extracts from <tt>element.h</tt>.)
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| 154 | <p>
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| 155 |
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| 156 | <pre> QTextStream &operator<<( <a href="qtextstream.html">QTextStream</a>&, const Element& );
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| 157 | QTextStream &operator>>( <a href="qtextstream.html">QTextStream</a>&, Element& );
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| 158 | </pre>
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| 159 | <p> To make our <tt>Element</tt> class more self-contained we provide overloads
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| 160 | for the << and >> operators so that <tt>Element</tt>s may be written to
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| 161 | and read from text streams. We could just as easily have used binary
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| 162 | streams, but using text makes it possible for users to manipulate
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| 163 | their data using a text editor and makes it easier to generate and
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| 164 | filter the data using a scripting language.
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| 165 | <p> (Extracts from <tt>element.cpp</tt>.)
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| 166 | <p>
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| 167 |
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| 168 | <pre> #include "element.h"
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| 169 |
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| 170 | #include <<a href="qstringlist-h.html">qstringlist.h</a>>
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| 171 | #include <<a href="qtextstream-h.html">qtextstream.h</a>>
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| 172 | </pre>
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| 173 | <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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| 174 | <p> <pre> const char FIELD_SEP = ':';
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| 175 | const char PROPOINT_SEP = ';';
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| 176 | const char XY_SEP = ',';
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| 177 | </pre>
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| 178 | <p> The format we are using to store the data is colon separated fields
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| 179 | and newline separated records. The proportional points are semi-colon
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| 180 | separated, with their x, y pairs being comma separated. The field
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| 181 | order is value, value color, value pattern, label color, label points,
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| 182 | label text. For example:
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| 183 | <pre>
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| 184 | 20:#ff0000:14:#000000:0.767033,0.412946;0,0.75;0,0:Red :with colons:!
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| 185 | 70:#00ffff:2:#ffff00:0.450549,0.198661;0.198516,0.125954;0,0.198473:Cyan
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| 186 | 35:#0000ff:8:#555500:0.10989,0.299107;0.397032,0.562977;0,0.396947:Blue
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| 187 | 55:#ffff00:1:#000080:0.0989011,0.625;0.595547,0.312977;0,0.59542:Yellow
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| 188 | 80:#ff00ff:1:#000000:0.518681,0.694196;0.794063,0;0,0.793893:Magenta or Violet
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| 189 | </pre>
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| 190 |
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| 191 | <p> There's no problem having whitespace and field separators in label
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| 192 | text due to the way we read <tt>Element</tt> data.
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| 193 | <p> <pre> QTextStream &operator<<( <a href="qtextstream.html">QTextStream</a> &s, const Element &element )
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| 194 | {
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| 195 | s << element.value() << FIELD_SEP
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| 196 | << element.valueColor().name() << FIELD_SEP
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| 197 | << element.valuePattern() << FIELD_SEP
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| 198 | << element.labelColor().name() << FIELD_SEP;
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| 199 |
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| 200 | for ( int i = 0; i < Element::MAX_PROPOINTS; ++i ) {
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| 201 | s << element.proX( i ) << XY_SEP << element.proY( i );
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| 202 | s << ( i == Element::MAX_PROPOINTS - 1 ? FIELD_SEP : PROPOINT_SEP );
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| 203 | }
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| 204 |
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| 205 | s << element.label() << '\n';
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| 206 |
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| 207 | return s;
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| 208 | }
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| 209 | </pre>
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| 210 | <p> Writing elements is straight-forward. Each member is written followed
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| 211 | 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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| 212 | separator. The final field is the label followed by a newline.
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| 213 | <p> <pre> QTextStream &operator>>( <a href="qtextstream.html">QTextStream</a> &s, Element &element )
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| 214 | {
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| 215 | <a name="x2553"></a> <a href="qstring.html">QString</a> data = s.<a href="qtextstream.html#readLine">readLine</a>();
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| 216 | element.setValue( Element::INVALID );
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| 217 |
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| 218 | int errors = 0;
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| 219 | bool ok;
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| 220 |
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| 221 | <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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| 222 | <a name="x2555"></a> if ( fields.<a href="qvaluelist.html#count">count</a>() >= 4 ) {
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| 223 | double value = fields[0].toDouble( &ok );
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| 224 | if ( !ok )
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| 225 | errors++;
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| 226 | <a href="qcolor.html">QColor</a> valueColor = QColor( fields[1] );
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| 227 | <a name="x2550"></a> if ( !valueColor.<a href="qcolor.html#isValid">isValid</a>() )
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| 228 | errors++;
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| 229 | int valuePattern = fields[2].toInt( &ok );
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| 230 | if ( !ok )
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| 231 | errors++;
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| 232 | <a href="qcolor.html">QColor</a> labelColor = QColor( fields[3] );
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| 233 | if ( !labelColor.<a href="qcolor.html#isValid">isValid</a>() )
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| 234 | errors++;
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| 235 | <a href="qstringlist.html">QStringList</a> propoints = QStringList::<a href="qstringlist.html#split">split</a>( PROPOINT_SEP, fields[4] );
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| 236 | <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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| 237 |
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| 238 | if ( !errors ) {
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| 239 | element.set( value, valueColor, valuePattern, label, labelColor );
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| 240 | int i = 0;
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| 241 | <a name="x2554"></a> for ( QStringList::iterator point = propoints.<a href="qvaluelist.html#begin">begin</a>();
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| 242 | <a name="x2556"></a> i < Element::MAX_PROPOINTS && point != propoints.<a href="qvaluelist.html#end">end</a>();
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| 243 | ++i, ++point ) {
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| 244 | errors = 0;
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| 245 | <a href="qstringlist.html">QStringList</a> xy = QStringList::<a href="qstringlist.html#split">split</a>( XY_SEP, *point );
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| 246 | double x = xy[0].toDouble( &ok );
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| 247 | if ( !ok || x <= 0.0 || x >= 1.0 )
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| 248 | errors++;
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| 249 | double y = xy[1].toDouble( &ok );
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| 250 | if ( !ok || y <= 0.0 || y >= 1.0 )
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| 251 | errors++;
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| 252 | if ( errors )
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| 253 | x = y = Element::NO_PROPORTION;
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| 254 | element.setProX( i, x );
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| 255 | element.setProY( i, y );
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| 256 | }
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| 257 | }
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| 258 | }
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| 259 |
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| 260 | return s;
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| 261 | }
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| 262 | </pre>
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| 263 | <p> To read an element we read one record (i.e. one line). We break the
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| 264 | data into fields using <a href="qstringlist.html#split">QStringList::split</a>(). Because it is possible
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| 265 | that a label will contain <tt>FIELD_SEP</tt> characters we use
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| 266 | <a href="qstring.html#section">QString::section</a>() to extract all the text from the last field to the
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| 267 | end of the line. If there are enough fields and the value, colors and
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| 268 | pattern data is valid we use <tt>Element::set()</tt> to write this data into
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| 269 | the element; otherwise we leave the element <tt>INVALID</tt>. We then
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| 270 | iterate through the points. If the x and y proportions are valid and
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| 271 | in range we set them for the element. If one or both proportions is
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| 272 | invalid they will hold the value zero; this is not suitable so we
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| 273 | change invalid (and out-of-range) proportional point values to <tt>NO_PROPORTION</tt>.
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| 274 | <p> Our <tt>Element</tt> class is now sufficient to store, manipulate, read and
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| 275 | write element data. We have also created an element vector typedef for
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| 276 | storing a collection of elements.
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| 277 | <p> We are now ready to create <tt>main.cpp</tt> and the user interface through
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| 278 | which our users will create, edit and visualise their data sets.
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| 279 | <p> <center><table cellpadding="4" cellspacing="2" border="0">
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| 280 | <tr bgcolor="#f0f0f0">
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| 281 | <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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| 282 | the source code for any of the Qt classes mentioned that are similar
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| 283 | to what you want to store.
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| 284 | </table></center>
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| 285 | <p> <p align="right">
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| 286 | <a href="tutorial2-02.html">« The 'Big Picture'</a> |
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| 287 | <a href="tutorial2.html">Contents</a> |
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| 288 | <a href="tutorial2-04.html">Mainly Easy »</a>
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| 289 | </p>
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| 290 | <p>
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| 291 | <!-- eof -->
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| 292 | <p><address><hr><div align=center>
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| 293 | <table width=100% cellspacing=0 border=0><tr>
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| 294 | <td>Copyright © 2007
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| 295 | <a href="troll.html">Trolltech</a><td align=center><a href="trademarks.html">Trademarks</a>
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| 296 | <td align=right><div align=right>Qt 3.3.8</div>
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| 297 | </table></div></address></body>
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| 298 | </html>
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