📄 sec2-3.htm
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style='font-size:12.0pt;font-family:宋体'><o:p></o:p></span></p>
<p class=MsoNormal style='margin-left:22.8pt;text-indent:24.0pt;mso-char-indent-count:
2.0;mso-char-indent-size:12pt'><span lang=EN-US style='font-size:12.0pt;
font-family:宋体;color:#FF9900'>[</span><span lang=ZH-CN style='font-size:12.0pt;
font-family:宋体;color:#FF9900'>构造名</span><span lang=EN-US style='font-size:12.0pt;
font-family:宋体;color:#FF9900'>:] IF(e<sub>1</sub>) THEN<o:p></o:p></span></p>
<p class=MsoNormal style='margin-left:45.6pt;text-indent:89.05pt;mso-char-indent-count:
7.42;mso-char-indent-size:12pt'><span lang=ZH-CN style='font-size:12.0pt;
font-family:宋体;color:#FF9900'>块</span><span lang=EN-US style='font-size:12.0pt;
font-family:宋体;color:#FF9900'>1<o:p></o:p></span></p>
<p class=MsoNormal style='margin-left:45.6pt;text-indent:60.7pt;mso-char-indent-count:
5.06;mso-char-indent-size:12pt'><span lang=EN-US style='font-size:12.0pt;
font-family:宋体;color:#FF9900'>[ELSE IF(e<sub>2</sub>) THEN [</span><span
lang=ZH-CN style='font-size:12.0pt;font-family:宋体;color:#FF9900'>构造名</span><span
lang=EN-US style='font-size:12.0pt;font-family:宋体;color:#FF9900'>]<o:p></o:p></span></p>
<p class=MsoNormal style='margin-left:45.6pt;text-indent:89.05pt;mso-char-indent-count:
7.42;mso-char-indent-size:12pt'><span lang=ZH-CN style='font-size:12.0pt;
font-family:宋体;color:#FF9900'>块</span><span lang=EN-US style='font-size:12.0pt;
font-family:宋体;color:#FF9900'>2]<o:p></o:p></span></p>
<p class=MsoNormal style='margin-left:45.6pt;text-indent:89.05pt;mso-char-indent-count:
7.42'><span lang=ZH-CN style='font-size:12.0pt;font-family:宋体;color:#FF9900'>…</span><span
lang=EN-US style='font-size:12.0pt;font-family:宋体;color:#FF9900'><o:p></o:p></span></p>
<p class=MsoNormal style='margin-left:45.6pt;text-indent:60.7pt;mso-char-indent-count:
5.06;mso-char-indent-size:12pt'><span lang=EN-US style='font-size:12.0pt;
font-family:宋体;color:#FF9900'>[ELSE IF(e<sub>n</sub>) THEN [</span><span
lang=ZH-CN style='font-size:12.0pt;font-family:宋体;color:#FF9900'>构造名</span><span
lang=EN-US style='font-size:12.0pt;font-family:宋体;color:#FF9900'>]<o:p></o:p></span></p>
<p class=MsoNormal style='margin-left:45.6pt;text-indent:89.05pt;mso-char-indent-count:
7.42;mso-char-indent-size:12pt'><span lang=ZH-CN style='font-size:12.0pt;
font-family:宋体;color:#FF9900'>块</span><span lang=EN-US style='font-size:12.0pt;
font-family:宋体;color:#FF9900'>n]<o:p></o:p></span></p>
<p class=MsoNormal style='margin-left:45.6pt;text-indent:60.7pt;mso-char-indent-count:
5.06'><span lang=EN-US style='font-size:12.0pt;font-family:宋体;color:#FF9900'>[ELSE
[</span><span lang=ZH-CN style='font-size:12.0pt;font-family:宋体;color:#FF9900'>构造名</span><span
lang=EN-US style='font-size:12.0pt;font-family:宋体;color:#FF9900'>]<o:p></o:p></span></p>
<p class=MsoNormal style='margin-left:45.6pt;text-indent:89.05pt;mso-char-indent-count:
7.42;mso-char-indent-size:12pt'><span lang=ZH-CN style='font-size:12.0pt;
font-family:宋体;color:#FF9900'>块</span><span lang=EN-US style='font-size:12.0pt;
font-family:宋体;color:#FF9900'>n+1]<o:p></o:p></span></p>
<p class=MsoNormal style='margin-left:45.6pt;text-indent:60.7pt;mso-char-indent-count:
5.06;mso-char-indent-size:12pt'><span lang=EN-US style='font-size:12.0pt;
font-family:宋体;color:#FF9900'>END IF [</span><span lang=ZH-CN style='font-size:
12.0pt;font-family:宋体;color:#FF9900'>构造名</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体;color:#FF9900'>]<o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:24.0pt;mso-char-indent-count:2.0;
line-height:17.0pt;mso-line-height-rule:exactly;mso-char-indent-size:12pt'><span
lang=ZH-CN style='font-size:12.0pt;font-family:宋体'>其中</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'>e<sub>1</sub></span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>，…，</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'>e<sub>n</sub></span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>是逻辑表达式，指出各种条件。语句块是一组语句，内容是当</span><span
lang=EN-US style='font-size:12.0pt;font-family:宋体'>e</span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>成立（</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'>e</span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>逻辑表达式值为真）时要执行的算法。</span><span
lang=EN-US style='font-size:12.0pt;font-family:宋体'>IF</span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>构造的控制机制为：</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'>(1) </span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>检查</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'>e<sub>1</sub></span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>真否。真，执行块</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'>1</span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>，绕过其它块，直接转出口语句</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'>END IF</span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>处出口；假，跳过块</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'>1</span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>，检查</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'>e<sub>2</sub></span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>真否。</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'>(2) e<sub>2</sub></span><span
lang=ZH-CN style='font-size:12.0pt;font-family:宋体'>真，执行块</span><span
lang=EN-US style='font-size:12.0pt;font-family:宋体'>2</span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>，而后直接转出口；假，跳过块</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'>2</span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>，检查</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'>e<sub>3</sub></span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>，以此类推；</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'>(3) </span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>如果所有</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'>ELSE IF</span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>语句的</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'>e</span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>都是假，那么必须执行</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'>ELSE</span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>语句下的块</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'>n</span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>＋</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'>1</span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>。如果</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'>IF</span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>构造中没有</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'>ELSE</span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>语句，就什么也不执行，转出口。由此可见，执行</span><span
lang=EN-US style='font-size:12.0pt;font-family:宋体'>IF</span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>构造，自上而下顺次检查每块前面的条件，满足条件的就执行该条件下面的块，执行完该块后立即转向出口。以后即使还有块满足条件（条件可以相交），也不予理睬。因此，一个</span><span
lang=EN-US style='font-size:12.0pt;font-family:宋体'>IF</span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>构造中最多只执行一块，也可能一块也不执行。</span><span
lang=EN-US style='font-size:12.0pt;font-family:宋体'><o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:24.0pt;mso-char-indent-count:2.0;
line-height:17.0pt;mso-line-height-rule:exactly'><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>例：输入实数</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'>x</span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>，求下面的三角波脉冲函数</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'>F(x)</span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>的值。</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'><a href="example/e_231_03.f90">[e_231_03.f90]</a><o:p></o:p></span></p>
<p class=MsoNormal style='margin-left:48.45pt'><span lang=EN-US
style='position:relative;top:25.0pt;mso-text-raise:-25.0pt'><!--[if gte vml 1]><v:shape
id="_x0000_i1026" type="#_x0000_t75" style='width:191.25pt;height:56.25pt'
o:ole="">
<v:imagedata src="Sec2-3.files/image005.wmz" o:title=""/>
</v:shape><![endif]--><![if !vml]><img border=0 width=255 height=75
src="Sec2-3.files/image007.gif" v:shapes="_x0000_i1026"><![endif]><!--[if gte mso 9]><xml>
<o:OLEObject Type="Embed" ProgID="Equation.3" ShapeID="_x0000_i1026"
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<p><a name="第2_3_1d"><span lang=EN-US style='font-family:宋体;color:red'>d)</span></a><span
lang=EN-US style='font-family:宋体;color:red'> IF</span><span lang=ZH-CN
style='font-family:宋体;color:red'>构造的缺省形式</span><span lang=EN-US
style='font-family:宋体;color:red'><o:p></o:p></span></p>
<p class=MsoNormal style='text-indent:24.0pt;mso-char-indent-count:2.0;
line-height:17.0pt;mso-line-height-rule:exactly;mso-char-indent-size:12pt'><span
lang=ZH-CN style='font-size:12.0pt;font-family:宋体'>在</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'>IF</span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>构造中，</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'>then</span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>块、</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'>else if</span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>块、</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'>else</span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>块都可以缺省。缺省有两种形式，一种是构造中出现该类语句，但后面没有算法语句块；一种是连该类语句一起省略。这两种情况使执行时可能结果不同，应按控制机理仔细分析。</span><span
lang=EN-US style='font-size:12.0pt;font-family:宋体'><o:p></o:p></span></p>
<p class=MsoNormal style='margin-left:49.65pt;text-indent:-24.0pt;line-height:
17.0pt;mso-line-height-rule:exactly;mso-list:l0 level1 lfo2;tab-stops:list 49.65pt 72.0pt'><![if !supportLists]><span
lang=EN-US style='font-size:12.0pt;font-family:Symbol;mso-fareast-font-family:
Symbol;mso-bidi-font-family:Symbol'><span style='mso-list:Ignore'><img
width=13 height=13 src="Sec2-3.files/image001.gif" alt="*"><span
style='font:7.0pt "Times New Roman"'>
</span></span></span><![endif]><span lang=ZH-CN style='font-size:12.0pt;
font-family:宋体;color:purple'>只有</span><span lang=EN-US style='font-size:12.0pt;
font-family:宋体;color:purple'>then</span><span lang=ZH-CN style='font-size:12.0pt;
font-family:宋体;color:purple'>块</span><span lang=EN-US style='font-size:12.0pt;
font-family:宋体;color:purple'><o:p></o:p></span></p>
<p class=MsoNormal style='margin-left:49.55pt;mso-para-margin-left:4.72gd;
text-indent:24.0pt;mso-char-indent-count:2.0;line-height:15.0pt;mso-line-height-rule:
exactly'><span lang=EN-US style='font-size:12.0pt;font-family:宋体'>IF(e) THEN<o:p></o:p></span></p>
<p class=MsoNormal style='margin-left:49.55pt;mso-para-margin-left:4.72gd;
text-indent:49.7pt;mso-char-indent-count:4.14;line-height:15.0pt;mso-line-height-rule:
exactly'><span lang=ZH-CN style='font-size:12.0pt;font-family:宋体'>块</span><span
lang=EN-US style='font-size:12.0pt;font-family:宋体'>1<o:p></o:p></span></p>
<p class=MsoNormal style='margin-left:49.55pt;mso-para-margin-left:4.72gd;
text-indent:24.0pt;mso-char-indent-count:2.0;line-height:15.0pt;mso-line-height-rule:
exactly'><span lang=EN-US style='font-size:12.0pt;font-family:宋体'>END IF<o:p></o:p></span></p>
<p class=MsoNormal style='margin-left:26.25pt;mso-para-margin-left:2.5gd;
text-indent:24.0pt;mso-char-indent-count:2.0;line-height:17.0pt;mso-line-height-rule:
exactly'><span lang=ZH-CN style='font-size:12.0pt;font-family:宋体'>当</span><span
lang=EN-US style='font-size:12.0pt;font-family:宋体'>e</span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>真，执行块</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'>1</span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>，</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'>e</span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>假，则不执行任何语句，此时</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'>IF</span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>构造相当于没有。</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'><o:p></o:p></span></p>
<p class=MsoNormal style='margin-left:49.65pt;text-indent:-24.0pt;line-height:
17.0pt;mso-line-height-rule:exactly;mso-list:l0 level1 lfo2;tab-stops:list 49.65pt 72.0pt'><![if !supportLists]><span
lang=EN-US style='font-size:12.0pt;font-family:Symbol;mso-fareast-font-family:
Symbol;mso-bidi-font-family:Symbol'><span style='mso-list:Ignore'><img
width=13 height=13 src="Sec2-3.files/image001.gif" alt="*"><span
style='font:7.0pt "Times New Roman"'>
</span></span></span><![endif]><span lang=ZH-CN style='font-size:12.0pt;
font-family:宋体;color:purple'>缺省</span><span lang=EN-US style='font-size:12.0pt;
font-family:宋体;color:purple'>else if</span><span lang=ZH-CN style='font-size:
12.0pt;font-family:宋体;color:purple'>块</span><span lang=EN-US style='font-size:
12.0pt;font-family:宋体;color:purple'><o:p></o:p></span></p>
<p class=MsoNormal style='margin-left:26.25pt;mso-para-margin-left:2.5gd;
text-indent:24.0pt;mso-char-indent-count:2.0;line-height:17.0pt;mso-line-height-rule:
exactly'><span lang=ZH-CN style='font-size:12.0pt;font-family:宋体'>如果连</span><span
lang=EN-US style='font-size:12.0pt;font-family:宋体'>ELSE IF</span><span
lang=ZH-CN style='font-size:12.0pt;font-family:宋体'>语句省去：</span><span
lang=EN-US style='font-size:12.0pt;font-family:宋体'><o:p></o:p></span></p>
<p class=MsoNormal style='margin-left:49.55pt;mso-para-margin-left:4.72gd;
text-indent:24.0pt;mso-char-indent-count:2.0;line-height:15.0pt;mso-line-height-rule:
exactly'><span lang=EN-US style='font-size:12.0pt;font-family:宋体'>IF(e) THEN<o:p></o:p></span></p>
<p class=MsoNormal style='margin-left:49.55pt;mso-para-margin-left:4.72gd;
text-indent:49.7pt;mso-char-indent-count:4.14;line-height:15.0pt;mso-line-height-rule:
exactly'><span lang=ZH-CN style='font-size:12.0pt;font-family:宋体'>块</span><span
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