📄 sec2-3.htm
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<body bgcolor="#DDFFEE" lang=ZH-TW link=blue vlink=purple style='tab-interval:
21.0pt;text-justify-trim:punctuation'>
<div class=Section1 style='layout-grid:15.6pt'>
<p class=MsoNormal align=center style='text-align:center'><b><span lang=ZH-CN
style='font-size:22.0pt;font-family:华文细黑;color:#333399'>选择结构</span></b><b><span
lang=EN-US style='font-size:22.0pt;font-family:华文细黑;color:#333399'><o:p></o:p></span></b></p>
<p class=MsoNormal align=center style='margin-top:15.6pt;margin-right:0cm;
margin-bottom:15.6pt;margin-left:0cm;mso-para-margin-top:1.0gd;mso-para-margin-right:
0cm;mso-para-margin-bottom:1.0gd;mso-para-margin-left:0cm;text-align:center;
mso-char-indent-size:12pt'><a name="第2_3_1"><span lang=EN-US style='font-size:
16.0pt;mso-bidi-font-size:12.0pt;mso-fareast-font-family:楷体_GB2312'>2.3.1</span></a><span
lang=EN-US style='font-size:16.0pt;mso-bidi-font-size:12.0pt;mso-fareast-font-family:
楷体_GB2312'> </span><span lang=ZH-CN style='font-size:16.0pt;mso-bidi-font-size:
12.0pt;font-family:楷体_GB2312;mso-ascii-font-family:"Times New Roman";
color:red'>块</span><span lang=EN-US style='font-size:16.0pt;mso-bidi-font-size:
12.0pt;mso-fareast-font-family:楷体_GB2312;color:red'>IF</span><span lang=ZH-CN
style='font-size:16.0pt;mso-bidi-font-size:12.0pt;font-family:楷体_GB2312;
mso-ascii-font-family:"Times New Roman";color:red'>构造</span><b><span
lang=EN-US style='font-size:16.0pt;mso-bidi-font-size:12.0pt;mso-fareast-font-family:
楷体_GB2312'><o:p></o:p></span></b></p>
<p style='mso-char-indent-size:10.5pt'><a name="第2_3_1a"><span lang=EN-US
style='font-family:宋体;color:red'>a)</span></a><span lang=EN-US
style='font-family:宋体;color:red'> </span><span lang=ZH-CN style='font-family:
宋体;color:red'>块</span><span lang=EN-US style='font-family:宋体;color:red'>IF</span><span
lang=ZH-CN style='font-family:宋体;color:red'>的组成和执行</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'>块</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'>的一般形式可写成：</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: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'>IF(</span><span lang=ZH-CN style='font-size:12.0pt;
font-family:宋体;color:#FF9900'>逻辑表达式</span><span lang=EN-US style='font-size:
12.0pt;font-family:宋体;color:#FF9900'>) THEN<span style='mso-tab-count:2'> </span></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'>IF</span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>语句</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:24.0pt;mso-char-indent-count:
2.0;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<span style='mso-tab-count:5'> </span></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'> 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;color:#FF9900'><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'>ELSE<o:p></o:p></span></p>
<p class=MsoNormal style='margin-left:45.6pt;text-indent:24.0pt;mso-char-indent-count:
2.0;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<span style='mso-tab-count:5'> </span></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'> else</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;color:#FF9900'><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'>END IF<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'>块</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'>语句不是一个单独语句。块</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'>语句、</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'>语句和</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'>ENDIF</span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>语句只能用在块</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'>中而不能单独使用，它们必须和块</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'>中其它语句联系起来共同起作用。可以说一个块</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'>是一个语句块</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'>(</span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>决不能只包括一个语句</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'>)</span><span lang=ZH-CN
style='font-size:12.0pt;font-family:宋体'>，用来实现选择结构。但是，块</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'>可以不包含</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'>语句和</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。</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'>块是当块</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'>语句中的逻辑表达式为真时执行的语句组。</span><span
lang=EN-US style='font-size:12.0pt;font-family:宋体'><a
href="example/e_231_01.f90">[e_231_01.f90]</a><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'>块</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'>的执行步骤为：先执行块</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'>语句，求出逻辑表达式的值，如果此值为“真”，则将流程转到</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。执行</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'>块中各个执行语句。执行完</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'>块后跳过</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'>语句和</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，流程转到</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'>语句处。如果逻辑表达式的值为“假”，则流程跳过</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，转到</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'>语句及</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。执行完</span><span lang=EN-US
style='font-size:12.0pt;font-family:宋体'>else</span><span lang=ZH-CN
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