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mso-char-indent-size:0cm'><![if !supportLists]><span lang=EN-US
style='font-family:Symbol;mso-fareast-font-family:Symbol;mso-bidi-font-family:
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height=13 src="Sec2-1.files/image002.gif" alt="*"><span style='font:7.0pt "Times New Roman"'>
</span></span></span><![endif]><span lang=ZH-CN style='color:#FF6600;
mso-font-kerning:0pt'>起止框</span><span lang=ZH-CN style='mso-font-kerning:0pt'>（圆弧形框），表示流程开始或结束。</span><span
lang=EN-US style='mso-font-kerning:0pt'><o:p></o:p></span></p>
<p class=MsoBodyTextIndent2 style='margin-left:48.0pt;text-indent:-24.0pt;
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mso-char-indent-size:0cm'><![if !supportLists]><span lang=EN-US
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</span></span></span><![endif]><span lang=ZH-CN style='color:#FF6600;
mso-font-kerning:0pt'>连接点</span><span lang=ZH-CN style='mso-font-kerning:0pt'>（圆圈），用于将画在不同地方的流程线连接起来。如图中有两个以</span><span
lang=EN-US style='mso-font-kerning:0pt'>1</span><span lang=ZH-CN
style='mso-font-kerning:0pt'>标志的连接点</span><span lang=EN-US style='mso-font-kerning:
0pt'>(</span><span lang=ZH-CN style='mso-font-kerning:0pt'>在连接点圈中写上“</span><span
lang=EN-US style='mso-font-kerning:0pt'>l</span><span lang=ZH-CN
style='mso-font-kerning:0pt'>”</span><span lang=EN-US style='mso-font-kerning:
0pt'>)</span><span lang=ZH-CN style='mso-font-kerning:0pt'>则表示这两个点是连接在一起的，相当于一个点一样。用连接点，可以避免流程线的交叉或过长，使流程图清晰。</span><span
lang=EN-US style='mso-font-kerning:0pt'><o:p></o:p></span></p>
<p class=MsoBodyTextIndent2 style='margin-left:48.0pt;text-indent:-24.0pt;
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mso-char-indent-size:0cm'><![if !supportLists]><span lang=EN-US
style='font-family:Symbol;mso-fareast-font-family:Symbol;mso-bidi-font-family:
Symbol;mso-font-kerning:0pt'><span style='mso-list:Ignore'><img width=13
height=13 src="Sec2-1.files/image002.gif" alt="*"><span style='font:7.0pt "Times New Roman"'>
</span></span></span><![endif]><span lang=ZH-CN style='color:#FF6600;
mso-font-kerning:0pt'>流程线</span><span lang=ZH-CN style='mso-font-kerning:0pt'>（指向线），表示流程的路径和方向。</span><span
lang=EN-US style='mso-font-kerning:0pt'><o:p></o:p></span></p>
<p class=MsoBodyTextIndent2 style='margin-left:48.0pt;text-indent:-24.0pt;
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mso-char-indent-size:0cm'><![if !supportLists]><span lang=EN-US
style='font-family:Symbol;mso-fareast-font-family:Symbol;mso-bidi-font-family:
Symbol;mso-font-kerning:0pt'><span style='mso-list:Ignore'><img width=13
height=13 src="Sec2-1.files/image002.gif" alt="*"><span style='font:7.0pt "Times New Roman"'>
</span></span></span><![endif]><span lang=ZH-CN style='color:#FF6600;
mso-font-kerning:0pt'>注释框</span><span lang=EN-US style='mso-font-kerning:0pt'>,
</span><span lang=ZH-CN style='mso-font-kerning:0pt'>是为了对流程图中某些框的操作做必要的补充说明，以帮助阅读流程图的人更好地理解流程图的作用。它不是流程图中必要的部分，不反映流程和操作。</span><span
lang=EN-US style='mso-font-kerning:0pt'><o:p></o:p></span></p>
<p class=MsoBodyTextIndent2 style='line-height:17.0pt;mso-line-height-rule:
exactly'><span lang=ZH-CN style='mso-font-kerning:0pt'>程序框图表示程序内各步骤的内容以及它们的关系和执行的顺序。它说明了程序的逻辑结构。框图应该足够详细，以便可以按照它顺利地写出程序，而不必在编写时临时构思，甚至出现逻辑错误。流程图不仅可以指导编写程序，而且可以在调试程序中用来检查程序的正确性。如果框图是正确的而结果不对，则按照框图逐步检查程序是很容易发现其错误的。流程图还能作为程序说明书的一部分提供给别人，以便帮助别人理解你编写程序的思路和结构。</span><span
lang=EN-US style='mso-font-kerning:0pt'><o:p></o:p></span></p>
<p class=MsoBodyTextIndent2 style='line-height:17.0pt;mso-line-height-rule:
exactly'><span lang=ZH-CN style='mso-font-kerning:0pt'>例：对一个大于或等于</span><span
lang=EN-US style='mso-font-kerning:0pt'>3</span><span lang=ZH-CN
style='mso-font-kerning:0pt'>的正整数，判断它是不是一个素数。</span><span lang=EN-US
style='mso-font-kerning:0pt'><o:p></o:p></span></p>
<p class=MsoBodyTextIndent2 style='line-height:17.0pt;mso-line-height-rule:
exactly'><span lang=ZH-CN style='mso-font-kerning:0pt'>所谓素数，是指除</span><span
lang=EN-US style='mso-font-kerning:0pt'>l</span><span lang=ZH-CN
style='mso-font-kerning:0pt'>和该数本身之外，不能被其它任何整数整除的数。例如，</span><span lang=EN-US
style='mso-font-kerning:0pt'>13</span><span lang=ZH-CN style='mso-font-kerning:
0pt'>是素数，因为它不能被</span><span lang=EN-US style='mso-font-kerning:0pt'>2</span><span
lang=ZH-CN style='mso-font-kerning:0pt'>，</span><span lang=EN-US
style='mso-font-kerning:0pt'>3</span><span lang=ZH-CN style='mso-font-kerning:
0pt'>，</span><span lang=EN-US style='mso-font-kerning:0pt'>4</span><span
lang=ZH-CN style='mso-font-kerning:0pt'>，…，</span><span lang=EN-US
style='mso-font-kerning:0pt'>12</span><span lang=ZH-CN style='mso-font-kerning:
0pt'>整除。</span><span lang=EN-US style='mso-font-kerning:0pt'><o:p></o:p></span></p>
<p class=MsoBodyTextIndent2 style='line-height:17.0pt;mso-line-height-rule:
exactly'><span lang=ZH-CN style='mso-font-kerning:0pt'>判断一个数</span><span
lang=EN-US style='mso-font-kerning:0pt'>N(N</span><span lang=ZH-CN
style='mso-font-kerning:0pt'>＞</span><span lang=EN-US style='mso-font-kerning:
0pt'>3)</span><span lang=ZH-CN style='mso-font-kerning:0pt'>是否素数的方法是很简单的：将</span><span
lang=EN-US style='mso-font-kerning:0pt'>N</span><span lang=ZH-CN
style='mso-font-kerning:0pt'>作为被除数，将</span><span lang=EN-US style='mso-font-kerning:
0pt'>2</span><span lang=ZH-CN style='mso-font-kerning:0pt'>到</span><span
lang=EN-US style='mso-font-kerning:0pt'>(N</span><span lang=ZH-CN
style='mso-font-kerning:0pt'>—</span><span lang=EN-US style='mso-font-kerning:
0pt'>1)</span><span lang=ZH-CN style='mso-font-kerning:0pt'>各个整数轮流作为除数，如果都不能被整除，则</span><span
lang=EN-US style='mso-font-kerning:0pt'>N</span><span lang=ZH-CN
style='mso-font-kerning:0pt'>为素数。算法可以表示如下：</span><span lang=EN-US
style='mso-font-kerning:0pt'><o:p></o:p></span></p>
<div style='mso-element:frame;mso-element-frame-width:197.9pt;mso-element-frame-height:
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<p class=MsoNormal style='tab-stops:45.8pt 91.6pt 137.4pt 183.2pt 229.0pt 274.8pt 320.6pt 366.4pt 412.2pt 458.0pt 503.8pt 549.6pt 595.4pt 641.2pt 687.0pt 732.8pt;
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<p class=MsoBodyTextIndent2 style='line-height:17.0pt;mso-line-height-rule:
exactly'><span lang=ZH-CN style='mso-font-kerning:0pt'>① 输入</span><span
lang=EN-US style='mso-font-kerning:0pt'>N</span><span lang=ZH-CN
style='mso-font-kerning:0pt'>的值。</span><span lang=EN-US style='mso-font-kerning:
0pt'><o:p></o:p></span></p>
<p class=MsoBodyTextIndent2 style='line-height:17.0pt;mso-line-height-rule:
exactly'><span lang=ZH-CN style='mso-font-kerning:0pt'>②</span><span
lang=EN-US style='mso-font-kerning:0pt'> I</span><span lang=ZH-CN
style='mso-font-kerning:0pt'>＝</span><span lang=EN-US style='mso-font-kerning:
0pt'>2</span><span lang=ZH-CN style='mso-font-kerning:0pt'>。</span><span
lang=EN-US style='mso-font-kerning:0pt'><o:p></o:p></span></p>
<p class=MsoBodyTextIndent2 style='line-height:17.0pt;mso-line-height-rule:
exactly'><span lang=ZH-CN style='mso-font-kerning:0pt'>③</span><span
lang=EN-US style='mso-font-kerning:0pt'> N</span><span lang=ZH-CN
style='mso-font-kerning:0pt'>被</span><span lang=EN-US style='mso-font-kerning:
0pt'>I</span><span lang=ZH-CN style='mso-font-kerning:0pt'>除。</span><span
lang=EN-US style='mso-font-kerning:0pt'><o:p></o:p></span></p>
<p class=MsoBodyTextIndent2 style='line-height:17.0pt;mso-line-height-rule:
exactly'><span lang=ZH-CN style='mso-font-kerning:0pt'>④ 如果余数为</span><span
lang=EN-US style='mso-font-kerning:0pt'>0</span><span lang=ZH-CN
style='mso-font-kerning:0pt'>，表示</span><span lang=EN-US style='mso-font-kerning:
0pt'>N</span><span lang=ZH-CN style='mso-font-kerning:0pt'>能被</span><span
lang=EN-US style='mso-font-kerning:0pt'>I</span><span lang=ZH-CN
style='mso-font-kerning:0pt'>整除，则打印</span><span lang=EN-US style='mso-font-kerning:
0pt'>N</span><span lang=ZH-CN style='mso-font-kerning:0pt'>“不是素数”，算法结束。否则继续。</span><span
lang=EN-US style='mso-font-kerning:0pt'><o:p></o:p></span></p>
<p class=MsoBodyTextIndent2 style='line-height:17.0pt;mso-line-height-rule:
exactly'><span lang=ZH-CN style='mso-font-kerning:0pt'>⑤</span><span
lang=EN-US style='mso-font-kerning:0pt'> I</span><span lang=ZH-CN
style='mso-font-kerning:0pt'>＝</span><span lang=EN-US style='mso-font-kerning:
0pt'>I+1</span><span lang=ZH-CN style='mso-font-kerning:0pt'>。</span><span
lang=EN-US style='mso-font-kerning:0pt'><o:p></o:p></span></p>
<p class=MsoBodyTextIndent2 style='line-height:17.0pt;mso-line-height-rule:
exactly'><span lang=ZH-CN style='mso-font-kerning:0pt'>⑥ 如果</span><span
lang=EN-US style='mso-font-kerning:0pt'>I</span><span lang=ZH-CN
style='mso-font-kerning:0pt'>≤</span><span lang=EN-US style='mso-font-kerning:
0pt'>N</span><span lang=ZH-CN style='mso-font-kerning:0pt'>－</span><span
lang=EN-US style='mso-font-kerning:0pt'>l</span><span lang=ZH-CN
style='mso-font-kerning:0pt'>，返回③。否则打印</span><span lang=EN-US style='mso-font-kerning:
0pt'>N</span><span lang=ZH-CN style='mso-font-kerning:0pt'>“是素数”。然后结束。</span><span
lang=EN-US style='mso-font-kerning:0pt'><o:p></o:p></span></p>
<p class=MsoBodyTextIndent2 style='line-height:17.0pt;mso-line-height-rule:
exactly'><span lang=ZH-CN style='mso-font-kerning:0pt'>实际上．</span><span
lang=EN-US style='mso-font-kerning:0pt'>N</span><span lang=ZH-CN
style='mso-font-kerning:0pt'>不必被</span><span lang=EN-US style='mso-font-kerning:
0pt'>2</span><span lang=ZH-CN style='mso-font-kerning:0pt'>到</span><span
lang=EN-US style='mso-font-kerning:0pt'>(N</span><span lang=ZH-CN
style='mso-font-kerning:0pt'>一</span><span lang=EN-US style='mso-font-kerning:
0pt'>1)</span><span lang=ZH-CN style='mso-font-kerning:0pt'>的整数除，只需被</span><span
lang=EN-US style='mso-font-kerning:0pt'>2</span><span lang=ZH-CN
style='mso-font-kerning:0pt'>到</span><span lang=EN-US style='mso-font-kerning:
0pt'>N/2</span><span lang=ZH-CN style='mso-font-kerning:0pt'>间整数除即可，甚至只需被</span><span
lang=EN-US style='mso-font-kerning:0pt'>2</span><span lang=ZH-CN
style='mso-font-kerning:0pt'>到</span><span lang=EN-US style='mso-font-kerning:
0pt'><span style='position:relative;top:4.0pt;mso-text-raise:-4.0pt'><!--[if gte vml 1]><v:shape
id="_x0000_i1026" type="#_x0000_t75" style='width:23.25pt;height:18pt' o:ole="">
<v:imagedata src="Sec2-1.files/image026.wmz" o:title=""/>
</v:shape><![endif]--><![if !vml]><img width=31 height=24
src="Sec2-1.files/image010.gif" v:shapes="_x0000_i1026"><![endif]></span><!--[if gte mso 9]><xml>
<o:OLEObject Type="Embed" ProgID="Equation.3" ShapeID="_x0000_i1026"
DrawAspect="Content" ObjectID="_1069676436">
</o:OLEObject>
</xml><![endif]--></span><span lang=ZH-CN style='mso-font-kerning:0pt'>之间的整数除即可。例如，判断</span><span
lang=EN-US style='mso-font-kerning:0pt'>13</span><span lang=ZH-CN
style='mso-font-kerning:0pt'>是否素数，只需将</span><span lang=EN-US style='mso-font-kerning:
0pt'>13</span><span lang=ZH-CN style='mso-font-kerning:0pt'>被</span><span
lang=EN-US style='mso-font-kerning:0pt'>2</span><span lang=ZH-CN
style='mso-font-kerning:0pt'>，</span><span lang=EN-US style='mso-font-kerning:
0pt'>3</span><span lang=ZH-CN style='mso-font-kerning:0pt'>除即可，如都除不尽，</span><span
lang=EN-US style='mso-font-kerning:0pt'>N</span><span lang=ZH-CN
style='mso-font-kerning:0pt'>必为素数。步骤⑥可改为：</span><span lang=EN-US
style='mso-font-kerning:0pt'><o:p></o:p></span></p>
<p class=MsoBodyTextIndent2 style='line-height:17.0pt;mso-line-height-rule:
exactly'><span lang=ZH-CN style='mso-font-kerning:0pt'>⑥：如果</span><span
lang=EN-US style='mso-font-kerning:0pt'>I</span><span lang=ZH-CN
style='mso-font-kerning:0pt'>≤</span><span lang=EN-US style='mso-font-kerning:
0pt'><span style='position:relative;top:4.0pt;mso-text-raise:-4.0pt'><!--[if gte vml 1]><v:shape
id="_x0000_i1027" type="#_x0000_t75" style='width:23.25pt;height:18pt' o:ole="">
<v:imagedata src="Sec2-1.files/image026.wmz" o:title=""/>
</v:shape><![endif]--><![if !vml]><img width=31 height=24
src="Sec2-1.files/image010.gif" v:shapes="_x0000_i1027"><![endif]></span><!--[if gte mso 9]><xml>
<o:OLEObject Type="Embed" ProgID="Equation.3" ShapeID="_x0000_i1027"
DrawAspect="Content" ObjectID="_1069676437">
</o:OLEObject>
</xml><![endif]--></span><span lang=ZH-CN style='mso-font-kerning:0pt'>，返回③。否则算法结束。</span><span
lang=EN-US style='mso-font-kerning:0pt'><o:p></o:p></span></p>
<p class=MsoBodyTextIndent2 style='line-height:17.0pt;mso-line-height-rule:
exactly'><span lang=EN-US style='mso-font-kerning:0pt'>Fortran</span><span
lang=ZH-CN style='mso-font-kerning:0pt'>代码文件为</span><span lang=EN-US
style='mso-font-kerning:0pt'><a href="example/e_212_01.f">[e_212_01.f]</a><a
href="example/e_212_02.f">[e_212_02.f]</a></span><span lang=ZH-CN
style='mso-font-kerning:0pt'>。</span><span lang=EN-US style='mso-font-kerning:
0pt'><o:p></o:p></span></p>
<p class=MsoBodyTextIndent2 style='line-height:17.0pt;mso-line-height-rule:
exactly'><span lang=EN-US style='mso-font-kerning:0pt'><o:p> </o:p></span></p>
<p class=MsoBodyTextIndent2 style='line-height:17.0pt;mso-line-height-rule:
exactly'><span lang=EN-US style='mso-font-kerning:0pt'><o:p> </o:p></span></p>
<p style='margin-left:18.0pt;text-indent:-18.0pt;mso-list:l4 level1 lfo6;
tab-stops:list 18.0pt'><a name="第2_1_2b"></a><![if !supportLists]><span
lang=EN-US style='font-family:宋体;mso-bidi-font-family:宋体;color:red'><span
style='mso-list:Ignore'>a)<span style='font:7.0pt "Times New Roman"'>
</span></span></span><![endif]><span lang=ZH-CN style='font-family:宋体;
color:red'>三种基本结构</span><span lang=EN-US style='font-family:宋体;color:red'><o:p></o:p></span></p>
<p class=MsoBodyTextIndent2 style='line-height:17.0pt;mso-line-height-rule:
exactly'><span lang=ZH-CN style='mso-font-kerning:0pt'>传统的流程图用流程线指出各框的执行顺序，对流程线的使用没有严格限制。因此，使用者可以毫不受限制地使流程随意地转来转去，使流程图变得毫无规律，阅读者要花很大精力去追踪流程，使人难以理解算法的逻辑。如果我们写出的算法能限制流程的无规律任意转向，而像一本书那样，由各章各节顺序组成，那样，阅读起来就很方便，不会有任何困难，只需从头到尾顺序地看下去即可。</span><span
lang=EN-US style='mso-font-kerning:0pt'><o:p></o:p></span></p>
<p class=MsoBodyTextIndent2 style='line-height:17.0pt;mso-line-height-rule:
exactly'><span lang=ZH-CN style='mso-font-kerning:0pt'>为了提高算法的质量，使算法的设计和阅读方便，必须限制箭头的滥用，即不允许无规律地使流程乱转向，只能按顺序地进行下去。但是，算法上难免会包含一些分支和循环，而不可能全部由一个一个框顺序组成。如上例不是由各框顺序进行的，包含一些流程的向前或向后的非顺序转移。为了解决这个问题，人们设想，如果规定出几种基本结构，然后由这些基本结构按一定规律组成一个算法结构，就如同用一些基本预制构件来搭成房屋一样，整个算法的结构是由上而下地将各个基本结构顺序排列起来的。</span><span
lang=EN-US style='mso-font-kerning:0pt'>1966</span><span lang=ZH-CN
style='mso-font-kerning:0pt'>年，</span><span lang=EN-US style='mso-font-kerning:
0pt'>Bohra</span><span lang=ZH-CN style='mso-font-kerning:0pt'>和</span><span
lang=EN-US style='mso-font-kerning:0pt'>Jacoplni</span><span lang=ZH-CN
style='mso-font-kerning:0pt'>提出了以下三种基本结构，用这三种基本结构作为表示一个良好算法的基本单元。</span><span
lang=EN-US style='mso-font-kerning:0pt'><o:p></o:p></span></p>
<p class=MsoBodyTextIndent2 style='margin-left:48.0pt;text-indent:-24.0pt;
mso-char-indent-count:0;line-height:17.0pt;mso-line-height-rule:exactly;
mso-list:l3 level1 lfo12;tab-stops:45.8pt list 48.0pt left 91.6pt 137.4pt 183.2pt 229.0pt 274.8pt 320.6pt 366.4pt 412.2pt 458.0pt 503.8pt 549.6pt 595.4pt 641.2pt 687.0pt 732.8pt;
mso-char-indent-size:0cm'><![if !supportLists]><span lang=EN-US
style='font-family:Symbol;mso-fareast-font-family:Symbol;mso-bidi-font-family:
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