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<div class="section" id="c-c">
<h1>c & c++<a class="headerlink" href="#c-c" title="Permalink to this headline">¶</a></h1>
<div class="section" id="printf-d-d-d-d-i-i-i-i">
<h2>printf(“%d,%d,%d,%d”,i++,++i,i++,i++)<a class="headerlink" href="#printf-d-d-d-d-i-i-i-i" title="Permalink to this headline">¶</a></h2>
<div class="highlight-c"><div class="highlight"><pre><span class="kt">int</span> <span class="n">i</span><span class="o">=</span><span class="mi">5</span><span class="p">;</span>
<span class="n">printf</span><span class="p">(</span><span class="s">"%d,%d,%d,%d"</span><span class="p">,</span><span class="n">i</span><span class="o">++</span><span class="p">,</span><span class="o">++</span><span class="n">i</span><span class="p">,</span><span class="n">i</span><span class="o">++</span><span class="p">,</span><span class="n">i</span><span class="o">++</span><span class="p">);</span>
</pre></div>
</div>
<p>下面给出几个编译器编译运行得到的结果:</p>
<div class="highlight-python"><div class="highlight"><pre>gcc:8,9,6,5(加-Wall选项得到warning:operation on ''i'' may be undefined
icc:5,7,7,8
clang:5,7,7,8 （warning: multiple unsequenced ）modifications
opencc: 5,9,7,8 (加-Wall编译选项也得到warning)
</pre></div>
</div>
<p>原因在于：在C/C++标准中，函数参数的运算顺序是没有特殊定义的，而是有编译器自己设定。
因此，不要纠结这种问题，而且编程中也应尽量避免这种情况的出现，即不要在一表达式（如：i=(i++)+(++i))或函数中出现多个这种计算。</p>
<p>可以查看C sequence points相关内容:<a class="reference external" href="http://msdn.microsoft.com/en-us/library/azk8zbxd.aspx">http://msdn.microsoft.com/en-us/library/azk8zbxd.aspx</a></p>
</div>
<div class="section" id="getsfgets">
<h2>字符串数组越界(gets、fgets）<a class="headerlink" href="#getsfgets" title="Permalink to this headline">¶</a></h2>
<p>编译时，不会检查是否越界，而putchar()或%s输出时只在遇到’‘0’‘时停止，当字符串数组越界时，越界较短情况下，可能不会报错，而且输出完整的值，但如果越界很多，则会报错，而越界后继续访问内存，很可能会造成严重后果。所以，需要人为控制内存溢出问题，给字符串用memset()赋初值，人为给字符串终止符’‘0’‘留个位置，注意下标等等。</p>
<div class="highlight-c"><div class="highlight"><pre><span class="kt">char</span> <span class="n">b</span><span class="p">[]</span> <span class="o">=</span> <span class="p">{</span><span class="sc">'i'</span><span class="p">,</span> <span class="sc">'s'</span><span class="p">};</span> <span class="c1">// 假设忘记添加结束符\0</span>
<span class="n">printf</span><span class="p">(</span><span class="s">"字符串b：%s"</span><span class="p">,</span> <span class="n">b</span><span class="p">);</span> <span class="c1">// 输出字符串b，很可能出现不可知的错误。</span>
<span class="kt">char</span> <span class="n">p</span><span class="p">[][</span><span class="mi">3</span><span class="p">]</span><span class="o">=</span><span class="p">{</span><span class="s">"abc"</span><span class="p">,</span><span class="s">"def"</span><span class="p">};</span>
<span class="n">printf</span><span class="p">(</span><span class="s">"%s"</span><span class="p">,</span><span class="n">p</span><span class="p">[</span><span class="mi">0</span><span class="p">]);</span> <span class="c1">// 将输出abcdef</span>
</pre></div>
</div>
<p>使用gets()时，可以无限读取，如果定义的数组空间较小，容易发生溢出，多出来的字符会覆盖堆栈中原先的内容，因此少用或不用gets()函数，而改用fgets()。</p>
</div>
<div class="section" id="getchar">
<h2>getchar()<a class="headerlink" href="#getchar" title="Permalink to this headline">¶</a></h2>
<p>程序运行到a=getchar()函数时，等待键盘输入，（行缓冲模式下）输入一串字符后回车，将字符串（含回车）送入缓存区并将第一个字符返回给变量a,“删除”第一个字符（实际是指针后移一位），如果缓存区内已有字符，则getchar()自动从缓冲区获得一个字符，而无需键盘输入。例:</p>
<div class="highlight-c"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre>1
2
3
4
5
6
7
8</pre></div></td><td class="code"><div class="highlight"><pre><span class="kt">int</span> <span class="nf">main</span><span class="p">()</span>
<span class="p">{</span>
<span class="kt">char</span> <span class="n">a</span><span class="p">,</span><span class="n">b</span><span class="p">;</span>
<span class="n">a</span><span class="o">=</span><span class="n">getchar</span><span class="p">();</span>
<span class="n">b</span><span class="o">=</span><span class="n">getchar</span><span class="p">();</span>
<span class="n">prinf</span><span class="p">(</span><span class="s">"%c%c"</span><span class="p">,</span><span class="n">a</span><span class="p">,</span><span class="n">b</span><span class="p">);</span>
<span class="k">return</span> <span class="mi">0</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</td></tr></table></div>
<p>键盘输入：</p>
<blockquote>
<div>c回车</div></blockquote>
<p>则缓冲区内字符为：”c\n”;</p>
<p>因此: a=’c’,b=’\n’;</p>
</div>
<div class="section" id="id1">
<h2>二维数组作为函数参数<a class="headerlink" href="#id1" title="Permalink to this headline">¶</a></h2>
<div class="highlight-c"><div class="highlight"><pre>int a[m][n]; // m,n已知
a[i][j]地址为：
a+i*n+j; // 因此对于多维数组，必须给定二维或更高维的大小。
</pre></div>
</div>
<p>传递二维数组的几种常见方法：</p>
<ul>
<li><p class="first">形参给出第二维的长度</p>
<div class="highlight-c"><div class="highlight"><pre><span class="kt">void</span> <span class="nf">func</span><span class="p">(</span><span class="kt">char</span> <span class="n">str</span><span class="p">[][</span><span class="mi">5</span><span class="p">]);</span>
<span class="kt">char</span> <span class="n">str</span><span class="p">[][</span><span class="mi">5</span><span class="p">]</span><span class="o">=</span><span class="p">{</span><span class="s">"abc"</span><span class="p">,</span><span class="s">"def"</span><span class="p">,</span><span class="s">"ghi"</span><span class="p">};</span>
<span class="n">func</span><span class="p">(</span><span class="n">str</span><span class="p">);</span>
</pre></div>
</div>
</li>
<li><p class="first">形参为指向数组的指针</p>
<div class="highlight-c"><div class="highlight"><pre><span class="kt">void</span> <span class="nf">func</span><span class="p">(</span><span class="kt">char</span> <span class="p">(</span><span class="o">*</span><span class="n">str</span><span class="p">)[</span><span class="mi">5</span><span class="p">]);</span> <span class="c1">// 注意(*str)前后括号</span>
<span class="c1">// [5]表示每个字符串最多5个字符，</span>
<span class="c1">// 由于字符串最后一个应为'\0'，因此实际上最好只存4个字符，否则用printf输出有问题。</span>
<span class="kt">char</span> <span class="n">str</span><span class="p">[][</span><span class="mi">5</span><span class="p">]</span><span class="o">=</span><span class="p">{</span><span class="s">"abc"</span><span class="p">,</span><span class="s">"def"</span><span class="p">,</span><span class="s">"ghi"</span><span class="p">};</span>
<span class="n">func</span><span class="p">(</span><span class="n">str</span><span class="p">);</span>
</pre></div>
</div>
</li>
<li><p class="first">形参为指针数组</p>
<div class="highlight-c"><div class="highlight"><pre><span class="kt">void</span> <span class="nf">func</span><span class="p">(</span><span class="kt">char</span> <span class="o">*</span> <span class="n">str</span><span class="p">[</span><span class="mi">5</span><span class="p">]);</span>
<span class="kt">char</span> <span class="o">*</span> <span class="n">str</span><span class="p">[</span><span class="mi">5</span><span class="p">]</span><span class="o">=</span><span class="p">{</span><span class="s">"abc"</span><span class="p">,</span><span class="s">"def"</span><span class="p">,</span><span class="s">"ghijklm"</span><span class="p">};</span> <span class="c1">// str为5个指向字符串的指针</span>
<span class="n">func</span><span class="p">(</span><span class="n">str</span><span class="p">);</span>
</pre></div>
</div>
</li>
<li><p class="first">形参为指针的指针</p>
<div class="highlight-c"><div class="highlight"><pre><span class="kt">void</span> <span class="nf">func</span><span class="p">(</span><span class="kt">char</span> <span class="o">**</span><span class="n">str</span><span class="p">);</span>
<span class="kt">char</span> <span class="n">str</span><span class="p">[][</span><span class="mi">5</span><span class="p">]</span><span class="o">=</span><span class="p">{</span><span class="s">"abc"</span><span class="p">,</span><span class="s">"def"</span><span class="p">,</span><span class="s">"ghi"</span><span class="p">};</span>
<span class="n">p</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span><span class="o">=&</span><span class="n">str</span><span class="p">[</span><span class="mi">0</span><span class="p">][</span><span class="mi">0</span><span class="p">];</span>
<span class="n">p</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span><span class="o">=</span><span class="n">str</span><span class="p">[</span><span class="mi">1</span><span class="p">];</span>
<span class="n">p</span><span class="p">[</span><span class="mi">2</span><span class="p">]</span><span class="o">=</span><span class="n">str</span><span class="p">[</span><span class="mi">2</span><span class="p">];</span>
<span class="n">func</span><span class="p">(</span><span class="n">p</span><span class="p">);</span>
<span class="c1">// 或</span>
<span class="kt">char</span> <span class="o">*</span><span class="n">str</span><span class="p">[</span><span class="mi">5</span><span class="p">]</span> <span class="c1">// 声明</span>
</pre></div>
</div>
</li>
<li><p class="first">c++里使用二维向量</p>
<div class="highlight-c++"><div class="highlight"><pre><span class="n">vector</span><span class="o"><</span><span class="n">vector</span><span class="o"><</span><span class="kt">int</span><span class="o">>></span> <span class="n">array</span><span class="p">[</span><span class="mi">5</span><span class="p">]</span>
</pre></div>
</div>
</li>
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<li><a class="reference internal" href="#">c & c++</a><ul>
<li><a class="reference internal" href="#printf-d-d-d-d-i-i-i-i">printf(“%d,%d,%d,%d”,i++,++i,i++,i++)</a></li>
<li><a class="reference internal" href="#getsfgets">字符串数组越界(gets、fgets）</a></li>
<li><a class="reference internal" href="#getchar">getchar()</a></li>
<li><a class="reference internal" href="#id1">二维数组作为函数参数</a></li>
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