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R语言从入门到放弃 (2). 向量(列表)及函数
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目录
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<div id="toc"><ul><li><a class="toc-href" href="#fancy-indexing" title="fancy indexing">fancy indexing</a></li><li><a class="toc-href" href="#names-dimnames" title="names / dimnames">names / dimnames</a></li><li><a class="toc-href" href="#c" title="c()">c()</a></li><li><a class="toc-href" href="#vector-vs-list-matrix-vs-dataframe" title="vector VS list ( matrix VS data.frame )">vector VS list ( matrix VS data.frame )</a></li><li><a class="toc-href" href="#functions" title="functions">functions</a></li><li><a class="toc-href" href="#applylapplysapply" title="apply/lapply/sapply">apply/lapply/sapply</a><ul><li><a class="toc-href" href="#apply" title="apply">apply</a></li><li><a class="toc-href" href="#lapplysapply" title="lapply/sapply">lapply/sapply</a></li></ul></li></ul></div>
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<p>首先, R似乎默认所有的变量都为向量vector, 即使一个单独的数字也是长度为1的, 所以<code>1</code>等价于<code>c(1)</code>. </p>
<div class="highlight"><pre><span class="code-line"><span></span><span class="o">></span> <span class="n">a</span> <span class="o"><-</span> <span class="m">1</span></span>
<span class="code-line"><span class="o">></span> <span class="n">a</span></span>
<span class="code-line"><span class="n">[1]</span> <span class="m">1</span></span>
<span class="code-line"><span class="o">></span> <span class="nf">length</span><span class="p">(</span><span class="n">a</span><span class="p">)</span></span>
<span class="code-line"><span class="n">[1]</span> <span class="m">1</span></span>
<span class="code-line"><span class="o">></span> <span class="n">a[1]</span></span>
<span class="code-line"><span class="n">[1]</span> <span class="m">1</span></span>
<span class="code-line"><span class="o">></span> <span class="nf">typeof</span><span class="p">(</span><span class="n">a</span><span class="p">)</span></span>
<span class="code-line"><span class="n">[1]</span> <span class="s">"double"</span> <span class="c1"># means "double vector" (I think)</span></span>
<span class="code-line"><span class="o">></span> <span class="m">1</span> <span class="o">==</span> <span class="nf">c</span><span class="p">(</span><span class="m">1</span><span class="p">)</span></span>
<span class="code-line"><span class="n">[1]</span> <span class="kc">TRUE</span></span>
</pre></div>
<h2 id="fancy-indexing">fancy indexing</h2>
<p>R的vector/list/matrix支持类似numpy(稍有不同)的fancy indexing, 以下是例子: </p>
<div class="highlight"><pre><span class="code-line"><span></span><span class="o">></span> <span class="n">v</span> <span class="o"><-</span> <span class="m">1</span><span class="o">:</span><span class="m">10</span></span>
<span class="code-line"><span class="o">></span> <span class="n">v[1</span><span class="o">:</span><span class="m">3</span><span class="n">]</span> <span class="c1"># slicing</span></span>
<span class="code-line"><span class="n">[1]</span> <span class="m">1</span> <span class="m">2</span> <span class="m">3</span></span>
<span class="code-line"><span class="o">></span> <span class="n">v[</span><span class="o">-</span><span class="p">(</span><span class="m">1</span><span class="o">:</span><span class="m">3</span><span class="p">)</span><span class="n">]</span> <span class="c1"># EXCLUDING first 3 elements</span></span>
<span class="code-line"><span class="n">[1]</span> <span class="m">4</span> <span class="m">5</span> <span class="m">6</span> <span class="m">7</span> <span class="m">8</span> <span class="m">9</span> <span class="m">10</span></span>
<span class="code-line"><span class="o">></span> <span class="n">v[</span><span class="m">-1</span><span class="n">]</span> <span class="c1"># EXCLUDING first element</span></span>
<span class="code-line"><span class="n">[1]</span> <span class="m">2</span> <span class="m">3</span> <span class="m">4</span> <span class="m">5</span> <span class="m">6</span> <span class="m">7</span> <span class="m">8</span> <span class="m">9</span> <span class="m">10</span></span>
<span class="code-line"><span class="o">></span> <span class="n">v</span><span class="nf">[c</span><span class="p">(</span><span class="m">2</span><span class="p">,</span><span class="m">4</span><span class="p">,</span><span class="m">9</span><span class="p">)</span><span class="n">]</span></span>
<span class="code-line"><span class="n">[1]</span> <span class="m">2</span> <span class="m">4</span> <span class="m">9</span></span>
<span class="code-line"><span class="o">></span> <span class="n">v[v</span><span class="o">%%</span><span class="m">2</span><span class="o">==</span><span class="m">0</span><span class="n">]</span> <span class="c1"># indexing using logical array</span></span>
<span class="code-line"><span class="n">[1]</span> <span class="m">2</span> <span class="m">4</span> <span class="m">6</span> <span class="m">8</span> <span class="m">10</span></span>
<span class="code-line"></span>
<span class="code-line"><span class="c1"># works also for lists</span></span>
<span class="code-line"><span class="o">></span> <span class="n">l</span> <span class="o"><-</span> <span class="nf">list</span><span class="p">(</span><span class="m">1</span><span class="p">,</span><span class="m">2</span><span class="p">,</span><span class="m">3</span><span class="p">,</span><span class="s">"aa"</span><span class="p">)</span></span>
<span class="code-line"><span class="o">></span> <span class="n">l[</span><span class="m">-1</span><span class="n">]</span></span>
<span class="code-line"><span class="n">[[1]]</span></span>
<span class="code-line"><span class="n">[1]</span> <span class="m">2</span></span>
<span class="code-line"></span>
<span class="code-line"><span class="n">[[2]]</span></span>
<span class="code-line"><span class="n">[1]</span> <span class="m">3</span></span>
<span class="code-line"></span>
<span class="code-line"><span class="n">[[3]]</span></span>
<span class="code-line"><span class="n">[1]</span> <span class="s">"aa"</span></span>
</pre></div>
<p>以下是矩阵的fancy indexing例子: </p>
<div class="highlight"><pre><span class="code-line"><span></span><span class="o">></span> <span class="n">mat</span> <span class="o"><-</span> <span class="nf">matrix</span><span class="p">(</span><span class="m">1</span><span class="o">:</span><span class="m">9</span><span class="p">,</span> <span class="n">nrow</span><span class="o">=</span><span class="m">3</span><span class="p">,</span> <span class="n">ncol</span><span class="o">=</span><span class="m">3</span><span class="p">)</span></span>
<span class="code-line"><span class="o">></span> <span class="n">mat</span></span>
<span class="code-line"> <span class="n">[</span><span class="p">,</span><span class="m">1</span><span class="n">]</span> <span class="n">[</span><span class="p">,</span><span class="m">2</span><span class="n">]</span> <span class="n">[</span><span class="p">,</span><span class="m">3</span><span class="n">]</span></span>
<span class="code-line"><span class="n">[1</span><span class="p">,</span><span class="n">]</span> <span class="m">1</span> <span class="m">4</span> <span class="m">7</span></span>
<span class="code-line"><span class="n">[2</span><span class="p">,</span><span class="n">]</span> <span class="m">2</span> <span class="m">5</span> <span class="m">8</span></span>
<span class="code-line"><span class="n">[3</span><span class="p">,</span><span class="n">]</span> <span class="m">3</span> <span class="m">6</span> <span class="m">9</span></span>
<span class="code-line"><span class="o">></span> <span class="n">mat[1</span><span class="p">,</span><span class="n">]</span></span>
<span class="code-line"><span class="n">[1]</span> <span class="m">1</span> <span class="m">4</span> <span class="m">7</span></span>
<span class="code-line"><span class="o">></span> <span class="n">mat[1</span><span class="p">,</span><span class="nf">c</span><span class="p">(</span><span class="m">2</span><span class="p">,</span><span class="m">3</span><span class="p">)</span><span class="n">]</span></span>
<span class="code-line"><span class="n">[1]</span> <span class="m">4</span> <span class="m">7</span></span>
<span class="code-line"><span class="o">></span> <span class="n">mat[</span><span class="m">-1</span><span class="p">,</span><span class="n">]</span></span>
<span class="code-line"> <span class="n">[</span><span class="p">,</span><span class="m">1</span><span class="n">]</span> <span class="n">[</span><span class="p">,</span><span class="m">2</span><span class="n">]</span> <span class="n">[</span><span class="p">,</span><span class="m">3</span><span class="n">]</span></span>
<span class="code-line"><span class="n">[1</span><span class="p">,</span><span class="n">]</span> <span class="m">2</span> <span class="m">5</span> <span class="m">8</span></span>
<span class="code-line"><span class="n">[2</span><span class="p">,</span><span class="n">]</span> <span class="m">3</span> <span class="m">6</span> <span class="m">9</span></span>
<span class="code-line"><span class="o">></span> <span class="n">mat[</span><span class="p">,</span><span class="m">2</span><span class="n">]</span></span>
<span class="code-line"><span class="n">[1]</span> <span class="m">4</span> <span class="m">5</span> <span class="m">6</span></span>
</pre></div>
<h2 id="names-dimnames">names / dimnames</h2>
<p>好玩的是可以用<code>names</code>/<code>dimnames</code>函数给每个值加上一个名字: </p>
<div class="highlight"><pre><span class="code-line"><span></span><span class="o">></span> <span class="nf">names</span><span class="p">(</span><span class="n">v</span><span class="p">)</span> <span class="o"><-</span> <span class="nf">paste</span><span class="p">(</span><span class="s">"elem"</span><span class="p">,</span> <span class="n">sep</span><span class="o">=</span><span class="s">"-"</span><span class="p">,</span> <span class="m">1</span><span class="o">:</span><span class="nf">length</span><span class="p">(</span><span class="n">v</span><span class="p">))</span></span>
<span class="code-line"><span class="o">></span> <span class="n">v</span></span>
<span class="code-line"> <span class="n">elem</span><span class="m">-1</span> <span class="n">elem</span><span class="m">-2</span> <span class="n">elem</span><span class="m">-3</span> <span class="n">elem</span><span class="m">-4</span> <span class="n">elem</span><span class="m">-5</span> <span class="n">elem</span><span class="m">-6</span> <span class="n">elem</span><span class="m">-7</span> <span class="n">elem</span><span class="m">-8</span> <span class="n">elem</span><span class="m">-9</span> <span class="n">elem</span><span class="m">-10</span> </span>
<span class="code-line"> <span class="m">1</span> <span class="m">2</span> <span class="m">3</span> <span class="m">4</span> <span class="m">5</span> <span class="m">6</span> <span class="m">7</span> <span class="m">8</span> <span class="m">9</span> <span class="m">10</span> </span>
<span class="code-line"><span class="o">></span> <span class="nf">names</span><span class="p">(</span><span class="n">v</span><span class="p">)</span> <span class="o"><-</span> <span class="nf">paste</span><span class="p">(</span><span class="s">"elem"</span><span class="p">,</span> <span class="n">sep</span><span class="o">=</span><span class="s">"_"</span><span class="p">,</span> <span class="m">1</span><span class="o">:</span><span class="nf">length</span><span class="p">(</span><span class="n">v</span><span class="p">))</span></span>
<span class="code-line"><span class="o">></span> <span class="n">v</span></span>
<span class="code-line"> <span class="n">elem_1</span> <span class="n">elem_2</span> <span class="n">elem_3</span> <span class="n">elem_4</span> <span class="n">elem_5</span> <span class="n">elem_6</span> <span class="n">elem_7</span> <span class="n">elem_8</span> <span class="n">elem_9</span> <span class="n">elem_10</span> </span>
<span class="code-line"> <span class="m">1</span> <span class="m">2</span> <span class="m">3</span> <span class="m">4</span> <span class="m">5</span> <span class="m">6</span> <span class="m">7</span> <span class="m">8</span> <span class="m">9</span> <span class="m">10</span> </span>
<span class="code-line"><span class="o">></span> <span class="n">v</span><span class="o">$</span><span class="n">elem_1</span></span>
<span class="code-line"><span class="n">Error</span> <span class="n">in</span> <span class="n">v</span><span class="o">$</span><span class="n">elem_1</span> <span class="o">:</span> <span class="o">$</span> <span class="n">operator</span> <span class="n">is</span> <span class="n">invalid</span> <span class="n">for</span> <span class="n">atomic</span> <span class="n">vectors</span></span>
<span class="code-line"><span class="o">></span> <span class="n">v[</span><span class="s">"elem_1"</span><span class="n">]</span></span>
<span class="code-line"><span class="n">elem_1</span> </span>
<span class="code-line"> <span class="m">1</span></span>
<span class="code-line"><span class="o">></span> <span class="nf">names</span><span class="p">(</span><span class="n">l</span><span class="p">)</span> <span class="o"><-</span> <span class="nf">paste</span><span class="p">(</span><span class="s">"elem"</span><span class="p">,</span> <span class="n">sep</span><span class="o">=</span><span class="s">"_"</span><span class="p">,</span> <span class="m">1</span><span class="o">:</span><span class="nf">length</span><span class="p">(</span><span class="n">l</span><span class="p">))</span></span>
<span class="code-line"><span class="o">></span> <span class="n">l</span><span class="o">$</span><span class="n">elem_1</span></span>
<span class="code-line"><span class="n">[1]</span> <span class="m">1</span></span>
<span class="code-line"><span class="o">></span> <span class="n">l[1]</span></span>
<span class="code-line"><span class="o">$</span><span class="n">elem_1</span></span>
<span class="code-line"><span class="n">[1]</span> <span class="m">1</span></span>
<span class="code-line"><span class="o">></span> <span class="n">l[</span><span class="s">"elem_1"</span><span class="n">]</span></span>
<span class="code-line"><span class="o">$</span><span class="n">elem_1</span></span>
<span class="code-line"><span class="n">[1]</span> <span class="m">1</span></span>
</pre></div>
<p>上面例子看到, vector不能使用<code>$</code>来获得"field", 但是list可以, 这是list和vector的一个区别. </p>
<p>下面是矩阵的例子: </p>
<div class="highlight"><pre><span class="code-line"><span></span><span class="o">></span> <span class="nf">dimnames</span><span class="p">(</span><span class="n">mat</span><span class="p">)</span> <span class="o"><-</span> <span class="nf">list</span><span class="p">(</span> <span class="nf">paste</span><span class="p">(</span><span class="s">"row"</span><span class="p">,</span><span class="n">sep</span><span class="o">=</span><span class="s">"_"</span><span class="p">,</span> <span class="m">1</span><span class="o">:</span><span class="nf">nrow</span><span class="p">(</span><span class="n">mat</span><span class="p">)),</span> <span class="nf">paste</span><span class="p">(</span><span class="s">"col"</span><span class="p">,</span><span class="n">sep</span><span class="o">=</span><span class="s">"_"</span><span class="p">,</span> <span class="m">1</span><span class="o">:</span><span class="nf">ncol</span><span class="p">(</span><span class="n">mat</span><span class="p">))</span> <span class="p">)</span></span>
<span class="code-line"><span class="o">></span> <span class="n">mat</span></span>
<span class="code-line"> <span class="n">col_1</span> <span class="n">col_2</span> <span class="n">col_3</span></span>
<span class="code-line"><span class="n">row_1</span> <span class="m">1</span> <span class="m">4</span> <span class="m">7</span></span>
<span class="code-line"><span class="n">row_2</span> <span class="m">2</span> <span class="m">5</span> <span class="m">8</span></span>
<span class="code-line"><span class="n">row_3</span> <span class="m">3</span> <span class="m">6</span> <span class="m">9</span></span>
<span class="code-line"><span class="o">></span> <span class="n">mat[</span><span class="s">"row_1"</span><span class="p">,</span><span class="s">"col_1"</span><span class="n">]</span></span>
<span class="code-line"><span class="n">[1]</span> <span class="m">1</span></span>
</pre></div>
<h2 id="c">c()</h2>
<p>关于<code>c</code>这个函数, 值得一提的除了它自动"展开"参数的list/vector以外(上次博客提到), 还有就是它会自动cast, 文档里是这么说的: </p>
<blockquote>
<p>The output type is determined from the highest type of the components in the hierarchy NULL < raw < logical < integer < double < complex < character < list < expression. </p>
</blockquote>
<p>其中的logical, integer, character都属于(atomic) vector, list和他们不同, 见后文. </p>
<p>以下是例子: </p>
<div class="highlight"><pre><span class="code-line"><span></span><span class="o">></span> <span class="k">c</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="k">TRUE</span><span class="p">)</span> <span class="o">#</span> <span class="n">logical</span> <span class="o"><</span> <span class="nb">integer</span></span>
<span class="code-line"><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="mi">1</span> <span class="mi">2</span> <span class="mi">1</span></span>
<span class="code-line"><span class="o">></span> <span class="k">c</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="ss">"char"</span><span class="p">)</span> <span class="o">#</span> <span class="nb">integer</span> <span class="o"><</span> <span class="nb">character</span></span>
<span class="code-line"><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="ss">"1"</span> <span class="ss">"2"</span> <span class="ss">"char"</span></span>
<span class="code-line"><span class="o">></span> <span class="k">c</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="n">list</span><span class="p">(</span><span class="mi">1</span><span class="p">))</span> <span class="o">#</span> <span class="nb">integer</span> <span class="o"><</span> <span class="n">list</span></span>
<span class="code-line"><span class="p">[[</span><span class="mi">1</span><span class="p">]]</span></span>
<span class="code-line"><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="mi">1</span></span>
<span class="code-line"></span>
<span class="code-line"><span class="p">[[</span><span class="mi">2</span><span class="p">]]</span></span>
<span class="code-line"><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="mi">2</span></span>
<span class="code-line"></span>
<span class="code-line"><span class="p">[[</span><span class="mi">3</span><span class="p">]]</span></span>
<span class="code-line"><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="mi">1</span></span>
</pre></div>
<h2 id="vector-vs-list-matrix-vs-dataframe">vector VS list ( matrix VS data.frame )</h2>
<p>用于集合主要是vector和list, 他们的区别是: <strong>vector只能存放同样类型的元素, 而list可以存放不同类型的元素</strong><em>.</em> </p>
<p>vector可以是: numeric, logical, char... 类比一下, vector类似java里的array, list类似python的list. </p>
<p>另外访问第i个元素, vector是 <code>v[i]</code>, 而list需要用两个括号 <code>l[[i]]</code>(<code>l[i]</code>还是一个list, <code>l[[i]]</code>才是想要的东西... )</p>
<p>看例子: </p>
<div class="highlight"><pre><span class="code-line"><span></span><span class="o">></span> <span class="k">c</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span><span class="p">)</span> <span class="o">#</span> <span class="nb">numeric</span> <span class="n">vector</span></span>
<span class="code-line"><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="mi">1</span> <span class="mi">2</span> <span class="mi">3</span></span>
<span class="code-line"><span class="o">></span> <span class="k">c</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="ss">"a"</span><span class="p">)</span> <span class="o">#</span> <span class="k">c</span><span class="p">()</span> <span class="err">自动</span><span class="k">cast</span><span class="err">把前两个数字转成了</span><span class="nb">char</span><span class="p">,</span> <span class="err">变成一个</span><span class="nb">char</span><span class="err">类型的</span><span class="n">vector</span></span>
<span class="code-line"><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="ss">"1"</span> <span class="ss">"2"</span> <span class="ss">"a"</span></span>
<span class="code-line"><span class="o">></span> <span class="n">list</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="ss">"a"</span><span class="p">)</span> <span class="o">#</span> <span class="n">list</span></span>
<span class="code-line"><span class="p">[[</span><span class="mi">1</span><span class="p">]]</span></span>
<span class="code-line"><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="mi">1</span></span>
<span class="code-line"></span>
<span class="code-line"><span class="p">[[</span><span class="mi">2</span><span class="p">]]</span></span>
<span class="code-line"><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="mi">2</span></span>
<span class="code-line"></span>
<span class="code-line"><span class="p">[[</span><span class="mi">3</span><span class="p">]]</span></span>
<span class="code-line"><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="ss">"a"</span></span>
</pre></div>
<p>同理, matrix和data.frame也类似, matrix的所有元素必须相同, 而data.frame可以每一列各不相同(不过一列之中需要相同). 另外data.frame也支持用<code>$</code>选取一列, matrix则不支持.</p>
<h2 id="functions">functions</h2>
<p>R的函数定义为如下形式, 注意, 函数体的最后一句就是返回值, 不用显示写"return" (类似scala).</p>
<div class="highlight"><pre><span class="code-line"><span></span><span class="n">myfunnction</span> <span class="o"><-</span> <span class="nf">function</span><span class="p">(</span><span class="n">params</span><span class="p">,</span> <span class="kc">...</span><span class="p">){</span></span>
<span class="code-line"> <span class="c1"># ...</span></span>
<span class="code-line"> <span class="n">the.return.value</span></span>
<span class="code-line"><span class="p">}</span></span>
</pre></div>
<p>另外注意到上面函数定义, 参数里有三个点<code>...</code>, 这个不是必须的, 它的的作用见下一节. </p>
<p>R是函数式语言: 一个function可以作为参数传递, 例子就是<code>apply</code>, 见下一节.</p>
<h2 id="applylapplysapply">apply/lapply/sapply</h2>
<h3 id="apply">apply</h3>
<p>apply这个函数的doc写到用法为: </p>
<p><code>apply(X, MARGIN, FUN, ...)</code></p>
<p><code>X</code>是操作的数据(<strong>一般为matrix</strong>), <code>MARGIN</code>为选择对行或列操作(类似numpy的<code>axis</code>参数), <code>FUN</code>就是作为参数传入的函数了. </p>
<div class="highlight"><pre><span class="code-line"><span></span><span class="o">></span> <span class="n">mat</span> <span class="o"><-</span> <span class="nf">matrix</span><span class="p">(</span><span class="m">1</span><span class="o">:</span><span class="m">12</span><span class="p">,</span> <span class="n">nrow</span><span class="o">=</span><span class="m">3</span><span class="p">,</span> <span class="n">ncol</span><span class="o">=</span><span class="m">4</span><span class="p">)</span></span>
<span class="code-line"><span class="o">></span> <span class="n">mat</span></span>
<span class="code-line"> <span class="n">[</span><span class="p">,</span><span class="m">1</span><span class="n">]</span> <span class="n">[</span><span class="p">,</span><span class="m">2</span><span class="n">]</span> <span class="n">[</span><span class="p">,</span><span class="m">3</span><span class="n">]</span> <span class="n">[</span><span class="p">,</span><span class="m">4</span><span class="n">]</span></span>
<span class="code-line"><span class="n">[1</span><span class="p">,</span><span class="n">]</span> <span class="m">1</span> <span class="m">4</span> <span class="m">7</span> <span class="m">10</span></span>
<span class="code-line"><span class="n">[2</span><span class="p">,</span><span class="n">]</span> <span class="m">2</span> <span class="m">5</span> <span class="m">8</span> <span class="m">11</span></span>
<span class="code-line"><span class="n">[3</span><span class="p">,</span><span class="n">]</span> <span class="m">3</span> <span class="m">6</span> <span class="m">9</span> <span class="m">12</span></span>
<span class="code-line"><span class="o">></span> <span class="nf">apply</span><span class="p">(</span><span class="n">mat</span><span class="p">,</span> <span class="m">1</span><span class="p">,</span> <span class="n">sum</span><span class="p">)</span> <span class="c1"># apply on row</span></span>
<span class="code-line"><span class="n">[1]</span> <span class="m">22</span> <span class="m">26</span> <span class="m">30</span></span>
<span class="code-line"><span class="o">></span> <span class="nf">apply</span><span class="p">(</span><span class="n">mat</span><span class="p">,</span> <span class="m">2</span><span class="p">,</span> <span class="n">sum</span><span class="p">)</span> <span class="c1"># apply on col</span></span>
<span class="code-line"><span class="n">[1]</span> <span class="m">6</span> <span class="m">15</span> <span class="m">24</span> <span class="m">33</span></span>
</pre></div>
<p>这里类似做reduce操作, 而MARGIN就是指定要reduce哪一个维度. </p>
<p>另外文档里的三个点<code>...</code>很有意思, 它是<strong>参数FUN的额外参数</strong> ! 下面是一个例子, 给FUN传入了一个匿名函数: <code>function(x,power) sum(x^power)</code>, 它计算x里元素的power次方, 然后加起来. 所以在apply里可以指定FUN这个<code>power</code>参数的数值, 这就对应着apply用法里的这三个点<code>...</code>.</p>
<div class="highlight"><pre><span class="code-line"><span></span><span class="err">> apply(mat, 1, function(x,power) sum(x^power), power=1 )</span></span>
<span class="code-line"><span class="err">[1] 22 26 30</span></span>
<span class="code-line"><span class="err">> apply(mat, 1, function(x,power) sum(x^power), power=2 )</span></span>
<span class="code-line"><span class="err">[1] 166 214 270</span></span>
</pre></div>
<h3 id="lapplysapply">lapply/sapply</h3>
<p>如果说上面<code>apply</code>一般用在matrix上, 用于将一个matrix <strong>reduce</strong>为向量的话, <code>lapply/sapply</code>就是<strong>map</strong>操作了: 作用在一个vector/list上, 返回对每一个元素进行操作后的新list. </p>
<p>它们的区别大概是: lapply返回list, sapply返回vector. </p>
<div class="highlight"><pre><span class="code-line"><span></span><span class="o">></span> <span class="n">sq</span> <span class="o"><-</span> <span class="nf">function</span><span class="p">(</span><span class="n">x</span><span class="p">)</span> <span class="n">x^2</span></span>
<span class="code-line"><span class="o">></span> <span class="n">l</span> <span class="o"><-</span> <span class="nf">list</span><span class="p">(</span><span class="m">1</span><span class="p">,</span><span class="m">2</span><span class="p">,</span><span class="m">3</span><span class="p">,</span><span class="m">4</span><span class="p">)</span></span>
<span class="code-line"><span class="o">></span> <span class="n">v</span> <span class="o"><-</span> <span class="m">1</span><span class="o">:</span><span class="m">4</span></span>
<span class="code-line"><span class="o">></span> <span class="nf">lapply</span><span class="p">(</span><span class="n">v</span><span class="p">,</span> <span class="n">sq</span><span class="p">)</span> <span class="c1"># lapply returns a list</span></span>
<span class="code-line"><span class="n">[[1]]</span></span>
<span class="code-line"><span class="n">[1]</span> <span class="m">1</span></span>
<span class="code-line"></span>
<span class="code-line"><span class="n">[[2]]</span></span>
<span class="code-line"><span class="n">[1]</span> <span class="m">4</span></span>
<span class="code-line"></span>
<span class="code-line"><span class="n">[[3]]</span></span>
<span class="code-line"><span class="n">[1]</span> <span class="m">9</span></span>
<span class="code-line"></span>
<span class="code-line"><span class="n">[[4]]</span></span>
<span class="code-line"><span class="n">[1]</span> <span class="m">16</span></span>
<span class="code-line"></span>
<span class="code-line"><span class="o">></span> <span class="nf">lapply</span><span class="p">(</span><span class="n">l</span><span class="p">,</span> <span class="n">sq</span><span class="p">)</span> <span class="c1"># lapply returns a list</span></span>
<span class="code-line"><span class="n">[[1]]</span></span>
<span class="code-line"><span class="n">[1]</span> <span class="m">1</span></span>
<span class="code-line"></span>
<span class="code-line"><span class="n">[[2]]</span></span>
<span class="code-line"><span class="n">[1]</span> <span class="m">4</span></span>
<span class="code-line"></span>
<span class="code-line"><span class="n">[[3]]</span></span>
<span class="code-line"><span class="n">[1]</span> <span class="m">9</span></span>
<span class="code-line"></span>
<span class="code-line"><span class="n">[[4]]</span></span>
<span class="code-line"><span class="n">[1]</span> <span class="m">16</span></span>
<span class="code-line"></span>
<span class="code-line"><span class="o">></span> <span class="nf">sapply</span><span class="p">(</span><span class="n">v</span><span class="p">,</span> <span class="n">sq</span><span class="p">)</span> <span class="c1"># sapply returns a vector</span></span>
<span class="code-line"><span class="n">[1]</span> <span class="m">1</span> <span class="m">4</span> <span class="m">9</span> <span class="m">16</span></span>
<span class="code-line"><span class="o">></span> <span class="nf">sapply</span><span class="p">(</span><span class="n">l</span><span class="p">,</span> <span class="n">sq</span><span class="p">)</span> <span class="c1"># sapply returns a vector</span></span>
<span class="code-line"><span class="n">[1]</span> <span class="m">1</span> <span class="m">4</span> <span class="m">9</span> <span class="m">16</span></span>
</pre></div>
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