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<h2>
Kỳ diệu range
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by Pymier0
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<p>Bất kỳ ai học Python cũng đều biết đến range, function giúp tạo 1 list các
số nguyên:</p>
<div class="highlight"><pre><span></span><code><span class="nb">range</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">100</span><span class="p">)</span>
</code></pre></div>
<p><img alt="img" src="https://images.unsplash.com/photo-1550031675-d8ce0062caf7?crop=entropy&cs=tinysrgb&fit=max&fm=jpg&ixid=MnwyMzI1MzN8MHwxfHJhbmRvbXx8fHx8fHx8fDE2MjU3OTU0MzY&ixlib=rb-1.2.1&q=80&w=600"></p>
<p><code>range</code> có trong Python từ rất lâu rồi, thời Python2, <code>range</code> có kèm 1 người
anh em song sinh tên gọi <code>xrange</code>. <code>xrange</code> này chính là <code>range</code> ở Python3.
Khác gì nhau?</p>
<ul>
<li>range (cũ) sẽ tạo ra 1 list</li>
<li>xrange hay range python3 sẽ tạo ra… 1 range</li>
</ul>
<div class="highlight"><pre><span></span><code><span class="o">>>></span> <span class="nb">range</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">100</span><span class="p">)</span>
<span class="nb">range</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">100</span><span class="p">)</span>
<span class="o">>>></span> <span class="nb">type</span><span class="p">(</span><span class="nb">range</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="mi">100</span><span class="p">))</span>
<span class="o"><</span><span class="k">class</span> <span class="err">'</span><span class="nc">range</span><span class="s1">'></span>
</code></pre></div>
<p>Vậy nên trong Python3, khi muốn tạo list, phải gõ <code>list(range(1, 100)</code>.
Tại sao?</p>
<p>Nếu dùng <code>range</code> Python2, sẽ khó mà tạo được list chứa 1 tỷ phần tử (do máy sẽ
hết <span class="caps">RAM</span>), nhưng <code>range</code> Python3 thì tạo cái gì cũng được, và nó chỉ trả
về phần tử khi yêu cầu - đây là ý tưởng chính của <a href="https://pp.pymi.vn/article/tuple_comps/">“generator”</a>.</p>
<p>Nhưng range không phải generator, range là kiểu range, vì nó còn đặc biệt hơn.
Một <a href="https://pp.pymi.vn/article/tuple_comps/">generator</a> không có len, range thì có:</p>
<div class="highlight"><pre><span></span><code><span class="o">>>></span> <span class="nb">len</span><span class="p">(</span><span class="nb">range</span><span class="p">(</span><span class="mi">2</span><span class="p">,</span> <span class="mi">1_000_000_000</span><span class="p">,</span> <span class="mi">3</span><span class="p">))</span>
<span class="mi">333333333</span>
<span class="o">>>></span> <span class="k">def</span> <span class="nf">gen</span><span class="p">():</span>
<span class="o">...</span> <span class="k">yield</span> <span class="mi">1</span>
<span class="o">...</span>
<span class="o">>>></span> <span class="nb">len</span><span class="p">(</span><span class="n">gen</span><span class="p">())</span>
<span class="n">Traceback</span> <span class="p">(</span><span class="n">most</span> <span class="n">recent</span> <span class="n">call</span> <span class="n">last</span><span class="p">):</span>
<span class="n">File</span> <span class="s2">"<stdin>"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">1</span><span class="p">,</span> <span class="ow">in</span> <span class="o"><</span><span class="n">module</span><span class="o">></span>
<span class="ne">TypeError</span><span class="p">:</span> <span class="nb">object</span> <span class="n">of</span> <span class="nb">type</span> <span class="s1">'generator'</span> <span class="n">has</span> <span class="n">no</span> <span class="nb">len</span><span class="p">()</span>
</code></pre></div>
<p>Bonus: đọc code tính len của range - code C:
<a href="https://github.com/python/cpython/blob/main/Objects/rangeobject.c#L182">https://github.com/python/cpython/blob/main/Objects/rangeobject.c#L182</a></p>
<h2>Hết</h2>
<p>Đăng ký ngay tại <a href="https://pymi.vn">PyMI.vn</a> để học Python tại Hà Nội <span class="caps">TP</span> <span class="caps">HCM</span> (Sài Gòn),
trở thành lập trình viên #python chuyên nghiệp ngay sau khóa học.</p>
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