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Tính căn số lớn
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by Pymier0
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<p><img alt="img" src="https://images.unsplash.com/photo-1594013755115-76c4fe78b165?crop=entropy&cs=tinysrgb&fit=max&fm=jpg&ixid=MnwyMzI1MzN8MHwxfHJhbmRvbXx8fHx8fHx8fDE2NDgwMzg1Nzg&ixlib=rb-1.2.1&q=80&w=600"></p>
<p>Trong Python, kiểu int có độ lớn tùy ý, như 2 mũ 2048:</p>
<div class="highlight"><pre><span></span><code><span class="o">>>></span> <span class="n">b</span> <span class="o">=</span> <span class="mi">2</span><span class="o">**</span><span class="mi">2048</span>
<span class="o">>>></span> <span class="n">b</span>
<span class="mi">32317006071311007300714876688669951960444102669715484032130345427524655138867890893197201411522913463688717960921898019494119559150490921095088152386448283120630877367300996091750197750389652106796057638384067568276792218642619756161838094338476170470581645852036305042887575891541065808607552399123930385521914333389668342420684974786564569494856176035326322058077805659331026192708460314150258592864177116725943603718461857357598351152301645904403697613233287231227125684710820209725157101726931323469678542580656697935045997268352998638215525166389437335543602135433229604645318478604952148193555853611059596230656</span>
</code></pre></div>
<p>cộng trừ vẫn thoải mái, nhưng… tính căn 2 (hoặc chia) thì sao? </p>
<div class="highlight"><pre><span></span><code><span class="o">>>></span> <span class="kn">from</span> <span class="nn">math</span> <span class="kn">import</span> <span class="n">sqrt</span>
<span class="o">>>></span> <span class="n">sqrt</span><span class="p">(</span><span class="n">b</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">OverflowError</span><span class="p">:</span> <span class="nb">int</span> <span class="n">too</span> <span class="n">large</span> <span class="n">to</span> <span class="n">convert</span> <span class="n">to</span> <span class="nb">float</span>
</code></pre></div>
<p>Nội dung error cho thấy số này quá lớn để biến thành kiểu float (kết quả của phép chia, căn đều là float). Giải pháp là dùng math.isqrt</p>
<div class="highlight"><pre><span></span><code><span class="o">>>></span> <span class="kn">from</span> <span class="nn">math</span> <span class="kn">import</span> <span class="n">isqrt</span>
<span class="o">>>></span> <span class="n">isqrt</span><span class="p">(</span><span class="n">b</span><span class="p">)</span>
<span class="mi">179769313486231590772930519078902473361797697894230657273430081157732675805500963132708477322407536021120113879871393357658789768814416622492847430639474124377767893424865485276302219601246094119453082952085005768838150682342462881473913110540827237163350510684586298239947245938479716304835356329624224137216</span>
<span class="o">>>></span> <span class="n">isqrt</span><span class="p">(</span><span class="n">b</span><span class="p">)</span> <span class="o">==</span> <span class="mi">2</span><span class="o">**</span><span class="mi">1024</span>
<span class="kc">True</span>
</code></pre></div>
<p>Docstring: </p>
<div class="highlight"><pre><span></span><code><span class="n">isqrt</span><span class="p">(</span><span class="n">n</span><span class="p">,</span> <span class="o">/</span><span class="p">)</span>
<span class="n">Return</span> <span class="n">the</span> <span class="n">integer</span> <span class="n">part</span> <span class="n">of</span> <span class="n">the</span> <span class="n">square</span> <span class="n">root</span> <span class="n">of</span> <span class="n">the</span> <span class="nb">input</span><span class="o">.</span>
</code></pre></div>
<p>kết quả không chắc CHÍNH XÁC là căn 2 (bởi không phải căn 2 số nào cũng là số nguyên), nên cần test lại nếu bình phương kết quả có bằng số ban đầu không.</p>
<div class="highlight"><pre><span></span><code><span class="o">>>></span> <span class="n">isqrt</span><span class="p">(</span><span class="mi">10</span><span class="p">)</span>
<span class="mi">3</span>
<span class="o">>>></span> <span class="mi">3</span> <span class="o">**</span> <span class="mi">2</span> <span class="o">==</span> <span class="mi">10</span>
<span class="kc">False</span>
<span class="o">>>></span> <span class="mi">2</span><span class="o">**</span><span class="mi">1000</span><span class="o">/</span><span class="mi">2</span>
<span class="mf">5.357543035931337e+300</span>
<span class="o">>>></span> <span class="n">isqrt</span><span class="p">(</span><span class="mi">2</span><span class="o">**</span><span class="mi">1000</span><span class="p">)</span> <span class="o">**</span> <span class="mi">2</span> <span class="o">==</span> <span class="mi">2</span><span class="o">**</span><span class="mi">1000</span>
<span class="kc">True</span>
</code></pre></div>
<p>Hết.</p>
<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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