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slice[::-1] là slice[len(slice):0:-1] hay slice[0:len(slice):-1]?
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<h3>Slice là gì</h3>
<p>Slice là cú pháp tiện dụng của Python để lấy 1 list con của list đã có.</p>
<div class="highlight"><pre><span></span><code><span class="o">>>></span> <span class="n">names</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'c'</span><span class="p">,</span> <span class="s1">'java'</span><span class="p">,</span> <span class="s1">'js'</span><span class="p">,</span> <span class="s1">'php'</span><span class="p">,</span> <span class="s1">'python'</span><span class="p">,</span> <span class="s1">'go'</span><span class="p">,</span> <span class="s1">'rust'</span><span class="p">,</span> <span class="s1">'elixir'</span><span class="p">]</span>
</code></pre></div>
<p>Với cú pháp <code>slice[START:STOP:STEP]</code>, <span class="caps">START</span> và <span class="caps">STOP</span> là index bắt đầu và kết thúc,
<span class="caps">STEP</span> là bước dịch chuyển của index, để chọn index tiếp theo.</p>
<div class="highlight"><pre><span></span><code><span class="o">>>></span> <span class="n">names</span><span class="p">[</span><span class="mi">0</span><span class="p">:</span><span class="nb">len</span><span class="p">(</span><span class="n">names</span><span class="p">):</span><span class="mi">2</span><span class="p">]</span>
<span class="p">[</span><span class="s1">'c'</span><span class="p">,</span> <span class="s1">'js'</span><span class="p">,</span> <span class="s1">'python'</span><span class="p">,</span> <span class="s1">'rust'</span><span class="p">]</span>
<span class="o">>>></span> <span class="n">names</span><span class="p">[::</span><span class="mi">2</span><span class="p">]</span>
<span class="p">[</span><span class="s1">'c'</span><span class="p">,</span> <span class="s1">'js'</span><span class="p">,</span> <span class="s1">'python'</span><span class="p">,</span> <span class="s1">'rust'</span><span class="p">]</span>
</code></pre></div>
<p>Ở ví dụ trên lấy các phần tử có index 0, 0+2 = 2, 2+2 = 4, 4+2 = 6.</p>
<ul>
<li><span class="caps">START</span> khi không ghi cụ thể giá trị trước dấu : có giá trị mặc định là 0.</li>
<li><span class="caps">STOP</span> mặc định là <code>len(slice)</code>, tức 8.</li>
<li><span class="caps">STEP</span> nếu không ghi, mặc định là <code>1</code>.</li>
</ul>
<p>Một tính năng thú vị của slice khiến nhiều người phỏng vấn tỏ ra khó chịu khi hỏi: hãy đảo ngược 1 list/str và câu trả lời là <code>names[::-1]</code>.</p>
<div class="highlight"><pre><span></span><code><span class="s s-Atom">>>></span> <span class="s s-Atom">names</span><span class="p">[</span><span class="o">::-</span><span class="mi">1</span><span class="p">]</span>
<span class="p">[</span><span class="s s-Atom">'elixir'</span><span class="p">,</span> <span class="s s-Atom">'rust'</span><span class="p">,</span> <span class="s s-Atom">'go'</span><span class="p">,</span> <span class="s s-Atom">'python'</span><span class="p">,</span> <span class="s s-Atom">'php'</span><span class="p">,</span> <span class="s s-Atom">'js'</span><span class="p">,</span> <span class="s s-Atom">'java'</span><span class="p">,</span> <span class="s s-Atom">'c'</span><span class="p">]</span>
</code></pre></div>
<p>Khi sử dụng <span class="caps">STEP</span> = -1, cách tính sẽ thế nào? names[::-1] là phương án a hay b?</p>
<ul>
<li>a) names[len(names):0:-1]</li>
<li>b) names[0:len(names):-1]</li>
</ul>
<h3>Tìm lời giải slice với step -1 bằng builtin function</h3>
<p>Theo reference,</p>
<p><a href="https://docs.python.org/3/reference/datamodel.html#objects-values-and-types">https://docs.python.org/3/reference/datamodel.html#objects-values-and-types</a></p>
<p>Thử sử dụng built-in function <code>slice</code> để test 2 trường hợp trên:</p>
<div class="highlight"><pre><span></span><code><span class="o">>>></span> <span class="n">S</span> <span class="o">=</span> <span class="nb">slice</span><span class="p">(</span><span class="kc">None</span><span class="p">,</span> <span class="kc">None</span><span class="p">,</span> <span class="o">-</span><span class="mi">1</span><span class="p">)</span>
<span class="o">>>></span> <span class="nb">print</span><span class="p">(</span><span class="n">S</span><span class="o">.</span><span class="n">indices</span><span class="o">.</span><span class="vm">__doc__</span><span class="p">)</span>
<span class="n">S</span><span class="o">.</span><span class="n">indices</span><span class="p">(</span><span class="nb">len</span><span class="p">)</span> <span class="o">-></span> <span class="p">(</span><span class="n">start</span><span class="p">,</span> <span class="n">stop</span><span class="p">,</span> <span class="n">stride</span><span class="p">)</span>
<span class="n">Assuming</span> <span class="n">a</span> <span class="n">sequence</span> <span class="n">of</span> <span class="n">length</span> <span class="nb">len</span><span class="p">,</span> <span class="n">calculate</span> <span class="n">the</span> <span class="n">start</span> <span class="ow">and</span> <span class="n">stop</span>
<span class="n">indices</span><span class="p">,</span> <span class="ow">and</span> <span class="n">the</span> <span class="n">stride</span> <span class="n">length</span> <span class="n">of</span> <span class="n">the</span> <span class="n">extended</span> <span class="nb">slice</span> <span class="n">described</span> <span class="n">by</span>
<span class="n">S</span><span class="o">.</span> <span class="n">Out</span> <span class="n">of</span> <span class="n">bounds</span> <span class="n">indices</span> <span class="n">are</span> <span class="n">clipped</span> <span class="ow">in</span> <span class="n">a</span> <span class="n">manner</span> <span class="n">consistent</span> <span class="k">with</span> <span class="n">the</span>
<span class="n">handling</span> <span class="n">of</span> <span class="n">normal</span> <span class="n">slices</span><span class="o">.</span>
<span class="o">>>></span> <span class="n">names</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'c'</span><span class="p">,</span> <span class="s1">'java'</span><span class="p">,</span> <span class="s1">'js'</span><span class="p">,</span> <span class="s1">'php'</span><span class="p">,</span> <span class="s1">'python'</span><span class="p">,</span> <span class="s1">'go'</span><span class="p">,</span> <span class="s1">'rust'</span><span class="p">,</span> <span class="s1">'elixir'</span><span class="p">]</span>
<span class="o">>>></span> <span class="n">start</span><span class="p">,</span> <span class="n">stop</span><span class="p">,</span> <span class="n">step</span> <span class="o">=</span> <span class="n">S</span><span class="o">.</span><span class="n">indices</span><span class="p">(</span><span class="nb">len</span><span class="p">(</span><span class="n">names</span><span class="p">))</span>
<span class="o">>>></span> <span class="nb">print</span><span class="p">(</span><span class="n">start</span><span class="p">,</span> <span class="n">stop</span><span class="p">,</span> <span class="n">step</span><span class="p">)</span>
<span class="mi">7</span> <span class="o">-</span><span class="mi">1</span> <span class="o">-</span><span class="mi">1</span>
</code></pre></div>
<p><strong>Câu trả lời là khi step = -1 thì start index là <code>len(names)-1</code>, stop index là <code>-1</code></strong>. Cả 2 phương án nêu từ đầu đều sai.</p>
<h3>Tìm lời giải slice với step -1 bằng cách đọc code CPython</h3>
<p>Tải code Python v3.12.0</p>
<div class="highlight"><pre><span></span><code>$ git clone https://github.com/python/cpython --depth <span class="m">1</span> --branch v3.12.0 python3120
Cloning into <span class="s1">'python3120'</span>...
...
Resolving deltas: <span class="m">100</span>% <span class="o">(</span><span class="m">513</span>/513<span class="o">)</span>, <span class="k">done</span>.
Note: switching to <span class="s1">'0fb18b02c8ad56299d6a2910be0bab8ad601ef24'</span>.
You are <span class="k">in</span> <span class="s1">'detached HEAD'</span> state. You can look around, make experimental
...
$ du -csh python3120
136M python3120
136M total
$ <span class="nb">cd</span> python3120
</code></pre></div>
<p>Tìm file code C chứa từ khóa <code>slice</code>:</p>
<div class="highlight"><pre><span></span><code>$ grep -Rin <span class="s1">'slice'</span> -l <span class="p">|</span> grep -F .c
Python/ast_opt.c
Python/import.c
Python/Python-ast.c
Python/compile.c
...
Objects/sliceobject.c
Objects/bytes_methods.c
Objects/listobject.c
...
</code></pre></div>
<p>trong file <code>sliceobject.c</code>:</p>
<div class="highlight"><pre><span></span><code><span class="cm">/* Implementation of slice.indices. */</span><span class="w"></span>
<span class="k">static</span><span class="w"> </span><span class="n">PyObject</span><span class="o">*</span><span class="w"></span>
<span class="nf">slice_indices</span><span class="p">(</span><span class="n">PySliceObject</span><span class="o">*</span><span class="w"> </span><span class="n">self</span><span class="p">,</span><span class="w"> </span><span class="n">PyObject</span><span class="o">*</span><span class="w"> </span><span class="n">len</span><span class="p">)</span><span class="w"></span>
<span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">PyObject</span><span class="w"> </span><span class="o">*</span><span class="n">start</span><span class="p">,</span><span class="w"> </span><span class="o">*</span><span class="n">stop</span><span class="p">,</span><span class="w"> </span><span class="o">*</span><span class="n">step</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="p">...</span><span class="w"></span>
<span class="w"> </span><span class="n">error</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">_PySlice_GetLongIndices</span><span class="p">(</span><span class="n">self</span><span class="p">,</span><span class="w"> </span><span class="n">length</span><span class="p">,</span><span class="w"> </span><span class="o">&</span><span class="n">start</span><span class="p">,</span><span class="w"> </span><span class="o">&</span><span class="n">stop</span><span class="p">,</span><span class="w"> </span><span class="o">&</span><span class="n">step</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="p">...</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>
<span class="p">...</span><span class="w"></span>
<span class="kt">int</span><span class="w"></span>
<span class="n">_PySlice_GetLongIndices</span><span class="p">(</span><span class="n">PySliceObject</span><span class="w"> </span><span class="o">*</span><span class="n">self</span><span class="p">,</span><span class="w"> </span><span class="n">PyObject</span><span class="w"> </span><span class="o">*</span><span class="n">length</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="n">PyObject</span><span class="w"> </span><span class="o">**</span><span class="n">start_ptr</span><span class="p">,</span><span class="w"> </span><span class="n">PyObject</span><span class="w"> </span><span class="o">**</span><span class="n">stop_ptr</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="n">PyObject</span><span class="w"> </span><span class="o">**</span><span class="n">step_ptr</span><span class="p">)</span><span class="w"></span>
<span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="cm">/* Convert step to an integer; raise for zero step. */</span><span class="w"></span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">self</span><span class="o">-></span><span class="n">step</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="n">Py_None</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">step</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">_PyLong_GetOne</span><span class="p">();</span><span class="w"></span>
<span class="w"> </span><span class="n">step_is_negative</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">step_sign</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="p">...</span><span class="w"></span>
<span class="w"> </span><span class="n">step_is_negative</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">step_sign</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="w"> </span><span class="cm">/* Find lower and upper bounds for start and stop. */</span><span class="w"></span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">step_is_negative</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">lower</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">PyLong_FromLong</span><span class="p">(</span><span class="mi">-1L</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="p">...</span><span class="w"></span>
<span class="w"> </span><span class="n">upper</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">PyNumber_Add</span><span class="p">(</span><span class="n">length</span><span class="p">,</span><span class="w"> </span><span class="n">lower</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">lower</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">_PyLong_GetZero</span><span class="p">();</span><span class="w"></span>
<span class="w"> </span><span class="n">upper</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Py_NewRef</span><span class="p">(</span><span class="n">length</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="w"> </span><span class="cm">/* Compute start. */</span><span class="w"></span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">self</span><span class="o">-></span><span class="n">start</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="n">Py_None</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">start</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Py_NewRef</span><span class="p">(</span><span class="n">step_is_negative</span><span class="w"> </span><span class="o">?</span><span class="w"> </span><span class="n">upper</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">lower</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="p">...</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="w"> </span><span class="cm">/* Compute stop. */</span><span class="w"></span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">self</span><span class="o">-></span><span class="n">stop</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="n">Py_None</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">stop</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Py_NewRef</span><span class="p">(</span><span class="n">step_is_negative</span><span class="w"> </span><span class="o">?</span><span class="w"> </span><span class="n">lower</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">upper</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">stop</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">evaluate_slice_index</span><span class="p">(</span><span class="n">self</span><span class="o">-></span><span class="n">stop</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="p">...</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="w"> </span><span class="p">...</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>
</code></pre></div>
<p>đoạn code này thực hiện tính toán khi step < 0, start = upper = length + lower và stop = lower = -1.</p>
<p>Code CPython khá phức tạp, <strong>việc in ra start, stop khi thực hiện slice[::-1] để lại như bài tập cho bạn đọc.</strong></p>
<p>Vậy code nào đơn giản hơn? Python có rất nhiều bản khác nhau và đơn giản nhất có thể kể tới MicroPython.</p>
<h3>MicroPython - khỏe như Python mà nhỏ cỡ micro</h3>
<div class="highlight"><pre><span></span><code>$ git clone https://github.com/micropython/micropython --depth <span class="m">1</span> --branch v1.23.0
Cloning into <span class="s1">'micropython'</span>...
remote: Enumerating objects: <span class="m">5784</span>, <span class="k">done</span>.
remote: Counting objects: <span class="m">100</span>% <span class="o">(</span><span class="m">5784</span>/5784<span class="o">)</span>, <span class="k">done</span>.
$ du -csh micropython
55M micropython
55M total
</code></pre></div>
<p>Mở file <code>py/objslice.c</code> tìm function <code>slice_indices</code> và thêm dòng printf để in start, stop, step:</p>
<div class="highlight"><pre><span></span><code><span class="nb nb-Type">void</span><span class="w"> </span><span class="n">mp_obj_slice_indices</span><span class="p">(</span><span class="n">mp_obj_t</span><span class="w"> </span><span class="n">self_in</span><span class="p">,</span><span class="w"> </span><span class="n">mp_int_t</span><span class="w"> </span><span class="n">length</span><span class="p">,</span><span class="w"> </span><span class="n">mp_bound_slice_t</span><span class="w"> </span><span class="o">*</span><span class="n">result</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">mp_obj_slice_t</span><span class="w"> </span><span class="o">*</span><span class="bp">self</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">MP_OBJ_TO_PTR</span><span class="p">(</span><span class="n">self_in</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="n">mp_int_t</span><span class="w"> </span><span class="n">start</span><span class="p">,</span><span class="w"> </span><span class="n">stop</span><span class="p">,</span><span class="w"> </span><span class="n">step</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="o">...</span><span class="w"></span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">step</span><span class="w"> </span><span class="o">></span><span class="w"> </span><span class="mi">0</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="o">//</span><span class="w"> </span><span class="n">Positive</span><span class="w"> </span><span class="n">step</span><span class="w"></span>
<span class="w"> </span><span class="o">...</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="o">//</span><span class="w"> </span><span class="n">Negative</span><span class="w"> </span><span class="n">step</span><span class="w"></span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="bp">self</span><span class="o">-></span><span class="n">start</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="n">mp_const_none</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">start</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">length</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="mi">1</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">start</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">mp_obj_get_int</span><span class="p">(</span><span class="bp">self</span><span class="o">-></span><span class="n">start</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">start</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="mi">0</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">start</span><span class="w"> </span><span class="o">+=</span><span class="w"> </span><span class="n">length</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="w"> </span><span class="n">start</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">MIN</span><span class="p">(</span><span class="n">length</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="n">MAX</span><span class="p">(</span><span class="n">start</span><span class="p">,</span><span class="w"> </span><span class="o">-</span><span class="mi">1</span><span class="p">));</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="bp">self</span><span class="o">-></span><span class="n">stop</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="n">mp_const_none</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">stop</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="o">-</span><span class="mi">1</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">stop</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">mp_obj_get_int</span><span class="p">(</span><span class="bp">self</span><span class="o">-></span><span class="n">stop</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">stop</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="mi">0</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">stop</span><span class="w"> </span><span class="o">+=</span><span class="w"> </span><span class="n">length</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="w"> </span><span class="n">stop</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">MIN</span><span class="p">(</span><span class="n">length</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="n">MAX</span><span class="p">(</span><span class="n">stop</span><span class="p">,</span><span class="w"> </span><span class="o">-</span><span class="mi">1</span><span class="p">));</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="w"> </span><span class="n">result</span><span class="o">-></span><span class="n">start</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">start</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="n">result</span><span class="o">-></span><span class="n">stop</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">stop</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="n">result</span><span class="o">-></span><span class="n">step</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">step</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="o">//</span><span class="w"> </span><span class="n">Thêm</span><span class="w"> </span><span class="n">dòng</span><span class="w"> </span><span class="n">này</span><span class="p">,</span><span class="w"> </span><span class="n">và</span><span class="w"> </span><span class="n">thêm</span><span class="w"> </span><span class="c1">#include <stdio.h> vào đầu file.</span><span class="w"></span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s2">"DEBUG: start </span><span class="si">%ld</span><span class="s2"> stop </span><span class="si">%ld</span><span class="s2"> step </span><span class="si">%ld</span><span class="se">\n</span><span class="s2">"</span><span class="p">,</span><span class="w"> </span><span class="n">start</span><span class="p">,</span><span class="w"> </span><span class="n">stop</span><span class="p">,</span><span class="w"> </span><span class="n">step</span><span class="p">);</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>
</code></pre></div>
<p>Build binary:</p>
<div class="highlight"><pre><span></span><code><span class="err">$</span> <span class="n">cd</span> <span class="n">micropython</span>
<span class="err">$</span> <span class="n">cd</span> <span class="n">ports</span><span class="o">/</span><span class="n">unix</span>
<span class="err">$</span> <span class="n">make</span> <span class="n">submodules</span>
<span class="err">$</span> <span class="n">sudo</span> <span class="n">apt</span> <span class="n">install</span> <span class="o">-</span><span class="n">y</span> <span class="n">libffi</span><span class="o">-</span><span class="n">dev</span>
<span class="err">$</span> <span class="n">make</span> <span class="o">-</span><span class="n">j</span>
<span class="o">...</span>
<span class="n">make</span> <span class="o">-</span><span class="n">j</span> <span class="mi">28</span><span class="p">,</span><span class="mi">59</span><span class="n">s</span> <span class="n">user</span> <span class="mi">4</span><span class="p">,</span><span class="mi">52</span><span class="n">s</span> <span class="n">system</span> <span class="mi">369</span><span class="o">%</span> <span class="n">cpu</span> <span class="mi">8</span><span class="p">,</span><span class="mi">970</span> <span class="n">total</span>
</code></pre></div>
<p>Chỉ mất 8 giây để build trên máy dùng <code>AMD Ryzen 3 4300U</code>.</p>
<div class="highlight"><pre><span></span><code><span class="err">$</span> <span class="p">.</span><span class="o">/</span><span class="s s-Atom">build</span><span class="o">-</span><span class="s s-Atom">standard</span><span class="o">/</span><span class="s s-Atom">micropython</span>
<span class="nv">MicroPython</span> <span class="s s-Atom">v1</span><span class="mf">.23.0</span><span class="o">-</span><span class="s s-Atom">dirty</span> <span class="s s-Atom">on</span> <span class="mi">2024</span><span class="o">-</span><span class="mi">08</span><span class="o">-</span><span class="mi">19</span><span class="p">;</span> <span class="s s-Atom">linux</span> <span class="p">[</span><span class="nv">GCC</span> <span class="mf">11.4.0</span><span class="p">]</span> <span class="s s-Atom">version</span>
<span class="nv">Use</span> <span class="nv">Ctrl</span><span class="o">-</span><span class="nv">D</span> <span class="s s-Atom">to</span> <span class="s s-Atom">exit</span><span class="p">,</span> <span class="nv">Ctrl</span><span class="o">-</span><span class="nv">E</span> <span class="s s-Atom">for</span> <span class="s s-Atom">paste</span> <span class="s s-Atom">mode</span>
<span class="s s-Atom">>>></span> <span class="s s-Atom">names</span> <span class="o">=</span> <span class="p">[</span><span class="s s-Atom">'c'</span><span class="p">,</span> <span class="s s-Atom">'java'</span><span class="p">,</span> <span class="s s-Atom">'js'</span><span class="p">,</span> <span class="s s-Atom">'php'</span><span class="p">,</span> <span class="s s-Atom">'python'</span><span class="p">,</span> <span class="s s-Atom">'go'</span><span class="p">,</span> <span class="s s-Atom">'rust'</span><span class="p">,</span> <span class="s s-Atom">'elixir'</span><span class="p">]</span>
<span class="s s-Atom">>>></span> <span class="s s-Atom">names</span><span class="p">[</span><span class="o">::-</span><span class="mi">1</span><span class="p">]</span>
<span class="nv">DEBUG</span><span class="o">:</span> <span class="s s-Atom">start</span> <span class="mi">7</span> <span class="s s-Atom">stop</span> <span class="o">-</span><span class="mi">1</span> <span class="s s-Atom">step</span> <span class="o">-</span><span class="mi">1</span>
<span class="p">[</span><span class="s s-Atom">'elixir'</span><span class="p">,</span> <span class="s s-Atom">'rust'</span><span class="p">,</span> <span class="s s-Atom">'go'</span><span class="p">,</span> <span class="s s-Atom">'python'</span><span class="p">,</span> <span class="s s-Atom">'php'</span><span class="p">,</span> <span class="s s-Atom">'js'</span><span class="p">,</span> <span class="s s-Atom">'java'</span><span class="p">,</span> <span class="s s-Atom">'c'</span><span class="p">]</span>
</code></pre></div>
<p>Vậy khi viết <code>slice[::-1]</code> start index là len(slice)-1, stop index là -1.</p>
<h3>Reverse thực hiện thế nào?</h3>
<p>Trong MicroPython <code>py/objlist.c</code> có code</p>
<div class="highlight"><pre><span></span><code><span class="k">static</span><span class="w"> </span><span class="n">mp_obj_t</span><span class="w"> </span><span class="nf">list_reverse</span><span class="p">(</span><span class="n">mp_obj_t</span><span class="w"> </span><span class="n">self_in</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">mp_check_self</span><span class="p">(</span><span class="n">mp_obj_is_type</span><span class="p">(</span><span class="n">self_in</span><span class="p">,</span><span class="w"> </span><span class="o">&</span><span class="n">mp_type_list</span><span class="p">));</span><span class="w"></span>
<span class="w"> </span><span class="n">mp_obj_list_t</span><span class="w"> </span><span class="o">*</span><span class="n">self</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">MP_OBJ_TO_PTR</span><span class="p">(</span><span class="n">self_in</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="n">mp_int_t</span><span class="w"> </span><span class="n">len</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">self</span><span class="o">-></span><span class="n">len</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="n">mp_int_t</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="n">len</span><span class="w"> </span><span class="o">/</span><span class="w"> </span><span class="mi">2</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="o">++</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">mp_obj_t</span><span class="w"> </span><span class="n">a</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">self</span><span class="o">-></span><span class="n">items</span><span class="p">[</span><span class="n">i</span><span class="p">];</span><span class="w"></span>
<span class="w"> </span><span class="n">self</span><span class="o">-></span><span class="n">items</span><span class="p">[</span><span class="n">i</span><span class="p">]</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">self</span><span class="o">-></span><span class="n">items</span><span class="p">[</span><span class="n">len</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="mi">1</span><span class="p">];</span><span class="w"></span>
<span class="w"> </span><span class="n">self</span><span class="o">-></span><span class="n">items</span><span class="p">[</span><span class="n">len</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="mi">1</span><span class="p">]</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">a</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">mp_const_none</span><span class="p">;</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>
</code></pre></div>
<p>Hay code Python</p>
<div class="highlight"><pre><span></span><code><span class="o">>>></span> <span class="n">names</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'c'</span><span class="p">,</span> <span class="s1">'java'</span><span class="p">,</span> <span class="s1">'js'</span><span class="p">,</span> <span class="s1">'php'</span><span class="p">,</span> <span class="s1">'python'</span><span class="p">,</span> <span class="s1">'go'</span><span class="p">,</span> <span class="s1">'rust'</span><span class="p">,</span> <span class="s1">'elixir'</span><span class="p">]</span>
<span class="o">>>></span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="n">length</span><span class="o">//</span><span class="mi">2</span><span class="p">):</span>
<span class="o">...</span> <span class="n">names</span><span class="p">[</span><span class="n">i</span><span class="p">],</span> <span class="n">names</span><span class="p">[</span><span class="n">length</span><span class="o">-</span><span class="n">i</span><span class="o">-</span><span class="mi">1</span><span class="p">]</span> <span class="o">=</span> <span class="n">names</span><span class="p">[</span><span class="n">length</span><span class="o">-</span><span class="n">i</span><span class="o">-</span><span class="mi">1</span><span class="p">],</span> <span class="n">names</span><span class="p">[</span><span class="n">i</span><span class="p">]</span>
<span class="o">...</span>
<span class="o">>>></span> <span class="n">names</span>
<span class="p">[</span><span class="s1">'elixir'</span><span class="p">,</span> <span class="s1">'rust'</span><span class="p">,</span> <span class="s1">'go'</span><span class="p">,</span> <span class="s1">'python'</span><span class="p">,</span> <span class="s1">'php'</span><span class="p">,</span> <span class="s1">'js'</span><span class="p">,</span> <span class="s1">'java'</span><span class="p">,</span> <span class="s1">'c'</span><span class="p">]</span>
</code></pre></div>
<h3>Kết luận</h3>
<p>slice với step=-1 có diễn biến bất thường. Nếu cần tìm xuống bên dưới thì đọc code CPython, nhỡ mà khó quá, thì qua MicroPython.</p>
<p><span class="caps">PS</span>: nhớ làm bài tập với CPython.</p>
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