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<meta name="description" content="In their amazing book “The 5 Elements of Effective Thinking” the authors Burger and Starbird share a story about how they observed Tony Plog, an internationally acclaimed trumpet virtuoso, conduct a master class for accomplished trumpet players. The students first played complex music phrases, which they played perfectly well. But …" />
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<meta property="og:description" content="In their amazing book “The 5 Elements of Effective Thinking” the authors Burger and Starbird share a story about how they observed Tony Plog, an internationally acclaimed trumpet virtuoso, conduct a master class for accomplished trumpet players. The students first played complex music phrases, which they played perfectly well. But …"/>
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<p>In their amazing book &#8220;The 5 Elements of Effective Thinking&#8221; the authors Burger and Starbird share a story about how they observed Tony Plog, an internationally acclaimed trumpet virtuoso, conduct a master class for accomplished trumpet players. The students first played complex music phrases, which they played perfectly well. But then they were asked to play very basic, simple notes. When they played the notes, the notes sounded childish compared to the previously played complex phrases. After they finished playing, the master teacher also played the same notes, but when he played them, they did not sound childish. The difference was stunning. Tony explained that mastering the performance of simple notes allows one to play complex pieces with greater control. The lesson was clear - to build true virtuosity one must focus on mastering simple, basic ideas.<sup id="fnref-1"><a class="footnote-ref" href="index.html#fn-1">1</a></sup></p>
<p>The lesson in the story clearly applies not only to music but also to software development. The story is a good reminder to all of us to not lose sight of the importance of deep work on simple, basic ideas even if it sometimes feels like a step back. While it is important to be proficient with a tool or framework you use, it is also extremely important to know the principles behind them. As Ralph Waldo Emerson&nbsp;said:</p>
<blockquote>
<p><em><span class="dquo">&#8220;</span>If you learn only methods, you’ll be tied to your methods. But if you learn principles, you can devise your own&nbsp;methods.&#8221;</em></p>
</blockquote>
<p>On that note, let’s dive into interpreters and compilers&nbsp;again.</p>
<p>Today I will show you a new version of the calculator from <a href="../lsbasi-part1/index.html" title="Part 1">Part 1</a> that will be able&nbsp;to:</p>
<ol>
<li>Handle whitespace characters anywhere in the input&nbsp;string</li>
<li>Consume multi-digit integers from the&nbsp;input</li>
<li>Subtract two integers (currently it can only add&nbsp;integers)</li>
</ol>
<p>Here is the source code for your new version of the calculator that can do all of the&nbsp;above:</p>
<div class="highlight"><pre><span></span><span class="c1"># Token types</span>
<span class="c1"># EOF (end-of-file) token is used to indicate that</span>
<span class="c1"># there is no more input left for lexical analysis</span>
<span class="n">INTEGER</span><span class="p">,</span> <span class="n">PLUS</span><span class="p">,</span> <span class="n">MINUS</span><span class="p">,</span> <span class="n">EOF</span> <span class="o">=</span> <span class="s1">&#39;INTEGER&#39;</span><span class="p">,</span> <span class="s1">&#39;PLUS&#39;</span><span class="p">,</span> <span class="s1">&#39;MINUS&#39;</span><span class="p">,</span> <span class="s1">&#39;EOF&#39;</span>
<span class="k">class</span> <span class="nc">Token</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="nb">type</span><span class="p">,</span> <span class="n">value</span><span class="p">):</span>
<span class="c1"># token type: INTEGER, PLUS, MINUS, or EOF</span>
<span class="bp">self</span><span class="o">.</span><span class="n">type</span> <span class="o">=</span> <span class="nb">type</span>
<span class="c1"># token value: non-negative integer value, &#39;+&#39;, &#39;-&#39;, or None</span>
<span class="bp">self</span><span class="o">.</span><span class="n">value</span> <span class="o">=</span> <span class="n">value</span>
<span class="k">def</span> <span class="fm">__str__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="sd">&quot;&quot;&quot;String representation of the class instance.</span>
<span class="sd"> Examples:</span>
<span class="sd"> Token(INTEGER, 3)</span>
<span class="sd"> Token(PLUS &#39;+&#39;)</span>
<span class="sd"> &quot;&quot;&quot;</span>
<span class="k">return</span> <span class="s1">&#39;Token({type}, {value})&#39;</span><span class="o">.</span><span class="n">format</span><span class="p">(</span>
<span class="nb">type</span><span class="o">=</span><span class="bp">self</span><span class="o">.</span><span class="n">type</span><span class="p">,</span>
<span class="n">value</span><span class="o">=</span><span class="nb">repr</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">value</span><span class="p">)</span>
<span class="p">)</span>
<span class="k">def</span> <span class="fm">__repr__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="fm">__str__</span><span class="p">()</span>
<span class="k">class</span> <span class="nc">Interpreter</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">text</span><span class="p">):</span>
<span class="c1"># client string input, e.g. &quot;3 + 5&quot;, &quot;12 - 5&quot;, etc</span>
<span class="bp">self</span><span class="o">.</span><span class="n">text</span> <span class="o">=</span> <span class="n">text</span>
<span class="c1"># self.pos is an index into self.text</span>
<span class="bp">self</span><span class="o">.</span><span class="n">pos</span> <span class="o">=</span> <span class="mi">0</span>
<span class="c1"># current token instance</span>
<span class="bp">self</span><span class="o">.</span><span class="n">current_token</span> <span class="o">=</span> <span class="bp">None</span>
<span class="bp">self</span><span class="o">.</span><span class="n">current_char</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">text</span><span class="p">[</span><span class="bp">self</span><span class="o">.</span><span class="n">pos</span><span class="p">]</span>
<span class="k">def</span> <span class="nf">error</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">raise</span> <span class="ne">Exception</span><span class="p">(</span><span class="s1">&#39;Error parsing input&#39;</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">advance</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="sd">&quot;&quot;&quot;Advance the &#39;pos&#39; pointer and set the &#39;current_char&#39; variable.&quot;&quot;&quot;</span>
<span class="bp">self</span><span class="o">.</span><span class="n">pos</span> <span class="o">+=</span> <span class="mi">1</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">pos</span> <span class="o">&gt;</span> <span class="nb">len</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">text</span><span class="p">)</span> <span class="o">-</span> <span class="mi">1</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">current_char</span> <span class="o">=</span> <span class="bp">None</span> <span class="c1"># Indicates end of input</span>
<span class="k">else</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">current_char</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">text</span><span class="p">[</span><span class="bp">self</span><span class="o">.</span><span class="n">pos</span><span class="p">]</span>
<span class="k">def</span> <span class="nf">skip_whitespace</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">while</span> <span class="bp">self</span><span class="o">.</span><span class="n">current_char</span> <span class="ow">is</span> <span class="ow">not</span> <span class="bp">None</span> <span class="ow">and</span> <span class="bp">self</span><span class="o">.</span><span class="n">current_char</span><span class="o">.</span><span class="n">isspace</span><span class="p">():</span>
<span class="bp">self</span><span class="o">.</span><span class="n">advance</span><span class="p">()</span>
<span class="k">def</span> <span class="nf">integer</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="sd">&quot;&quot;&quot;Return a (multidigit) integer consumed from the input.&quot;&quot;&quot;</span>
<span class="n">result</span> <span class="o">=</span> <span class="s1">&#39;&#39;</span>
<span class="k">while</span> <span class="bp">self</span><span class="o">.</span><span class="n">current_char</span> <span class="ow">is</span> <span class="ow">not</span> <span class="bp">None</span> <span class="ow">and</span> <span class="bp">self</span><span class="o">.</span><span class="n">current_char</span><span class="o">.</span><span class="n">isdigit</span><span class="p">():</span>
<span class="n">result</span> <span class="o">+=</span> <span class="bp">self</span><span class="o">.</span><span class="n">current_char</span>
<span class="bp">self</span><span class="o">.</span><span class="n">advance</span><span class="p">()</span>
<span class="k">return</span> <span class="nb">int</span><span class="p">(</span><span class="n">result</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">get_next_token</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="sd">&quot;&quot;&quot;Lexical analyzer (also known as scanner or tokenizer)</span>
<span class="sd"> This method is responsible for breaking a sentence</span>
<span class="sd"> apart into tokens.</span>
<span class="sd"> &quot;&quot;&quot;</span>
<span class="k">while</span> <span class="bp">self</span><span class="o">.</span><span class="n">current_char</span> <span class="ow">is</span> <span class="ow">not</span> <span class="bp">None</span><span class="p">:</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">current_char</span><span class="o">.</span><span class="n">isspace</span><span class="p">():</span>
<span class="bp">self</span><span class="o">.</span><span class="n">skip_whitespace</span><span class="p">()</span>
<span class="k">continue</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">current_char</span><span class="o">.</span><span class="n">isdigit</span><span class="p">():</span>
<span class="k">return</span> <span class="n">Token</span><span class="p">(</span><span class="n">INTEGER</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">integer</span><span class="p">())</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">current_char</span> <span class="o">==</span> <span class="s1">&#39;+&#39;</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">advance</span><span class="p">()</span>
<span class="k">return</span> <span class="n">Token</span><span class="p">(</span><span class="n">PLUS</span><span class="p">,</span> <span class="s1">&#39;+&#39;</span><span class="p">)</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">current_char</span> <span class="o">==</span> <span class="s1">&#39;-&#39;</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">advance</span><span class="p">()</span>
<span class="k">return</span> <span class="n">Token</span><span class="p">(</span><span class="n">MINUS</span><span class="p">,</span> <span class="s1">&#39;-&#39;</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">error</span><span class="p">()</span>
<span class="k">return</span> <span class="n">Token</span><span class="p">(</span><span class="n">EOF</span><span class="p">,</span> <span class="bp">None</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">eat</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">token_type</span><span class="p">):</span>
<span class="c1"># compare the current token type with the passed token</span>
<span class="c1"># type and if they match then &quot;eat&quot; the current token</span>
<span class="c1"># and assign the next token to the self.current_token,</span>
<span class="c1"># otherwise raise an exception.</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">current_token</span><span class="o">.</span><span class="n">type</span> <span class="o">==</span> <span class="n">token_type</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">current_token</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">get_next_token</span><span class="p">()</span>
<span class="k">else</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">error</span><span class="p">()</span>
<span class="k">def</span> <span class="nf">expr</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="sd">&quot;&quot;&quot;Parser / Interpreter</span>
<span class="sd"> expr -&gt; INTEGER PLUS INTEGER</span>
<span class="sd"> expr -&gt; INTEGER MINUS INTEGER</span>
<span class="sd"> &quot;&quot;&quot;</span>
<span class="c1"># set current token to the first token taken from the input</span>
<span class="bp">self</span><span class="o">.</span><span class="n">current_token</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">get_next_token</span><span class="p">()</span>
<span class="c1"># we expect the current token to be an integer</span>
<span class="n">left</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">current_token</span>
<span class="bp">self</span><span class="o">.</span><span class="n">eat</span><span class="p">(</span><span class="n">INTEGER</span><span class="p">)</span>
<span class="c1"># we expect the current token to be either a &#39;+&#39; or &#39;-&#39;</span>
<span class="n">op</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">current_token</span>
<span class="k">if</span> <span class="n">op</span><span class="o">.</span><span class="n">type</span> <span class="o">==</span> <span class="n">PLUS</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">eat</span><span class="p">(</span><span class="n">PLUS</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">eat</span><span class="p">(</span><span class="n">MINUS</span><span class="p">)</span>
<span class="c1"># we expect the current token to be an integer</span>
<span class="n">right</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">current_token</span>
<span class="bp">self</span><span class="o">.</span><span class="n">eat</span><span class="p">(</span><span class="n">INTEGER</span><span class="p">)</span>
<span class="c1"># after the above call the self.current_token is set to</span>
<span class="c1"># EOF token</span>
<span class="c1"># at this point either the INTEGER PLUS INTEGER or</span>
<span class="c1"># the INTEGER MINUS INTEGER sequence of tokens</span>
<span class="c1"># has been successfully found and the method can just</span>
<span class="c1"># return the result of adding or subtracting two integers,</span>
<span class="c1"># thus effectively interpreting client input</span>
<span class="k">if</span> <span class="n">op</span><span class="o">.</span><span class="n">type</span> <span class="o">==</span> <span class="n">PLUS</span><span class="p">:</span>
<span class="n">result</span> <span class="o">=</span> <span class="n">left</span><span class="o">.</span><span class="n">value</span> <span class="o">+</span> <span class="n">right</span><span class="o">.</span><span class="n">value</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">result</span> <span class="o">=</span> <span class="n">left</span><span class="o">.</span><span class="n">value</span> <span class="o">-</span> <span class="n">right</span><span class="o">.</span><span class="n">value</span>
<span class="k">return</span> <span class="n">result</span>
<span class="k">def</span> <span class="nf">main</span><span class="p">():</span>
<span class="k">while</span> <span class="bp">True</span><span class="p">:</span>
<span class="k">try</span><span class="p">:</span>
<span class="c1"># To run under Python3 replace &#39;raw_input&#39; call</span>
<span class="c1"># with &#39;input&#39;</span>
<span class="n">text</span> <span class="o">=</span> <span class="nb">raw_input</span><span class="p">(</span><span class="s1">&#39;calc&gt; &#39;</span><span class="p">)</span>
<span class="k">except</span> <span class="ne">EOFError</span><span class="p">:</span>
<span class="k">break</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">text</span><span class="p">:</span>
<span class="k">continue</span>
<span class="n">interpreter</span> <span class="o">=</span> <span class="n">Interpreter</span><span class="p">(</span><span class="n">text</span><span class="p">)</span>
<span class="n">result</span> <span class="o">=</span> <span class="n">interpreter</span><span class="o">.</span><span class="n">expr</span><span class="p">()</span>
<span class="k">print</span><span class="p">(</span><span class="n">result</span><span class="p">)</span>
<span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s1">&#39;__main__&#39;</span><span class="p">:</span>
<span class="n">main</span><span class="p">()</span>
</pre></div>
<p>Save the above code into the <em>calc2.py</em> file or download it directly from <a href="https://github.com/rspivak/lsbasi/blob/master/part2/calc2.py">GitHub</a>. Try it out. See for yourself that it works as expected: it can handle whitespace characters anywhere in the input; it can accept multi-digit integers, and it can also subtract two integers as well as add two&nbsp;integers.</p>
<p>Here is a sample session that I ran on my&nbsp;laptop:</p>
<div class="highlight"><pre><span></span>$ python calc2.py
calc&gt; <span class="m">27</span> + <span class="m">3</span>
<span class="m">30</span>
calc&gt; <span class="m">27</span> - <span class="m">7</span>
<span class="m">20</span>
calc&gt;
</pre></div>
<p>The major code changes compared with the version from <a href="../lsbasi-part1/index.html" title="Part 1">Part 1</a>&nbsp;are:</p>
<ol>
<li>The <em>get_next_token</em> method was refactored a bit. The logic to increment the <em>pos</em> pointer was factored into a separate method <em>advance</em>.</li>
<li>Two more methods were added: <em>skip_whitespace</em> to ignore whitespace characters and <em>integer</em> to handle multi-digit integers in the&nbsp;input.</li>
<li>The <em>expr</em> method was modified to recognize <span class="caps">INTEGER</span> -&gt; <span class="caps">MINUS</span> -&gt; <span class="caps">INTEGER</span> phrase in addition to <span class="caps">INTEGER</span> -&gt; <span class="caps">PLUS</span> -&gt; <span class="caps">INTEGER</span> phrase. The method now also interprets both addition and subtraction after having successfully recognized the corresponding&nbsp;phrase.</li>
</ol>
<p>In <a href="../lsbasi-part1/index.html" title="Part 1">Part 1</a> you learned two important concepts, namely that of a <strong>token</strong> and a <strong>lexical analyzer</strong>. Today I would like to talk a little bit about <strong>lexemes</strong>, <strong>parsing</strong>, and <strong>parsers</strong>.</p>
<p>You already know about tokens. But in order for me to round out the discussion of tokens I need to mention lexemes. What is a lexeme? A <strong>lexeme</strong> is a sequence of characters that form a token. In the following picture you can see some examples of tokens and sample lexemes and hopefully it will make the relationship between them&nbsp;clear:</p>
<p><img alt="" src="lsbasi_part2_lexemes.png"></p>
<p>Now, remember our friend, the <em>expr</em> method? I said before that that’s where the interpretation of an arithmetic expression actually happens. But before you can interpret an expression you first need to recognize what kind of phrase it is, whether it is addition or subtraction, for example. That’s what the <em>expr</em> method essentially does: it finds the structure in the stream of tokens it gets from the <em>get_next_token</em> method and then it interprets the phrase that is has recognized, generating the result of the arithmetic&nbsp;expression.</p>
<p>The process of finding the structure in the stream of tokens, or put differently, the process of recognizing a phrase in the stream of tokens is called <strong>parsing</strong>. The part of an interpreter or compiler that performs that job is called a <strong>parser</strong>.</p>
<p>So now you know that the <em>expr</em> method is the part of your interpreter where both <strong>parsing</strong> and <strong>interpreting</strong> happens - the <em>expr</em> method first tries to recognize (<strong>parse</strong>) the <span class="caps">INTEGER</span> -&gt; <span class="caps">PLUS</span> -&gt; <span class="caps">INTEGER</span> or the <span class="caps">INTEGER</span> -&gt; <span class="caps">MINUS</span> -&gt; <span class="caps">INTEGER</span> phrase in the stream of tokens and after it has successfully recognized (<strong>parsed</strong>) one of those phrases, the method interprets it and returns the result of either addition or subtraction of two integers to the&nbsp;caller.</p>
<p>And now it’s time for exercises&nbsp;again.</p>
<p><img alt="" src="lsbasi_part2_exercises.png" width="280"></p>
<ol>
<li>Extend the calculator to handle multiplication of two&nbsp;integers</li>
<li>Extend the calculator to handle division of two&nbsp;integers</li>
<li>Modify the code to interpret expressions containing an arbitrary number of additions and subtractions, for example &#8220;9 - 5 + 3 +&nbsp;11&#8221;</li>
</ol>
<p><strong>Check your&nbsp;understanding.</strong></p>
<ol>
<li>What is a&nbsp;lexeme?</li>
<li>What is the name of the process that finds the structure in the stream of tokens, or put differently, what is the name of the process that recognizes a certain phrase in that stream of&nbsp;tokens?</li>
<li>What is the name of the part of the interpreter (compiler) that does&nbsp;parsing?</li>
</ol>
<p><br/>
I hope you liked today’s material. In the next article of the series you will extend your calculator to handle more complex arithmetic expressions. Stay&nbsp;tuned.</p>
<p>And here is a list of books I recommend that will help you in your study of interpreters and&nbsp;compilers:</p>
<ol>
<li>
<p><a href="http://www.amazon.com/gp/product/193435645X/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=193435645X&linkCode=as2&tag=russblo0b-20&linkId=MP4DCXDV6DJMEJBL">Language Implementation Patterns: Create Your Own Domain-Specific and General Programming Languages (Pragmatic Programmers)</a><img src="http://ir-na.amazon-adsystem.com/e/ir?t=russblo0b-20&l=as2&o=1&a=193435645X" width="1" height="1" border="0" alt="" style="border:none !important; margin:0px !important;" /></p>
</li>
<li>
<p><a href="http://www.amazon.com/gp/product/0470177071/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0470177071&linkCode=as2&tag=russblo0b-20&linkId=UCLGQTPIYSWYKRRM">Writing Compilers and Interpreters: A Software Engineering Approach</a><img src="http://ir-na.amazon-adsystem.com/e/ir?t=russblo0b-20&l=as2&o=1&a=0470177071" width="1" height="1" border="0" alt="" style="border:none !important; margin:0px !important;" /></p>
</li>
<li>
<p><a href="http://www.amazon.com/gp/product/052182060X/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=052182060X&linkCode=as2&tag=russblo0b-20&linkId=ZSKKZMV7YWR22NMW">Modern Compiler Implementation in Java</a><img src="http://ir-na.amazon-adsystem.com/e/ir?t=russblo0b-20&l=as2&o=1&a=052182060X" width="1" height="1" border="0" alt="" style="border:none !important; margin:0px !important;" /></p>
</li>
<li>
<p><a href="http://www.amazon.com/gp/product/1461446988/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=1461446988&linkCode=as2&tag=russblo0b-20&linkId=PAXWJP5WCPZ7RKRD">Modern Compiler Design</a><img src="http://ir-na.amazon-adsystem.com/e/ir?t=russblo0b-20&l=as2&o=1&a=1461446988" width="1" height="1" border="0" alt="" style="border:none !important; margin:0px !important;" /></p>
</li>
<li>
<p><a href="http://www.amazon.com/gp/product/0321486811/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0321486811&linkCode=as2&tag=russblo0b-20&linkId=GOEGDQG4HIHU56FQ">Compilers: Principles, Techniques, and Tools (2nd Edition)</a><img src="http://ir-na.amazon-adsystem.com/e/ir?t=russblo0b-20&l=as2&o=1&a=0321486811" width="1" height="1" border="0" alt="" style="border:none !important; margin:0px !important;" /></p>
</li>
</ol>
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</p>
<p><br/>
<strong>All articles in this series:</strong>
<ul>
<li>
<a href="../lsbasi-part1/index.html">Let's Build A Simple Interpreter. Part 1.</a>
</li>
<li>
<a href="index.html">Let's Build A Simple Interpreter. Part 2.</a>
</li>
<li>
<a href="../lsbasi-part3/index.html">Let's Build A Simple Interpreter. Part 3.</a>
</li>
<li>
<a href="../lsbasi-part4/index.html">Let's Build A Simple Interpreter. Part 4.</a>
</li>
<li>
<a href="../lsbasi-part5/index.html">Let's Build A Simple Interpreter. Part 5.</a>
</li>
<li>
<a href="../lsbasi-part6/index.html">Let's Build A Simple Interpreter. Part 6.</a>
</li>
<li>
<a href="../lsbasi-part7/index.html">Let's Build A Simple Interpreter. Part 7: Abstract Syntax Trees</a>
</li>
<li>
<a href="../lsbasi-part8/index.html">Let's Build A Simple Interpreter. Part 8.</a>
</li>
<li>
<a href="../lsbasi-part9/index.html">Let's Build A Simple Interpreter. Part 9.</a>
</li>
<li>
<a href="../lsbasi-part10/index.html">Let's Build A Simple Interpreter. Part 10.</a>
</li>
<li>
<a href="../lsbasi-part11/index.html">Let's Build A Simple Interpreter. Part 11.</a>
</li>
<li>
<a href="../lsbasi-part12/index.html">Let's Build A Simple Interpreter. Part 12.</a>
</li>
<li>
<a href="../lsbasi-part13.html">Let's Build A Simple Interpreter. Part 13: Semantic Analysis</a>
</li>
<li>
<a href="../lsbasi-part14/index.html">Let's Build A Simple Interpreter. Part 14: Nested Scopes and a Source-to-Source Compiler</a>
</li>
<li>
<a href="../lsbasi-part15/index.html">Let's Build A Simple Interpreter. Part 15.</a>
</li>
<li>
<a href="../lsbasi-part16/index.html">Let's Build A Simple Interpreter. Part 16: Recognizing Procedure Calls</a>
</li>
<li>
<a href="../lsbasi-part17.html">Let's Build A Simple Interpreter. Part 17: Call Stack and Activation Records</a>
</li>
<li>
<a href="../lsbasi-part18/index.html">Let's Build A Simple Interpreter. Part 18: Executing Procedure Calls</a>
</li>
<li>
<a href="../lsbasi-part19/index.html">Let's Build A Simple Interpreter. Part 19: Nested Procedure Calls</a>
</li>
</ul>
</p>
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<p><a href="http://www.amazon.com/gp/product/0691156662/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0691156662&linkCode=as2&tag=russblo0b-20&linkId=B7GSVLONUPCIBIVY">The 5 Elements of Effective Thinking</a><img src="http://ir-na.amazon-adsystem.com/e/ir?t=russblo0b-20&l=as2&o=1&a=0691156662" width="1" height="1" border="0" alt="" style="border:none !important; margin:0px !important;" />&#160;<a class="footnote-backref" href="index.html#fnref-1" title="Jump back to footnote 1 in the text">&#8617;</a></p>
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