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<h1 class="title">Gnu Awk - Part 8 (HPR Show 2438)</h1>
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<h2 class="author">Dave Morriss</h2>
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<hr/>
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<ul>
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<li><a href="#introduction">Introduction</a></li>
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<li><a href="#recap-of-the-last-episode">Recap of the last episode</a></li>
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<li><a href="#some-more-statements">Some more statements</a><ul>
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<li><a href="#the-switch-statement">The <code>switch</code> statement</a></li>
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<li><a href="#the-break-statement">The <code>break</code> statement</a></li>
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<li><a href="#the-continue-statement">The <code>continue</code> statement</a></li>
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<li><a href="#the-next-statement">The <code>next</code> statement</a></li>
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</ul></li>
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<li><a href="#links">Links</a></li>
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</ul>
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</nav>
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</header>
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<h2 id="introduction">Introduction</h2>
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<p>This is the eighth episode of the “<a href="http://hackerpublicradio.org/series.php?id=94" title="Learning Awk">Learning Awk</a>” series that <a href="http://hackerpublicradio.org/correspondents.php?hostid=300" title="Mr. Young">Mr. Young</a> and I are doing.</p>
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<h2 id="recap-of-the-last-episode">Recap of the last episode</h2>
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<ul>
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<li><p>The <code>while</code> loop: tests a condition and performs commands <em>while</em> the test returns true</p></li>
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<li><p>The <code>do while</code> loop: performs commands after the <code>do</code>, then tests afterwards, repeating the commands <em>while</em> the test is true.</p></li>
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<li><p>The <code>for</code> loop (type 1): initialises a variable, performs a test, and increments the variable all together, performing commands while the test is true.</p></li>
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<li><p>The <code>for</code> loop (type 2): sets a variable to successive indices of an array, preforming a collection of commands for each index.</p></li>
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</ul>
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<p>These types of loops were demonstrated by examples in the <a href="http://hackerpublicradio.org/eps/hpr2330" title="Awk Part 7">last episode</a>.</p>
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<p>Note that the example for ‘<code>do while</code>’ was an infinite loop (perhaps as a test of the alertness of the audience!):</p>
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<pre><code>#!/usr/bin/awk -f
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BEGIN {
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i=2;
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do {
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print "The square of ", i, " is ", i*i;
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i = i + 1
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}
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while (i != 2)
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exit;
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}</code></pre>
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<p>The condition in the <code>while</code> is always true:</p>
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<pre><code>The square of 2 is 4
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The square of 3 is 9
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The square of 4 is 16
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The square of 5 is 25
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The square of 6 is 36
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The square of 7 is 49
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The square of 8 is 64
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The square of 9 is 81
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The square of 10 is 100
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...
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The square of 1269630 is 1611960336900
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The square of 1269631 is 1611962876161
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The square of 1269632 is 1611965415424
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The square of 1269633 is 1611967954689
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The square of 1269634 is 1611970493956
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...</code></pre>
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<p>The variable <code>i</code> is set to 2, the <code>print</code> is executed, then <code>i</code> is set to 3. The test “<code>i != 2</code>” is true and will be <em>ad infinitum</em>.</p>
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<h2 id="some-more-statements">Some more statements</h2>
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<p>We will come back to loops later in this episode, but first this seems like a good point to describe another statement: the <code>switch</code> statement.</p>
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<h3 id="the-switch-statement">The <code>switch</code> statement</h3>
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<p>This is specific to <code>gawk</code>, and can be disabled if non-GNU <code>awk</code>-compatibility is required. The <code>switch</code> statement in <code>gawk</code> is very similar to the one in <code>C</code> and many other languages.</p>
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<p>The layout of the <code>switch</code> statement is as follows:</p>
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<p><code>switch</code> (<em>expression</em>) {<br/> <code>case</code> <em>value</em>:<br/> <em>case-body</em><br/> <code>default</code>:<br/> <em>default-body</em><br/> }</p>
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<p>The ‘<code>expression</code>’ part is an expression, which returns a numeric or string result. The ‘<code>value</code>’ part after the <code>case</code> is a numeric or string constant or a regular expression.</p>
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<p>The <code>expression</code> is evaluated and the result matched against the case <code>value</code>s in turn. If there is a match the <code>case-body</code> statements are executed. If there is no match the <code>default-body</code> statements are executed.</p>
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<p>The following example is included as one of the files associated with this show, called <code>switch_example.awk</code>:</p>
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<div class="sourceCode"><pre class="sourceCode awk"><code class="sourceCode awk"><span class="co">#!/usr/bin/awk -f</span>
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<span class="co">#</span>
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<span class="co"># Example of the use of 'switch' in GNU Awk.</span>
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<span class="co">#</span>
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<span class="co"># Should be run against the data file 'file1.txt' included with the second</span>
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<span class="co"># show in the series: http://hackerpublicradio.org/eps/hpr2129/file1.txt</span>
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<span class="co">#</span>
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<span class="bu">NR</span> <span class="op">></span> <span class="dv">1</span> <span class="kw">{</span>
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<span class="kw">printf</span> <span class="st">"The %s is classified as: "</span><span class="op">,</span><span class="dt">$1</span>
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switch (<span class="dt">$1</span>) <span class="kw">{</span>
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case <span class="st">"apple"</span><span class="op">:</span>
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<span class="kw">print</span> <span class="st">"a fruit, pome"</span>
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<span class="kw">break</span>
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case <span class="st">"banana"</span><span class="op">:</span>
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case <span class="st">"grape"</span><span class="op">:</span>
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case <span class="st">"kiwi"</span><span class="op">:</span>
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<span class="kw">print</span> <span class="st">"a fruit, berry"</span>
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<span class="kw">break</span>
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case <span class="st">"strawberry"</span><span class="op">:</span>
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<span class="kw">print</span> <span class="st">"not a true fruit, pseudocarp"</span>
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<span class="kw">break</span>
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case <span class="st">"plum"</span><span class="op">:</span>
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<span class="kw">print</span> <span class="st">"a fruit, drupe"</span>
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<span class="kw">break</span>
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case <span class="st">"pineapple"</span><span class="op">:</span>
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<span class="kw">print</span> <span class="st">"a fruit, fused berries (syncarp)"</span>
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<span class="kw">break</span>
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case <span class="st">"potato"</span><span class="op">:</span>
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<span class="kw">print</span> <span class="st">"a vegetable, tuber"</span>
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<span class="kw">break</span>
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default<span class="op">:</span>
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<span class="kw">print</span> <span class="st">"[unclassified]"</span>
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<span class="kw">}</span>
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<span class="kw">}</span></code></pre></div>
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<p>The result of running this script against the “fruit” file presented in show 2129 is the following (<code>switch_example.out</code>): <small></p>
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<pre><code>The apple is classified as: a fruit, pome
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The banana is classified as: a fruit, berry
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The strawberry is classified as: not a true fruit, pseudocarp
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The grape is classified as: a fruit, berry
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The apple is classified as: a fruit, pome
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The plum is classified as: a fruit, drupe
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The kiwi is classified as: a fruit, berry
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The potato is classified as: a vegetable, tuber
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The pineapple is classified as: a fruit, fused berries (syncarp)</code></pre>
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<p></small></p>
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<p>What this simple example does is:</p>
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<ul>
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<li>It ignores the first line of the file (a header)</li>
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<li>It prints the first field (the name of a fruit - mostly) in the string “The %s is classified as:”. There is no newline so whatever is printed next is appended to the line.</li>
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<li>It uses the first field in a <code>switch</code> statement. Each <code>case</code> is an exact match with the contents of the field. If there is a match a <code>print</code> statement is used to print out the Botanical classification. If there are no matches then the <code>default</code> instance would print “[unclassified]”, but that doesn’t happen in this example.</li>
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<li>All <code>print</code> statements are followed by <code>break</code>. If this hadn’t been there the next <code>case</code> would be executed and so forth. This can be desirable in some instances. See the next section for a discussion of <code>break</code>.</li>
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<li>Note that banana, grape and kiwi are all Botanically classified as a berry, so there are three <code>case</code> parts associated with one <code>print</code>.</li>
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</ul>
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<h3 id="the-break-statement">The <code>break</code> statement</h3>
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<p>This statement is mainly for “breaking out of” a <code>for</code>, <code>while</code> or <code>do-while</code> loop, though, as we have seen it can interrupt the flow of execution in a <code>switch</code> statement also. Outside of these statements <code>break</code> has no effect.</p>
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<p>In a loop a <code>break</code> statement is often used where it’s not possible to determine the number of iterations of the loop beforehand. Invoking <code>break</code> completely terminates the enclosing loop (relevant when there are nested loops, or loops within loops).</p>
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<p>The following example (available for download as <code>divisor.awk</code>) is from the Gnu Awk manual and shows a method of finding the smallest divisor:</p>
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<div class="sourceCode"><pre class="sourceCode awk"><code class="sourceCode awk"><span class="co">#!/usr/bin/awk -f</span>
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<span class="co"># find smallest divisor of num</span>
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<span class="kw">{</span>
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num <span class="op">=</span> <span class="dt">$1</span>
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<span class="co">#</span>
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<span class="co"># Make an infinite loop using the for loop</span>
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<span class="co">#</span>
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<span class="kw">for</span> (divisor <span class="op">=</span> <span class="dv">2</span><span class="op">;</span> <span class="op">;</span> divisor<span class="op">++</span>) <span class="kw">{</span>
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<span class="co">#</span>
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<span class="co"># If the number is divisible by 'divisor' then we're done</span>
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<span class="co">#</span>
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<span class="kw">if</span> (num <span class="op">%</span> divisor <span class="op">==</span> <span class="dv">0</span>) <span class="kw">{</span>
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<span class="kw">printf</span> <span class="st">"Smallest divisor of %d is %d</span><span class="sc">\n</span><span class="st">"</span><span class="op">,</span> num<span class="op">,</span> divisor
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<span class="kw">break</span>
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<span class="kw">}</span>
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<span class="co">#</span>
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<span class="co"># If the value of 'divisor' has got too large the number has no</span>
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<span class="co"># divisors and is therefore a prime number</span>
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<span class="co">#</span>
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<span class="kw">if</span> (divisor <span class="op">*</span> divisor <span class="op">></span> num) <span class="kw">{</span>
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<span class="kw">printf</span> <span class="st">"%d is prime</span><span class="sc">\n</span><span class="st">"</span><span class="op">,</span> num
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<span class="kw">break</span>
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<span class="kw">}</span>
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<span class="kw">}</span>
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<span class="kw">}</span></code></pre></div>
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<p>I have added some comments to this script to (hopefully) make it clearer.</p>
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<p>Running this in a pipeline with the number presented to it as shown results in the following type of output (<code>divisor.out</code>):</p>
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<pre><code>$ echo 67 | ./divisor.awk
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67 is prime
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$ echo 69 | ./divisor.awk
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Smallest divisor of 69 is 3</code></pre>
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<h3 id="the-continue-statement">The <code>continue</code> statement</h3>
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<p>This is similar to <code>break</code> in that it is used a <code>for</code>, <code>while</code> or <code>do-while</code> loop. It is <strong>not</strong> relevant in <code>switch</code> statements however.</p>
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<p>Invoking <code>continue</code> skips the rest of the enclosing loop and begins the next cycle.</p>
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<p>The following example (available for download as <code>continue_example.awk</code>) is from the Gnu Awk manual and demonstrates a possible use of <code>continue</code>:</p>
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<div class="sourceCode"><pre class="sourceCode awk"><code class="sourceCode awk"><span class="co">#!/usr/bin/awk -f</span>
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<span class="co">#</span>
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<span class="co"># Loop, printing numbers from 0-20, except for 5</span>
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<span class="co"># (From the GNU Awk User's Guide)</span>
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<span class="co">#</span>
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<span class="cf">BEGIN</span> <span class="kw">{</span>
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<span class="kw">for</span> (x <span class="op">=</span> <span class="dv">0</span><span class="op">;</span> x <span class="op"><=</span> <span class="dv">20</span><span class="op">;</span> x<span class="op">++</span>) <span class="kw">{</span>
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<span class="kw">if</span> (x <span class="op">==</span> <span class="dv">5</span>)
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<span class="kw">continue</span>
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<span class="kw">printf</span> <span class="st">"%d "</span><span class="op">,</span> x
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<span class="kw">}</span>
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<span class="kw">print</span> <span class="st">""</span>
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<span class="kw">}</span></code></pre></div>
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<h3 id="the-next-statement">The <code>next</code> statement</h3>
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<p>This statement is not related to loops in the same way as <code>break</code> and <code>continue</code> but to the main record processing cycle of Awk. The <code>next</code> statement causes Awk to stop processing the current input record and go on to the next one.</p>
|
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<p>As we know from earlier episodes in this series, Awk reads records from its input stream and applies rules to them. The <code>next</code> statement stops the execution of further rules for the current record, and moves on to the next one.</p>
|
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<p>The following example (available for download as <code>next_example.awk</code>) is demonstrates a use of <code>next</code>:</p>
|
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<div class="sourceCode"><pre class="sourceCode awk"><code class="sourceCode awk"><span class="co">#!/usr/bin/awk -f</span>
|
||
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<span class="co">#</span>
|
||
<span class="co"># Ignore the header</span>
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<span class="co">#</span>
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<span class="bu">NR</span> <span class="op">==</span> <span class="dv">1</span> <span class="kw">{</span> <span class="kw">next</span> <span class="kw">}</span>
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||
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||
<span class="co">#</span>
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<span class="co"># If field 2 (colour) is less than 6 characters then save it with its line</span>
|
||
<span class="co"># number and skip it</span>
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||
<span class="co">#</span>
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||
<span class="fu">length</span>(<span class="dt">$2</span>) <span class="op"><</span> <span class="dv">6</span> <span class="kw">{</span>
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||
skip[<span class="bu">NR</span>] <span class="op">=</span> <span class="dt">$0</span>
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||
<span class="kw">next</span>
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<span class="kw">}</span>
|
||
|
||
<span class="co">#</span>
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||
<span class="co"># It's not the header and the colour name is > 6 characters, so print the line</span>
|
||
<span class="co">#</span>
|
||
<span class="kw">{</span>
|
||
<span class="kw">print</span>
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||
<span class="kw">}</span>
|
||
|
||
<span class="co">#</span>
|
||
<span class="co"># At the end show what was skipped</span>
|
||
<span class="co">#</span>
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<span class="cf">END</span> <span class="kw">{</span>
|
||
<span class="kw">printf</span> <span class="st">"</span><span class="sc">\n</span><span class="st">Skipped:</span><span class="sc">\n</span><span class="st">"</span>
|
||
<span class="kw">for</span> (n <span class="kw">in</span> skip)
|
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<span class="kw">print</span> n<span class="st">": "</span>skip[n]
|
||
<span class="kw">}</span></code></pre></div>
|
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<ul>
|
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<li>The script uses <code>next</code> in the first rule to avoid the first line of the file (a header).</li>
|
||
<li>The second rule skips lines where the colour name is less than 6 characters long, but it also saves that line in an array called <code>skip</code> using the line number as the key (index).</li>
|
||
<li>The third rule prints anything it sees, but it will not be invoked if either rule 1 or rule 2 cause it to be skipped.</li>
|
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<li>Finally, and <code>END</code> rule prints the contents of the array.</li>
|
||
</ul>
|
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<p>Running this with the file we have used many times before, <code>file1.txt</code>, results in the following output (<code>next_example.out</code>): <small></p>
|
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<pre><code>$ next_example.awk file1.txt
|
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banana yellow 6
|
||
grape purple 10
|
||
plum purple 2
|
||
pineapple yellow 5
|
||
|
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Skipped:
|
||
2: apple red 4
|
||
4: strawberry red 3
|
||
6: apple green 8
|
||
8: kiwi brown 4
|
||
9: potato brown 9</code></pre>
|
||
<p></small></p>
|
||
<h2 id="links">Links</h2>
|
||
<ul>
|
||
<li><a href="https://www.gnu.org/software/gawk/manual/html_node/index.html"><em>GNU Awk User’s Guide</em></a></li>
|
||
<li>Previous shows in this series on HPR:
|
||
<ul>
|
||
<li><a href="http://hackerpublicradio.org/eps/hpr2114">“<em>Gnu Awk - Part 1</em>”</a> - episode 2114</li>
|
||
<li><a href="http://hackerpublicradio.org/eps/hpr2129">“<em>Gnu Awk - Part 2</em>”</a> - episode 2129</li>
|
||
<li><a href="http://hackerpublicradio.org/eps/hpr2143">“<em>Gnu Awk - Part 3</em>”</a> - episode 2143</li>
|
||
<li><a href="http://hackerpublicradio.org/eps/hpr2163">“<em>Gnu Awk - Part 4</em>”</a> - episode 2163</li>
|
||
<li><a href="http://hackerpublicradio.org/eps/hpr2184">“<em>Gnu Awk - Part 5</em>”</a> - episode 2184</li>
|
||
<li><a href="http://hackerpublicradio.org/eps/hpr2238">“<em>Gnu Awk - Part 6</em>”</a> - episode 2238</li>
|
||
<li><a href="http://hackerpublicradio.org/eps/hpr2330">“<em>Gnu Awk - Part 7</em>”</a> - episode 2330</li>
|
||
</ul></li>
|
||
<li>Resources:
|
||
<ul>
|
||
<li><a href="hpr2438_full_shownotes.epub">ePub version of these notes</a></li>
|
||
<li><a href="hpr2438_full_shownotes.pdf">PDF version of these notes</a></li>
|
||
<li>Demonstration of the <code>switch</code> statement:
|
||
<ul>
|
||
<li>Script: <a href="hpr2438_switch_example.awk">switch_example.awk</a></li>
|
||
<li>Output: <a href="hpr2438_switch_example.out">switch_example.out</a></li>
|
||
</ul></li>
|
||
<li>Demonstration of the <code>break</code> statement:
|
||
<ul>
|
||
<li>Script: <a href="hpr2438_divisor.awk">divisor.awk</a></li>
|
||
<li>Output: <a href="hpr2438_divisor.out">divisor.out</a></li>
|
||
</ul></li>
|
||
<li>Demonstration of the <code>continue</code> statement:
|
||
<ul>
|
||
<li>Script: <a href="hpr2438_continue_example.awk">continue_example.awk</a></li>
|
||
</ul></li>
|
||
<li>Demonstration of the <code>next</code> statement:
|
||
<ul>
|
||
<li>Script: <a href="hpr2438_next_example.awk">next_example.awk</a></li>
|
||
<li>Output: <a href="hpr2438_next_example.out">next_example.out</a></li>
|
||
</ul></li>
|
||
</ul></li>
|
||
</ul>
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