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<div class='vhub' style='margin-top:10px;margin-right:20px;text-align:left;background-color:white;'><a href='https://rimstone-lang.com' style='border-bottom:0px'><img src='https://rimstone-lang.com/rimstone.png' style='width:180px;height:auto'/></a></div><div class='vhub' style='margin-top:10px;'>Reverse string: recursion in RimStone</div><hr/><br/>
This example will reverse a string, i.e. turn it around so that it's like in the mirror. This will be done by using recursion; of course it can be done in other ways too, but this will demonstrate the use of recursion to manipulate a string. For a different recursion example (that calculates a factorial of a number), see <a href='https://rimstone-lang.com/article-recursion-factorial.html'>article-recursion-factorial</a>.<br/>
<br/>
First, create a directory for this example, and an application named "recursive":<br/>
<div class="shcode" style='position:relative;padding-right:16px;'>
<pre id='code_301' class=notranslate>
<span class="Statement">mkdir</span> <span class="Special">-p</span> recsv
<span class="Statement">cd</span> recsv
rim <span class="Special">-k</span> recursive</pre>
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This will create the file "rev.rim", which contains two handlers, one which is public (and that's the one we'll call) and the other, recursive one:<br/>
<div class='code'  style='position:relative;padding-right:16px;'>
<pre id='code_300' class=notranslate>
cat &lt;&lt; '<span class="Constant">EOF</span>' &gt; rev.rim
<span class="Statement">%%</span> /rev/string<span class="Identifier"> get-param</span> str<span class="Identifier"> type string</span>, str_beg<span class="Identifier"> type number</span>, str_end<span class="Identifier"> type number</span>
<span class="Statement">    if-true</span> str_beg<span class="Identifier"> lesser-than</span> str_end
<span class="Statement">        set-number</span> end = str[str_end]
<span class="Statement">        set-string</span> str[str_end] = str[str_beg]
<span class="Statement">        set-string</span> str[str_beg] = end
<span class="Statement">        call-handler</span> <span class="Constant">&quot;/rev/string&quot;</span><span class="Identifier"> set-param</span> str, str_beg=str_beg+<span class="Constant">1</span>, str_end=str_end-<span class="Constant">1</span>
<span class="Statement">    end-if</span>
<span class="Statement">%%</span>

<span class="Statement">%%</span> /rev<span class="Identifier"> public</span>
<span class="Statement">    set-string</span> str=<span class="Constant">&quot;This is some string to reverse&quot;</span>
<span class="Statement">    string-length</span> str<span class="Type"> to</span> str_len
<span class="Statement">    call-handler</span> <span class="Constant">&quot;/rev/string&quot;</span><span class="Identifier"> set-param</span> str, str_beg=<span class="Constant">0</span>, str_end=str_len-<span class="Constant">1</span><span class="Identifier"> get-param</span> str<span class="Identifier"> type string</span>
<span class="Statement">    print-out</span> str<span class="Identifier"> new-line</span>
<span class="Statement">%%</span>
<span class="Constant">EOF</span></pre>
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The "/rev/string" handler takes in the string itself (which is changed, but if you don't want that make a copy of it first in "/rev" caller handler), and also the index of the first character to reverse and the index of the last one. The index starts with 0 (the first byte), and ends with the length of the string minus 1 (i.e. the same indexing scheme as in most other languages).<br/>
<br/>
The code itself is pretty self-explanatory: the first and last character are swapped, then the "/rev/string" handler is called again but this time with the substring whic starts one byte after the character we swapped, and ends one byte before the other swapping character. This is repeated until the beginning is equal or greater than the ending.<br/>
<br/>
Make the application:<br/>
<div class="shcode" style='position:relative;padding-right:16px;'>
<pre id='code_302' class=notranslate>
rim <span class="Special">-q</span></pre>
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Test it (we will reverse the string "This is some string to reverse"):<br/>
<div class="shcode" style='position:relative;padding-right:16px;'>
<pre id='code_303' class=notranslate>
rim <span class="Special">-r</span> <span class="Special">--req</span><span class="Statement">=</span><span class="Statement">&quot;</span><span class="Constant">/rev</span><span class="Statement">&quot;</span> <span class="Special">--exec</span> <span class="Special">--silent-header</span></pre>
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As expected, the result is a reversion of this string:<br/>
<div class='ncode' style='position:relative;padding-right:16px;'>
<pre id='code_299' class=notranslate>
esrever ot gnirts emos si sihT</pre>
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