retroforth/doc/html/chapters/techniques/loops.html
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<p><span class="h1">Loops</span>
<br/><br/>
RETRO provides several words for creating loops.
<br/><br/>
<span class="h2">Unconditional Loops</span>
<br/><br/>
An unconditional loop begins with <span class="tt">repeat</span> and ends with <span class="tt">again</span>.
<br/><br/>
<tt class='indentedcode'>:test&nbsp;repeat&nbsp;#1&nbsp;n:put&nbsp;sp&nbsp;again&nbsp;;</tt>
<tt class='indentedcode'>test</tt>
<br/><br/>
Unconditional loops must be inside a definition or quote. To exit
one of these, use <span class="tt">0;</span>, <span class="tt">-if;</span> or <span class="tt">if;</span>.
<br/><br/>
<tt class='indentedcode'>:test&nbsp;#100&nbsp;repeat&nbsp;0;&nbsp;dup&nbsp;n:put&nbsp;sp&nbsp;n:dec&nbsp;again&nbsp;;</tt>
<tt class='indentedcode'>test</tt>
<br/><br/>
<tt class='indentedcode'>:test&nbsp;#100&nbsp;repeat&nbsp;dup&nbsp;#50&nbsp;eq?&nbsp;[&nbsp;'done!&nbsp;s:put&nbsp;nl&nbsp;]&nbsp;if;&nbsp;n:dec&nbsp;again&nbsp;;</tt>
<tt class='indentedcode'>test</tt>
<br/><br/>
You can also achieve this via recursion:
<br/><br/>
<tt class='indentedcode'>:test&nbsp;0;&nbsp;dup&nbsp;n:put&nbsp;sp&nbsp;n:dec&nbsp;test&nbsp;;</tt>
<tt class='indentedcode'>#100&nbsp;test</tt>
<br/><br/>
Be careful with recursion as the virtual machine will have a limited
amount of space for the address stack and recursing too many times
can cause a stack overflow.
<br/><br/>
<span class="h2">Conditional Loops</span>
<br/><br/>
There are two conditional looping combinators: <span class="tt">while</span> and <span class="tt">until</span>.
Both take a quote and execute it, checking a returned flag to decide
when to stop running.
<br/><br/>
<tt class='indentedcode'>#0&nbsp;[&nbsp;dup&nbsp;n:put&nbsp;sp&nbsp;n:inc&nbsp;dup&nbsp;#10&nbsp;eq?&nbsp;]&nbsp;until</tt>
<tt class='indentedcode'>#10&nbsp;[&nbsp;dup&nbsp;n:put&nbsp;sp&nbsp;n:dec&nbsp;dup&nbsp;n:-zero?&nbsp;]&nbsp;while</tt>
<br/><br/>
<span class="h2">Counted Loops</span>
<br/><br/>
There are two combinators for counted loops. These are <span class="tt">times</span> and
<span class="tt">indexed-times</span>.
<br/><br/>
<tt class='indentedcode'>#0&nbsp;#10&nbsp;[&nbsp;dup&nbsp;n:put&nbsp;sp&nbsp;n:inc&nbsp;]&nbsp;times&nbsp;nl</tt>
<tt class='indentedcode'>#10&nbsp;[&nbsp;I&nbsp;n:put&nbsp;sp&nbsp;]&nbsp;indexed-times</tt>
<br/><br/>
The <span class="tt">indexed-times</span> provides an index via the <span class="tt">I</span>, <span class="tt">J</span>, and
<span class="tt">K</span> words. <span class="tt">I</span> will be the index of the current loop, with <span class="tt">J</span> and
<span class="tt">K</span> being the indexes of the next two older loops.
<br/><br/>
The loop indexes can be accessed outside the loop body:
<br/><br/>
<tt class='indentedcode'>:display&nbsp;I&nbsp;n:square&nbsp;n:put&nbsp;sp&nbsp;;</tt>
<tt class='indentedcode'>:squares&nbsp;[&nbsp;display&nbsp;]&nbsp;indexed-times&nbsp;nl&nbsp;;</tt>
<tt class='indentedcode'>#100&nbsp;squares</tt>
<br/><br/>
<span class="h2">Tradeoffs</span>
<br/><br/>
The unconditional loop form is more efficient as it's just a
simple jump operation. The <span class="tt">times</span> counted loops are a little
slower, but can be cleaner and more readable in many cases. The
<span class="tt">indexed-times</span> form is significantly slower than the other
two forms.
</p>
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