retroforth/doc/html/chapters/techniques/arrays.html
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<p><span class="h1">Working With Arrays</span>
<br/><br/>
RETRO offers a number of words for operating on statically sized
arrays.
<br/><br/>
<span class="h2">Namespace</span>
<br/><br/>
The words operating on arrays are kept in an <span class="tt">a:</span> namespace.
<br/><br/>
<span class="h2">Creating Arrays</span>
<br/><br/>
The easiest way to create an array is to wrap the values in a
<span class="tt">{</span> and <span class="tt">}</span> pair:
<br/><br/>
<span class='codeblock'><span class="tt">```</span><br/><span class="tt"><span class='imm'>{</span> <span class='num'>#1</span> <span class='num'>#2</span> <span class='num'>#3</span> <span class='num'>#4</span> <span class='imm'>}</span> </span><br/>
<span class="tt"><span class='imm'>{</span> <span class='str'>'this</span> <span class='str'>'is</span> <span class='str'>'an</span> <span class='str'>'array</span> <span class='str'>'of</span> <span class='str'>'strings</span> <span class='imm'>}</span> </span><br/>
<span class="tt"><span class='imm'>{</span> <span class='str'>'this</span> <span class='str'>'is</span> <span class='str'>'a</span> <span class='str'>'mixed</span> <span class='str'>'array</span> <span class='num'>#1</span> <span class='num'>#2</span> <span class='num'>#3</span> <span class='imm'>}</span> </span><br/>
<span class="tt">```</span></span><br/><br/>
You can also make an array from a quotation which returns
values and the number of values to store in the a:
<br/><br/>
<span class='codeblock'><span class="tt">```</span><br/><span class="tt"><span class='imm'>[</span> <span class='num'>#1</span> <span class='num'>#2</span> <span class='num'>#3</span> &nbsp;<span class='num'>#3</span> <span class='imm'>]</span> a:counted-results </span><br/>
<span class="tt"><span class='imm'>[</span> <span class='num'>#1</span> <span class='num'>#2</span> <span class='num'>#3</span> &nbsp;<span class='num'>#3</span> <span class='imm'>]</span> a:make </span><br/>
<span class="tt">```</span></span><br/><br/>
<span class="h2">Accessing Elements</span>
<br/><br/>
You can access a specific value with <span class="tt">a:th</span> and <span class="tt">fetch</span> or
<span class="tt">store</span>:
<br/><br/>
<span class='codeblock'><span class="tt">```</span><br/><span class="tt"><span class='imm'>{</span> <span class='num'>#1</span> <span class='num'>#2</span> <span class='num'>#3</span> <span class='num'>#4</span> <span class='imm'>}</span> <span class='num'>#3</span> a:th <span class='prim'>fetch</span> </span><br/>
<span class="tt">```</span></span><br/><br/>
<span class="h2">Find The Length</span>
<br/><br/>
Use <span class="tt">a:length</span> to find the size of the array.
<br/><br/>
<span class='codeblock'><span class="tt">```</span><br/><span class="tt"><span class='imm'>{</span> <span class='num'>#1</span> <span class='num'>#2</span> <span class='num'>#3</span> <span class='num'>#4</span> <span class='imm'>}</span> a:length </span><br/>
<span class="tt">```</span></span><br/><br/>
<span class="h2">Duplicate</span>
<br/><br/>
Use <span class="tt">a:dup</span> to make a copy of an a:
<br/><br/>
<span class='codeblock'><span class="tt">```</span><br/><span class="tt"><span class='imm'>{</span> <span class='num'>#1</span> <span class='num'>#2</span> <span class='num'>#3</span> <span class='num'>#4</span> <span class='imm'>}</span> a:dup </span><br/>
<span class="tt">```</span></span><br/><br/>
<span class="h2">Filtering</span>
<br/><br/>
RETRO provides <span class="tt">a:filter</span> which extracts matching values
from an array. This is used like:
<br/><br/>
<span class='codeblock'><span class="tt">```</span><br/><span class="tt"><span class='imm'>{</span> <span class='num'>#1</span> <span class='num'>#2</span> <span class='num'>#3</span> <span class='num'>#4</span> <span class='num'>#5</span> <span class='num'>#6</span> <span class='num'>#7</span> <span class='num'>#8</span> <span class='imm'>}</span> <span class='imm'>[</span> n:even? <span class='imm'>]</span> a:filter </span><br/>
<span class="tt">```</span></span><br/><br/>
The quote will be passed each value in the array and should
return TRUE or FALSE. Values that lead to TRUE will be collected
into a new array.
<br/><br/>
<span class="h2">Mapping</span>
<br/><br/>
<span class="tt">a:map</span> applies a quotation to each item in an array and
constructs a new array from the returned values.
<br/><br/>
Example:
<br/><br/>
<span class='codeblock'><span class="tt">```</span><br/><span class="tt"><span class='imm'>{</span> <span class='num'>#1</span> <span class='num'>#2</span> <span class='num'>#3</span> <span class='imm'>}</span> <span class='imm'>[</span> <span class='num'>#10</span> <span class='prim'>*</span> <span class='imm'>]</span> a:map </span><br/>
<span class="tt">```</span></span><br/><br/>
<span class="h2">Reduce</span>
<br/><br/>
<span class="tt">a:reduce</span> takes an array, a starting value, and a quote. It
executes the quote once for each item in the array, passing the
item and the value to the quote. The quote should consume both
and return a new value.
<br/><br/>
<span class='codeblock'><span class="tt">```</span><br/><span class="tt"><span class='imm'>{</span> <span class='num'>#1</span> <span class='num'>#2</span> <span class='num'>#3</span> <span class='imm'>}</span> <span class='num'>#0</span> <span class='imm'>[</span> <span class='prim'>+</span> <span class='imm'>]</span> a:reduce </span><br/>
<span class="tt">```</span></span><br/><br/>
<span class="h2">Search</span>
<br/><br/>
RETRO provides <span class="tt">a:contains?</span> and <span class="tt">a:contains/string?</span>
to search an array for a value (either a number or string) and
return either TRUE or FALSE.
<br/><br/>
<span class='codeblock'><span class="tt">```</span><br/><span class="tt"><span class='num'>#100</span> &nbsp;<span class='imm'>{</span> <span class='num'>#1</span> <span class='num'>#2</span> <span class='num'>#3</span> <span class='imm'>}</span> a:contains? </span><br/>
<span class="tt"><span class='str'>'test</span> <span class='imm'>{</span> <span class='str'>'abc</span> <span class='str'>'def</span> <span class='str'>'test</span> <span class='str'>'ghi</span> <span class='imm'>}</span> a:contains/string? </span><br/>
<span class="tt">```</span></span><br/><br/>
<span class="h2">Implementation</span>
<br/><br/>
In memory, an array is a count followed by the values. As an
example, if you have an array:
<br/><br/>
<tt class='indentedcode'>{&nbsp;#10&nbsp;#20&nbsp;#30&nbsp;}</tt>
<br/><br/>
In memory this would be setup as:
<br/><br/>
<tt class='indentedcode'>|&nbsp;Offset&nbsp;|&nbsp;Value&nbsp;|</tt>
<tt class='indentedcode'>|&nbsp;------&nbsp;|&nbsp;-----&nbsp;|</tt>
<tt class='indentedcode'>|&nbsp;000&nbsp;&nbsp;&nbsp;&nbsp;|&nbsp;3&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;|</tt>
<tt class='indentedcode'>|&nbsp;001&nbsp;&nbsp;&nbsp;&nbsp;|&nbsp;10&nbsp;&nbsp;&nbsp;&nbsp;|</tt>
<tt class='indentedcode'>|&nbsp;002&nbsp;&nbsp;&nbsp;&nbsp;|&nbsp;20&nbsp;&nbsp;&nbsp;&nbsp;|</tt>
<tt class='indentedcode'>|&nbsp;003&nbsp;&nbsp;&nbsp;&nbsp;|&nbsp;30&nbsp;&nbsp;&nbsp;&nbsp;|</tt>
<br/><br/>
You can construct one on the fly by keeping a pointer to
<span class="tt">here</span> and using <span class="tt">,</span> to place the values. E.g.,
<br/><br/>
<tt class='indentedcode'>here&nbsp;[&nbsp;#3&nbsp;,&nbsp;#10&nbsp;,&nbsp;#20&nbsp;,&nbsp;#30&nbsp;,&nbsp;]&nbsp;dip</tt>
<br/><br/>
An example of this can be seen in this excerpt from an example
(<strong>example/Primes.forth</strong>):
<br/><br/>
<tt class='indentedcode'>:create-set&nbsp;(-a)&nbsp;</tt>
<tt class='indentedcode'>&nbsp;&nbsp;&nbsp;&nbsp;here&nbsp;#3000&nbsp;,&nbsp;#2&nbsp;#3002&nbsp;[&nbsp;dup&nbsp;,&nbsp;n:inc&nbsp;]&nbsp;times&nbsp;drop&nbsp;;</tt>
</p>
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