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    <p>There are three styles of loops available in Racket: </p> <h3>#1 The for-loop style</h3> <p>This style uses the syntax described in Chapter 11 of the Racket Guide, <a href="http://docs.racket-lang.org/guide/for.html" rel="nofollow">"Iterations and Comprehensions"</a>. It looks like this:</p> <pre><code>(require slideshow/pict) (for/fold ([result (blank)]) ([i (in-range 9)]) (hc-append (circle 10) result)) </code></pre> <p>In this style, in the first parenthesis after <em>for</em> defines the loop's temporary variables. There is one such variable in my example, called <em>result</em>. Then you define the iteration variables and what they iterate over. So here, <em>i</em> loops over the numbers for 0 to 8. The body of the loop runs once for each <em>i</em>, each time the result is assigned to <em>result</em>, and the final value of the loop is the value of <em>result</em> at the end.</p> <h3>#2 The map and fold style.</h3> <p>This style is described in <a href="http://docs.racket-lang.org/guide/pairs.html" rel="nofollow">Section 3.8 of the Guide</a>. It looks like this:</p> <pre><code>(require slideshow/pict) (foldl hc-append (blank) (build-list 9 (lambda (i) (circle 10)))) </code></pre> <p>This code start by making a list of 9 circles:</p> <pre><code>(define o (circle 10)) (build-list 9 (lambda (i) o) ---&gt; (list o o o o o o o o o) </code></pre> <p>The <em>list</em> notation is a shorthand for the more verbose <em>cons</em> notation.</p> <pre><code>(list o o o o o o o o o) ---&gt; (cons o (cons o (cons o (cons o (cons o (cons o (cons o (cons o (cons o empty))))))))) </code></pre> <p>The way to think of the <em>foldl</em> function is that it turns a piece of data into a piece of computation. It takes a list as an input and it turns it into a collection of function calls.</p> <p>The <em>foldl</em> function takes the list and replaces each <em>cons</em> in the list with the first argument, and replaces the <em>empty</em> at the end of the list with the second argument. In the example, I pass the <em>hc-append</em> function and (blank), so the replacement looks like this:</p> <pre><code>(foldl hc-append (blank) ...) ---&gt; (hc-append o (hc-append o (hc-append o (hc-append o (hc-append o (hc-append o (hc-append o (hc-append o (hc-append o (blank)))))))))) </code></pre> <p>This sequence of function calls is exactly the one you wanted to avoid writing by hand. <em>Foldl</em> computes it for you.</p> <h3>#3 The recursion style</h3> <p>This is the style Roddy described in his answer. As a general matter of coding style, you should only use the recursion style if there is no simple *for, <em>map</em>, or <em>fold</em> possible, such as when traversing a recursive data structure.</p>
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    1. POWhat is the correct way to append via loop in racket?
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    1. USGuillaume Marceau
    1. USGuillaume Marceau
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    1. COIn case it's interesting for user1553081: #3 The recursion style is the most fundamental, and what most people learn first. #2 map/fold are the "traditional" Scheme functions, and you can write them yourself using recursion. #1 is the "newest", and is built using some macros. In other words, these 3 styles are listed from newest to oldest. If you're coming from some other language, you could dive in with #1 and you'll get some results quickly. However it might be worth taking some time to start with recursion and marinade yourself in that.
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      1. USGreg Hendershott
 

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