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    <p>The first thing you need to realize is that "looping" in Racket is really just recursion. In this case, you want to chain a bunch of drawing calls together. If we wrote this out, our target goal would be this:</p> <pre><code>(hc-append (circle 10) (hc-append (circle 10) (hc-append (circle 10) (hc-append (circle 10) (hc-append (circle 10) (hc-append (circle 10) (hc-append (circle 10) (hc-append (circle 10) (hc-append (circle 10)))))))))) </code></pre> <p>I am assuming all of our circles are going to be the same radius.</p> <p>Now, since we're going to be writing a recursive method, we need to think of our base case. We want exactly nine circles drawn. Let's call this maximum number of circles <code>max</code>. Our base case, when we break out of our "loop" will be when we reach <code>max</code> iterations, or when <code>(= iterations max)</code>.</p> <p>Now for the recursion itself. We already know we need to pass in at least two variables, the current iteration <code>iterations</code>, and the maximum iteration <code>max</code>. If you look at the code above, you'll notice that the repeating element in all of the "loops" is the <code>(circle 10)</code>. Now there are a number of ways you could pass that along -- some people would choose to just pass the radius for example -- but I think the easiest way would be to pass in the pict of the circle. </p> <p>Finally, we also have to pass along the picture we've done so far. That is, as we append a circle to our chain, we need to pass this back into the recursive method so we can keep appending.</p> <p>Now that we've got that squared away, we can define the structure of our recursive method, which we shall call <code>circle-chain-recursive</code>:</p> <pre><code>(define (circle-chain-recursive iteration max crcle output) ; body here ) </code></pre> <p>The "guts" of our method will be an <code>if</code>. If we've reached the max iteration, return the output. Otherwise append another circle, increment <code>iteration</code>, and call the method again.</p> <pre><code>(define (circle-chain-recursive iteration max crcle output) (if (= iteration max) output (circle-chain-recursive (+ 1 iteration) max crcle (hc-append crcle output)))) </code></pre> <p>I personally don't like calling recursive looping methods like this directly, so I would write a helper method like this:</p> <pre><code>(define (circle-chain num radius) (circle-chain-recursive 0 num (circle radius) (circle 0))) </code></pre> <p>Now if I want a series of 9 circles with radius 10, all I have to do is call <code>(circle-chain 9 10)</code>.</p> <p>You'll notice that I passed <code>(circle 0)</code> as in as the parameter called <code>output</code>. This is because the <code>hc-append</code> method requires a <code>pict</code> parameter. Since we're not starting out with any circles, I passed it the equivalent of a "blank" or nil pict. There may be some other way of passing a "blank" pict, but I'm not too familiar with the <code>slideshow/pict</code> libraries to know it.</p> <p>I hope that clears it up a bit.</p>
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    1. POWhat is the correct way to append via loop in racket?
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    1. USSam Tobin-Hochstadt
    1. USRoddy of the Frozen Peas
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    1. COThanks so much for this explanation. A lot of my confusion was being unable to correctly implement a recursive function, but your explanation was perfect.
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