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    <p>You are right about the <code>maximum</code> part. To get the <em>list</em> (that's what Haskell's []s are, arrays are different structures) you need to use the <code>map</code> higher-order function, like this:</p> <pre><code>chainLength n = length (chain n) lengths = map chainLength [1..1000000] </code></pre> <p>Essentially, <code>map</code> takes as arguments a function and a list. It applies the function to each element in the list and returns the list of the results.</p> <p>Since you will be needing the number whose chain has that length, you may want change the <code>chainLength</code> function to return the number as well, like this:</p> <pre><code>chainLength n = (n, length (chain n)) </code></pre> <p>That way you will have an array of pairs, with each number and its chain length.</p> <p>Now you need to get the pair with the largest second component. That's where the <code>maximumBy</code> function comes in. It works just like <code>maximum</code> but takes a function as a parameter to select how to compare the values. In this case, the second component of the pair. This comparison function takes two numbers and returns a value of type <code>Ordering</code>. This type has only three possible values: <code>LT</code>, <code>EQ</code>, <code>GT</code>, for less than, equal, and greater than, respectively.</p> <p>So, we need a function that given two pairs tells us how the second components compare to each other:</p> <pre><code>compareSnd (_, y1) (_, y2) = compare y1 y2 -- Or, if you import Data.Function, you can write it like this (thanks alexey_r): compareSnd = compare `on` snd -- reads nicely </code></pre> <p>I used the default <code>compare</code> function that compares numbers (well, <a href="http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data-Ord.html#v%3Acompare" rel="nofollow noreferrer">not just numbers</a>).</p> <p>Now we only need to get the maximum using this function:</p> <pre><code>longestChain = maximumBy compareSnd lengths </code></pre> <p>That gets you a pair of the number with the longest chain and the corresponding length. Feel free to apply <code>fst</code> and <code>snd</code> as you please.</p> <p>Note that this could be more much more concisely using <code>zip</code> and composition, but since you tagged the question as newbie, I thought it better to break it down like this.</p>
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    1. POIterating a function and analysing the result in haskell
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    1. COThis worked but how do I find how far along the result array it is?
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    2. COI noticed you said *below* 1,000,000 in your question, so feel free to use 999999 instead of 1000000 :)
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    3. COI was using `maximumBy` in a wrong way. My Haskell times are long gone, and my memory ain't what it used to be.
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