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POHow should I remove the first occurrence of an object from a list in Scala?
 

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  1. POHow should I remove the first occurrence of an object from a list in Scala?
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    <p>What is the best way to remove the first occurrence of an object from a list in Scala?</p> <p>Coming from Java, I'm accustomed to having a <a href="http://download.oracle.com/javase/7/docs/api/java/util/List.html#remove(java.lang.Object)" rel="nofollow"><code>List.remove(Object o)</code></a> method that removes the first occurrence of an element from a list. Now that I'm working in Scala, I would expect the method to return a new immutable <code>List</code> instead of mutating a given list. I might also expect the <code>remove()</code> method to take a predicate instead of an object. Taken together, I would expect to find a method like this:</p> <pre><code>/** * Removes the first element of the given list that matches the given * predicate, if any. To remove a specific object &lt;code&gt;x&lt;/code&gt; from * the list, use &lt;code&gt;(_ == x)&lt;/code&gt; as the predicate. * * @param toRemove * a predicate indicating which element to remove * @return a new list with the selected object removed, or the same * list if no objects satisfy the given predicate */ def removeFirst(toRemove: E =&gt; Boolean): List[E] </code></pre> <p>Of course, I can implement this method myself several different ways, but none of them jump out at me as being obviously the best. I would rather not convert my list to a Java list (or even to a Scala mutable list) and back again, although that would certainly work. I could use <code>List.indexWhere(p: (A) ⇒ Boolean)</code>:</p> <pre><code>def removeFirst[E](list: List[E], toRemove: (E) =&gt; Boolean): List[E] = { val i = list.indexWhere(toRemove) if (i == -1) list else list.slice(0, i) ++ list.slice(i+1, list.size) } </code></pre> <p>However, using indices with linked lists is usually not the most efficient way to go.</p> <p>I can write a more efficient method like this:</p> <pre><code>def removeFirst[T](list: List[T], toRemove: (T) =&gt; Boolean): List[T] = { def search(toProcess: List[T], processed: List[T]): List[T] = toProcess match { case Nil =&gt; list case head :: tail =&gt; if (toRemove(head)) processed.reverse ++ tail else search(tail, head :: processed) } search(list, Nil) } </code></pre> <p>Still, that's not exactly succinct. It seems strange that there's not an existing method that would let me do this efficiently and succinctly. So, am I missing something, or is my last solution really as good as it gets?</p>
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    1. COShoot, I searched for this question for a while before asking it, but I only found a duplicate after I posted it: http://stackoverflow.com/questions/5636717/what-is-an-idiomatic-scala-way-to-remove-one-element-from-an-immutable-list
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