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    <p>A sorting algorithm is said to be <strong>stable</strong> if two objects with equal keys appear in the same order in sorted output as they appear in the input array to be sorted. Some sorting algorithms are stable by nature like Insertion sort, Merge Sort, Bubble Sort, etc. And some sorting algorithms are not, like Heap Sort, Quick Sort, etc.</p> <p><em>Background</em>: a "stable" sorting algorithm keeps the items with the same sorting key in order. Suppose we have a list of 5-letter words:</p> <pre><code>peach straw apple spork </code></pre> <p>If we sort the list by just the first letter of each word then a stable-sort would produce:</p> <pre><code>apple peach straw spork </code></pre> <p>In an <strong>unstable</strong> sort algorithm, <code>straw</code> or <code>spork</code> may be interchanged, but in a stable one, they stay in the same relative positions (that is, since <code>straw</code> appears before <code>spork</code> in the input, it also appears before <code>spork</code> in the output).</p> <p>We could sort the list of words using this algorithm: stable sorting by column 5, then 4, then 3, then 2, then 1. In the end, it will be correctly sorted. Convince yourself of that. (by the way, that algorithm is called radix sort)</p> <p>Now to answer your question, suppose we have a list of first and last names. We are asked to sort "by last name, then by first". We could first sort (stable or unstable) by the first name, then stable sort by the last name. After these sorts, the list is primarily sorted by the last name. However, where last names are the same, the first names are sorted.</p> <p>You can't stack unstable sorts in the same fashion.</p>
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    1. POWhat is stability in sorting algorithms and why is it important?
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    1. USSergio
    1. USJoey Adams
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    1. POWhat is stability in sorting algorithms and why is it important?
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    1. COSounds good. Any other reasons ?
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    2. COSo, what would the sort be called to make the words in correct sorting order of apple peach sport straw? The stable sort gave us apple peach straw spork however st should be after sp (alphabetically correct), so the ultimate correct sort should be apple peach sport straw
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    3. CO@user1416486: We're sorting by the first letter only. With that assumption, `straw` and `spork` compare equal. Stable sort will preserve the order of input, whereas unstable sort does not make that guarantee. "Correct" depends on the application. The sort function in most programming languages lets the user supply a custom ordering function. If the user's function treats different items as equal (e.g. same first name, different last name), it helps to know if the original order will be preserved. See [OCaml's array sorting functions](http://is.gd/7gDhtD) for a real-world example.
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