Python: How to sort dictionary by value

It is not possible to sort a dict, only to get a representation of a dict that is sorted. Dicts are inherently orderless, but other types, such as lists and tuples, are not. So you need a sorted representation, which will be a lis t— probably a list of tuples.

For instance,

import operator
x = {1: 2, 3: 4, 4:3, 2:1, 0:0}
sorted_x = sorted(x.iteritems(), key=operator.itemgetter(1))

sorted_x will be a list of tuples sorted by the second element in each tuple. dict(sorted_x) == x.

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Sort list alphabetically with jQuery

This can be done without using jQuery:

function sortUnorderedList(ul, sortDescending) {
  if(typeof ul == "string")
    ul = document.getElementById(ul);

  // Idiot-proof, remove if you want
  if(!ul) {
    alert("The UL object is null!");
    return;
  }

  // Get the list items and setup an array for sorting
  var lis = ul.getElementsByTagName("LI");
  var vals = [];

  // Populate the array
  for(var i = 0, l = lis.length; i < l; i++)
    vals.push(lis[i].innerHTML);

  // Sort it
  vals.sort();

  // Sometimes you gotta DESC
  if(sortDescending)
    vals.reverse();

  // Change the list on the page
  for(var i = 0, l = lis.length; i < l; i++)
    lis[i].innerHTML = vals[i];
}

Easy to use:

sortUnorderedList("ID_OF_LIST");

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How to: sort arrays and data in PHP

How do I sort an array in PHP?
How do I sort a complex array in PHP?
How do I sort an array of objects in PHP?

Basic one dimensional arrays

$array = array(3, 5, 2, 8);

Applicable sort functions:

• sort
• rsort
• asort
• arsort
• natsort
• natcasesort
• ksort
• krsort

The difference between those is merely whether key-value associations are kept (the "a" functions), whether it sorts low-to-high or reverse ("r"), whether it sorts values or keys ("k") and how it compares values ("nat" vs. normal). See http://php.net/manual/en/array.sorting.php for an overview and links to further details.

Multi dimensional arrays, including arrays of objects

$array = array(
    array('foo' => 'bar', 'baz' => 42),
    array('foo' => ...,   'baz' => ...),
    ...
);

If you want to sort $array  by the key 'foo' of each entry, you need a custom comparison function. The above sort and related functions work on simple values that they know how to compare and sort. PHP does not simply "know" what to do with a complex value likearray('foo' => 'bar', 'baz' => 42) though; so you need to tell it.

To do that, you need to create a comparison function. That function takes two elements and must return 0 if these elements are considered equal, a value lower than 0 if the first value is lower and a value higher than 0 if the first value is higher. That's all that's needed:

function cmp(array $a, array $b) {
    if ($a['foo'] < $b['foo']) {
        return -1;
    } else if ($a['foo'] > $b['foo']) {
        return 1;
    } else {
        return 0;
    }
}

You then use one of these functions:

• usort
• uasort
• uksort

Again, they only differ in whether they keep key-value associations and sort by values or keys. Read their documentation for details.

Example usage:

usort($array, 'cmp');

usort will take two items from the array and call your cmp function with them. So cmp() will be called with $a as array('foo' => 'bar', 'baz' => 42) and $b as another array('foo' => ..., 'baz' => ...) The function then returns to usort which of the values was larger or whether they were equal. usort repeats this process passing different values for $a and $b until the array is sorted. The cmp function will be called many times, at least as many times as there are values in $array, with different combinations of values for $a and $b every time.

To get used to this idea, try this:

function cmp($a, $b) {
    echo 'cmp called with $a:', PHP_EOL;
    var_dump($a);
    echo 'and $b:', PHP_EOL;
    var_dump($b);
}

All you did was define a custom way to compare two items, that's all you need. That works with all sorts of values.

By the way, this works on any value, the values don't have to be complex arrays. If you have a custom comparison you want to do, you can do it on a simple array of numbers too.

Custom numeric comparisons

If you want to sort by the baz key, which is numeric, all you need to do is:

function cmp(array $a, array $b) {
    return $a['baz'] - $b['baz'];
}

Thanks to The PoWEr oF MATH this returns a value < 0, 0 or > 0 depending on whether $a is lower than, equal to or larger than $b.

NOTE: return $a['baz'] - $b['baz']; // to sort ASCENDING (1,2,3...) return $b['baz'] - $a['baz']; // to sort DESCENDING (9,8,7...)

Note that this won't work well for float values, since they'll be reduced to an int and lose precision. Use explicit -1, 0 and 1 return values instead.

Objects

If you have an array of objects, it works the same way:

function cmp($a, $b) {
    return $a->baz - $b->baz;
}

Functions

You can do anything you need inside a comparison function, including calling functions:

function cmp(array $a, array $b) {
    return someFunction($a['baz']) - someFunction($b['baz']);
}

Strings

A shortcut for the first string comparison version:

function cmp(array $a, array $b) {
    return strcmp($a['foo'], $b['foo']);
}

strcmp does exactly what's expected of cmp here, it returns -1, 0 or 1.

Sorting by multiple fields

If you want to sort primarily by foo, but if foo is equal for two elements sort by baz:

function cmp(array $a, array $b) {
    if (($cmp = strcmp($a['foo'], $b['foo'])) !== 0) {
        return $cmp;
    } else {
        return $a['baz'] - $b['baz'];
    }
}

For those familiar, this is equivalent to an SQL query with ORDER BY foo, baz.

Also see this very neat shorthand version and how to create such a comparison function dynamically for an arbitrary number of keys.

Sorting into a manual, static order

If you want to sort elements into a "manual order" like "foo", "bar", "baz":

function cmp(array $a, array $b) {
    static $order = array('foo', 'bar', 'baz');
    return array_search($a['foo'], $order) - array_search($b['foo'], $order);
}

For all the above, if you're using PHP 5.3 or higher (and you really should), use anonymous functions for shorter code and to avoid having another global function floating around:

usort($array, function (array $a, array $b) { return $a['baz'] - $b['baz']; });

That's how simple sorting a complex multi-dimensional array can be. Again, just think in terms of teaching PHP how to tell which of two items is "greater"; let PHP do the actual sorting.

Sorting one array based on another:

And then there's the peculiar array_multisort, which lets you sort one array based on another:

$array1 = array( 4,   6,   1);
$array2 = array('a', 'b', 'c');

The expected result here would be:

$array2 = array('c', 'a', 'b');  // the sorted order of $array1

Use array_multisort to get there:

array_multisort($array1, $array2);

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