array_reduce() can't work as associative-array

问题:

I have an associative array $assoc, and need to reduce to it to a string, in this context

$OUT = "<row";
foreach($assoc as $k=>$v) $OUT.= " $k=\"$v\"";
$OUT.= '/>';

How to do in an elegant way the same thing, but using array_reduce()

---

Near the same algorithm (lower performance and lower legibility) with array_walk() function,

 array_walk(  $row, function(&$v,$k){$v=" $k=\"$v\"";}  );
 $OUT.= "\n\t<row". join('',array_values($row)) ."/>";

Ugly solution with array_map() (and again join() as reducer):

  $row2 = array_map( 
    function($a,$b){return array(" $a=\"$b\"",1);},
    array_keys($row),
    array_values($row)
  ); // or  
  $OUT ="<row ". join('',array_column($row2,0)) ."/>";

PS: apparently PHP's array_reduce() not support associative arrays (why??).

回答1:

First, array_reduce() works with associative arrays, but you don't have any chance to access the key in the callback function, only the value.

You could use the use keyword to access the $result by reference in the closure like in the following example with array_walk(). This would be very similar to array_reduce():

$array = array(
    'foo' => 'bar',
    'hello' => 'world'
);

// Inject reference to `$result` into closure scope.
// $result will get initialized on it's first usage.
array_walk($array, function($key, $val) use(&$result) {
    $result .= "$key=\"$val\"";
});
echo $result;

Btw, imo your original foreach solution looks elegant too. Also there will be no significant performance issues as long as the array stays at small to medium size.

回答2:

$array = array(
    'foo' => 'bar',
    'hello' => 'world'
);

$OUT = join(" ", array_reduce(array_keys($array), function($as, $a) use ($array) {
    $as[] = sprintf('%s="%s"', $a, $array[$a]); return $as;
}, array()));

回答3:

I personally see nothing wrong with the foreach thing, but if you want one single expression, your map snippet can be simplified down to

$OUT = sprintf("<row %s/>",
    join(" ", array_map(
        function($a, $b) { return "$a=\"$b\""; },
        array_keys($assoc),
        array_values($assoc)
)));

Also, since you're generating XML, it's better to use a dedicated tool, for example:

$doc = new SimpleXMLElement("<row/>");
foreach($assoc as $k => $v)
    $doc->addAttribute($k, $v);
echo $doc->asXML();

回答4:

you can prepare your input array using array-chunk and use array_reduce like this:

$a = ["a" => 123, "b" => 234, "c" => 55]; 

echo array_reduce(
   array_chunk($a, 1, true), 
   function ($r, $i) { 
     return $r . key($i) ." = ". current($i) . PHP_EOL;
   }, "");

will show - ideal for array-as-text representation:

a = 123
b = 234
c = 55

array_chunk will create an array of single associative array entries. please note that is is probably not the the most performant solution - just a rather short one.

回答5:

If you are set on array_reduce, and the values of your array are unique, then you can access the key by passing the associative array to the callback function and using array_search.

// Pass $assoc to your anonymous function and use array_search to get the key.
$OUT .= array_reduce($assoc, function($o, $v) use($assoc) {
    return sprintf('%s %s="%s"', $o, array_search($v, $assoc), $v);
}, '');

Personally, I feel that array_map and join would be more effective in this situation.

$OUT .= join(' ', array_map(function($v, $k){
    return sprintf('%s="%s"', $k, $v);
}, $assoc, array_keys($assoc)));

回答6:

Strictly using array_reduce this is the simplest algorithm I can think of (also both anonymous functions are pure functions):

$out =
    '<row '.
        array_reduce(
            array_map  (
                function ($e, $k) { return [$e, $k];  },
                array_keys($assoc),
                $assoc
            ),
            function ( $props, $e ) { return $props." {$e[0]}=\"{$e[1]}\"";  }
        )
    .' />';

In one line...

$out = '<row '. array_reduce( array_map( function ($e, $k) { return [$e, $k];  }, array_keys($assoc), $assoc), function ( $props, $e ) { return $props." {$e[0]}=\"{$e[1]}\""; }).' />';

回答7:

$ar = array(
  array("month"=>'8', "revenue"=>300),
  array("month"=>'2',"revenue"=>500),
  array("month"=>'10',"revenue"=>100),
  array("month"=>'3',"revenue"=>200),
  array("month"=>'5',"revenue"=>600)
);

// where $a stores the result and $b is the new element of the array to add
function reduce_target_ar($a,$b){
  return $a + $b['revenue'];
}

$sum = array_reduce($ar,"reduce_target_ar");

echo $sum;