php – usort()排序算法如何工作？
作者：互联网
我有一个usort()示例,我添加了一些echo语句来查看代码是如何工作的：
<?php
function list_cmp($a, $b) {
global $order;
echo "\$a=$a, \$b=$b </br>";
foreach ($order as $key => $value) {
echo "\$value=$value </br>";
if ($a == $value) {
echo "\$a=\$value, returing 0. </br>";
return 0;
}
if ($b == $value) {
echo "\$b=\$value, returing 1. </br>";
return 1;
}
}
}
$order[0] = 1;
$order[1] = 3;
$order[2] = 4;
$order[3] = 2;
$array[0] = 2;
$array[1] = 1;
$array[2] = 3;
$array[3] = 4;
$array[4] = 2;
array[5] = 1;
$array[6] = 2;
usort($array, "list_cmp");
?>
代码的输出是这样的：
$a=2, $b=1
$value=1
$b=$value, returing 1.
$a=2, $b=3
$value=1
$value=3
$b=$value, returing 1.
$a=1, $b=3
$value=1
$a=$value, returing 0.
$a=2, $b=4
$value=1
$value=3
$value=4
$b=$value, returing 1.
$a=3, $b=4
$value=1
$value=3
$a=$value, returing 0.
$a=2, $b=2
$value=1
$value=3
$value=4
$value=2
$a=$value, returing 0.
$a=2, $b=1
$value=1
$b=$value, returing 1.
$a=2, $b=1
$value=1
$b=$value, returing 1.
$a=4, $b=1
$value=1
$b=$value, returing 1.
$a=3, $b=1
$value=1
$b=$value, returing 1.
$a=1, $b=1
$value=1
$a=$value, returing 0.
$a=2, $b=2
$value=1
$value=3
$value=4
$value=2
$a=$value, returing 0.
创造12 $a-$b对的机制是什么 – 2-1,2-3,1-3,2-4,3-4,2-2,2-1,2-1(同样再次) ？),4-1,3-1,1-1,2-2.上面的代码返回1,1,0,1,0,0,1,1,1,1,0,0.还有什么是根据返回的值对数组进行排序的机制？我试图了解usort()机制是如何工作的.
谢谢.
解决方法:
>比较器如何工作？
我不确定这是问题的一部分,但要清楚比较器是如何工作的：
您有一个由ordered list $order指定的订单和一个特殊的比较器回调list_cmp,它应该返回wether参数
> $a小于$b(返回-1或值<0)
> $a大于$b(返回1或值> 0)
> $a等于$b(返回0)
list_cmp通过查找其订单表而不是检查是否来完成此操作
> $a具有较小(或相等)的顺序,在这种情况下,循环会在返回0或if时退出
> $b的顺序较小,在这种情况下循环会在返回1时提前退出.
请注意,根据PHP文档,这是错误的,它声明它需要正/负/ 0作为返回值.只有当您知道内部仅检查比较器($a,$b)>时,这才是正确的. 0,意味着它只检查$b是否小于且不等于$a,使其成为$a< = $b的订单的比较顺序.如果代码开始检查$a是否小于且不等于$b,它可能很容易破坏.
>快速排序分类如何工作？
首先,我假设您使用的是PHP 7或更高版本.在这种情况下,你遇到了一个特殊情况,有6-15个元素大小的数组. PHP 7似乎不使用快速排序用于短列表,而是使用插入排序变体(由于硬件相关的东西,如缓存/代码位置,它最有可能更快).您可以检查排序源代码f.e.在Github PHP Mirror (as an example: PHP 7.0 Zend/Zend_sort.c Line 177-198).
该代码分为3个步骤：
>比较：比较邻居元素array [j]和array [j 1],如果array [j]< = array [j 1]继续,则转到2.
>查找插入点：现在如果array [j]> array [j 1],向后扫描以找到array [x]<对于x<数组[j 1]< = array [x 1] j(显然只有在点击开始之前)
> insert：shift elements x 1 … j one up,使其变为x 2 … j 1并将前一元素插入位置x 1
如果将该代码应用于配对(2-1,2-3,1-3,2-4,3-4,2-2,2-1,2-1,4-1,3-1,1) -1,2-2)它明显代码的作用.
-- [2,1],3,4,2,1,2 -> 1./2./3. compare [2,1], find and insert 1 before 2
-- 1,[2,3],4,2,1,2 -> 1./2. compare [2,3], find insert point for 3 (since order of 3 < order of 2)
-- [1,3],2,4,2,1,2 -> 3. compare [1,3], found insert point for 3 before 2
-- 1,3,[2,4],2,1,2 -> 1./2. compare [2,4], find insert point for 4 (since order of 4 < order of 2)
-- 1,[3,4],2,2,1,2 -> 3. compare [3,4], found insert point for 4 before 2
-- 1,3,4,[2,2],1,2 -> 1. compare [2,2], skip
-- 1,3,4,2,[2,1],2 -> 1./2. compare [2,1], find insert point for 1
-- 1,3,4,[2,1],2,2 -> 2. compare [2,1], find insert point for 1
-- 1,3,[4,1],2,2,2 -> 2. compare [4,1], find insert point for 1
-- 1,[3,1],4,2,2,2 -> 2. compare [3,1], find insert point for 1
-- [1,1],3,4,2,2,2 -> 3. compare [1,1], fond insert point for 1 before 3
-- 1,1,3,4,2,[2,2] -> 1. compare [2,2], skip
-- sorted: 1,1,3,4,2,2,2
PS：
在这里,您已经看到,通过其比较模式推导出简单排序算法(22行代码)的工作非常复杂. PHP 7快速排序的实现大约是代码行的10倍,并且有一些奇怪的优化(除了由于数据透视选择和递归而导致的正常疯狂).
大多数情况下,最好忽略深度实现细节,只将其减少到需要的东西.排序算法的典型问题是它是稳定/不稳定的,并且在O(log n)中执行O(n)内存消耗.有更简单的方法来学习这些优化实现背后的核心算法,如Quicksort Dance或任何其他可视化或旧的(e)书籍或带有示例的网页.
– 编辑
为插入排序添加了一个(糟糕的,未经优化的,不安全的)php实现,以实现其工作方式的另一个可视化：
<?php
function my_usort($A, $comparator) {
// Start .. End Positions
$current_pos = 0;
$last_pos = count($A)-1;
// Outer Loop: each step checks that A[0] up to A[current_pos] is sorted.
// When the algorithm finishes we know that A[0] ... A[last_pos] is sorted
while($current_pos < $last_pos) {
echo "Sorted Subarray from \$A is " . json_encode(array_slice($A, 0, $current_pos+1)) . "<br>\n";
echo "\$A looks like this now: " . json_encode($A) .
", comparing [" . $A[$current_pos] . "," . $A[$current_pos +1] . "] (verify step)<br>\n";
// "Verification Step"
// At this point A[0] ... A[current_pos] is sorted.
// Check A[current_pos] <= A[current_pos +1]
if($comparator($A[$current_pos], $A[$current_pos +1]) > 0) {
// nope, A[current_pos] > A[current_pos +1] (list_cmp/comparator returns value > 0)
// "Insertion Step" start, find the correct position for A[current_pos+1] in the already
// sorted list A[0] ... A[current_pos]
$insert_point = $current_pos;
// Swap the missmatching Neighbor pair
echo "swapping: \$A[" . $insert_point . "] and \$A[" . ($insert_point+1) . "]<br>\n";
$tmp = $A[$insert_point +1];
$A[$insert_point +1] = $A[$insert_point];
$A[$insert_point] = $tmp;
$sorted_up_to_current_pos = false;
// Inner Loop: find correct insertion point
while($insert_point > 0 && !$sorted_up_to_current_pos) {
echo "\$A looks like this now: " . json_encode($A) .
", comparing [" . $A[$insert_point-1] . "," . $A[$insert_point] . "] (insertion step)<br>\n";
// "Insertion Step", Swap the missmatching Neighbor pairs until A[0] ... A[current_pos] is sorted again
if($comparator($A[$insert_point-1], $A[$insert_point]) > 0) {
// Swap the missmatching Neighbor pair
echo "swapping: \$A[" . ($insert_point-1) . "] and \$A[" . $insert_point . "]<br>\n";
$tmp = $A[$insert_point];
$A[$insert_point] = $A[$insert_point-1];
$A[$insert_point-1] = $tmp;
// goto new pair
$insert_point = $insert_point -1;
} else {
// found correct spot, done
$sorted_up_to_current_pos = true;
}
}
$A[$insert_point] = $tmp;
echo "\$A looks like this now: " . json_encode($A) . ", insertion done<br>\n";
}
$current_pos = $current_pos + 1;
}
echo "Sorted Array \$A is " . json_encode(array_slice($A, 0, $current_pos+1)) . "<br>\n";
}
function list_cmp($a, $b) {
global $order;
//echo "\$a=$a, \$b=$b </br>\n";
foreach ($order as $key => $value) {
//echo "\$value=$value </br>\n";
if ($a == $value) {
echo "\$a=\$value, returing 0. </br>\n";
return 0;
}
if ($b == $value) {
echo "\$b=\$value, returing 1. </br>\n";
return 1;
}
}
}
$order[0] = 1;
$order[1] = 3;
$order[2] = 4;
$order[3] = 2;
$array[0] = 2;
$array[1] = 1;
$array[2] = 3;
$array[3] = 4;
$array[4] = 2;
$array[5] = 1;
$array[6] = 2;
my_usort($array, "list_cmp");
输出现在已完成,当前已排序的数组,位置：
Sorted Subarray from $A is [2]
$A looks like this now: [2,1,3,4,2,1,2], comparing [2,1] (verify step)
$b=$value, returing 1.
swapping: $A[0] and $A[1]
$A looks like this now: [1,2,3,4,2,1,2], insertion done
Sorted Subarray from $A is [1,2]
$A looks like this now: [1,2,3,4,2,1,2], comparing [2,3] (verify step)
$b=$value, returing 1.
swapping: $A[1] and $A[2]
$A looks like this now: [1,3,2,4,2,1,2], comparing [1,3] (insertion step)
$a=$value, returing 0.
$A looks like this now: [1,3,2,4,2,1,2], insertion done
Sorted Subarray from $A is [1,3,2]
$A looks like this now: [1,3,2,4,2,1,2], comparing [2,4] (verify step)
$b=$value, returing 1.
swapping: $A[2] and $A[3]
$A looks like this now: [1,3,4,2,2,1,2], comparing [3,4] (insertion step)
$a=$value, returing 0.
$A looks like this now: [1,3,4,2,2,1,2], insertion done
Sorted Subarray from $A is [1,3,4,2]
$A looks like this now: [1,3,4,2,2,1,2], comparing [2,2] (verify step)
$a=$value, returing 0.
Sorted Subarray from $A is [1,3,4,2,2]
$A looks like this now: [1,3,4,2,2,1,2], comparing [2,1] (verify step)
$b=$value, returing 1.
swapping: $A[4] and $A[5]
$A looks like this now: [1,3,4,2,1,2,2], comparing [2,1] (insertion step)
$b=$value, returing 1.
swapping: $A[3] and $A[4]
$A looks like this now: [1,3,4,1,2,2,2], comparing [4,1] (insertion step)
$b=$value, returing 1.
swapping: $A[2] and $A[3]
$A looks like this now: [1,3,1,4,2,2,2], comparing [3,1] (insertion step)
$b=$value, returing 1.
swapping: $A[1] and $A[2]
$A looks like this now: [1,1,3,4,2,2,2], comparing [1,1] (insertion step)
$a=$value, returing 0.
$A looks like this now: [1,1,3,4,2,2,2], insertion done
Sorted Subarray from $A is [1,1,3,4,2,2]
$A looks like this now: [1,1,3,4,2,2,2], comparing [2,2] (verify step)
$a=$value, returing 0.
Sorted Array $A is [1,1,3,4,2,2,2]
标签：php,quicksort,usort 来源： https://codeday.me/bug/20190627/1306167.html