经典的笛卡尔乘积代码，要怎么修改它？
笛卡尔乘积代码：
<?PHP
function Descartes() {
$t = func_get_args();
if(func_num_args() == 1) return call_user_func_array( __FUNCTION__, $t[0] );
$a = array_shift($t);
if(! is_array($a)) $a = array($a);
$a = array_chunk($a, 1);
do {
$r = array();
$b = array_shift($t);
if(! is_array($b)) $b = array($b);
foreach($a as $p)
foreach(array_chunk($b, 1) as $q)
$r[] = array_merge($p, $q);
$a = $r;
}while($t);
return $r;
}
$arr = array(
array('a1','a2'),
'b',
array('c1','c2'),
array('d1','d2','d3'),
array('e1','e2','e3')
//......省略其它元素（也可以是数组），
);
$r = Descartes( $arr );
print_r($r);
?>
//////////////////////////////////////
输出：
Array
(
[0] => Array
(
[0] => a1
[1] => b
[2] => c1
[3] => d1
[4] => e1
)
[1] => Array
(
[0] => a1
[1] => b
[2] => c1
[3] => d1
[4] => e2
)
...。。。
）
////////////////////
现在我只想要3个元素，第一个元素要有a1或a2。那段代码要怎么改？
Array
(
[0] => Array
(
[0] => a1
[1] => b
[2] => c1
)
......
[1] => Array
(
[0] => a2
[1] => c2
[2] => d3
)
......
）
[解决办法]
对于
$arr = array(
array('a1','a2'),
'b',
array('c1','c2'),
array('d1','d2','d3'),
array('e1','e2','e3')
//......省略其它元素（也可以是数组），
);
需先对
$arr = array(
'b',
array('c1','c2'),
array('d1','d2','d3'),
array('e1','e2','e3')
//......省略其它元素（也可以是数组），
);
求 M 取 2 的组合
$c = combination($arr, 2);
然后对组合的结果求笛卡尔积
$r = array();
foreach($c as $v)
$r = array_merge($r, Descartes( array('a1','a2'), $v[0], $v[1] ));
print_r($r);
Array( [0] => Array ( [0] => a1 [1] => d1 [2] => e1 ) [1] => Array ( [0] => a1 [1] => d1 [2] => e2 ) [2] => Array ( [0] => a1 [1] => d1 [2] => e3 ) [3] => Array ( [0] => a1 [1] => d2 [2] => e1 ) [4] => Array ( [0] => a1 [1] => d2 [2] => e2 ) [5] => Array ( [0] => a1 [1] => d2 [2] => e3 ) [6] => Array ( [0] => a1 [1] => d3 [2] => e1 ) [7] => Array ( [0] => a1 [1] => d3 [2] => e2 ) [8] => Array ( [0] => a1 [1] => d3 [2] => e3 ) [9] => Array ( [0] => a2 [1] => d1 [2] => e1 ) [10] => Array ( [0] => a2 [1] => d1 [2] => e2 ) [11] => Array ( [0] => a2 [1] => d1 [2] => e3 ) [12] => Array ( [0] => a2 [1] => d2 [2] => e1 ) [13] => Array ( [0] => a2 [1] => d2 [2] => e2 ) [14] => Array ( [0] => a2 [1] => d2 [2] => e3 ) [15] => Array ( [0] => a2 [1] => d3 [2] => e1 ) [16] => Array ( [0] => a2 [1] => d3 [2] => e2 ) [17] => Array ( [0] => a2 [1] => d3 [2] => e3 ) [18] => Array ( [0] => a1 [1] => c1 [2] => e1 ) [19] => Array ( [0] => a1 [1] => c1 [2] => e2 ) [20] => Array ( [0] => a1 [1] => c1 [2] => e3 ) [21] => Array ( [0] => a1 [1] => c2 [2] => e1 ) [22] => Array ( [0] => a1 [1] => c2 [2] => e2 ) [23] => Array ( [0] => a1 [1] => c2 [2] => e3 ) [24] => Array ( [0] => a2 [1] => c1 [2] => e1 ) [25] => Array ( [0] => a2 [1] => c1 [2] => e2 ) [26] => Array ( [0] => a2 [1] => c1 [2] => e3 ) [27] => Array ( [0] => a2 [1] => c2 [2] => e1 ) [28] => Array ( [0] => a2 [1] => c2 [2] => e2 ) [29] => Array ( [0] => a2 [1] => c2 [2] => e3 ) [30] => Array ( [0] => a1 [1] => c1 [2] => d1 ) [31] => Array ( [0] => a1 [1] => c1 [2] => d2 ) [32] => Array ( [0] => a1 [1] => c1 [2] => d3 ) [33] => Array ( [0] => a1 [1] => c2 [2] => d1 ) [34] => Array ( [0] => a1 [1] => c2 [2] => d2 ) [35] => Array ( [0] => a1 [1] => c2 [2] => d3 ) [36] => Array ( [0] => a2 [1] => c1 [2] => d1 ) [37] => Array ( [0] => a2 [1] => c1 [2] => d2 ) [38] => Array ( [0] => a2 [1] => c1 [2] => d3 ) [39] => Array ( [0] => a2 [1] => c2 [2] => d1 ) [40] => Array ( [0] => a2 [1] => c2 [2] => d2 ) [41] => Array ( [0] => a2 [1] => c2 [2] => d3 ) [42] => Array ( [0] => a1 [1] => b [2] => e1 ) [43] => Array ( [0] => a1 [1] => b [2] => e2 ) [44] => Array ( [0] => a1 [1] => b [2] => e3 ) [45] => Array ( [0] => a2 [1] => b [2] => e1 ) [46] => Array ( [0] => a2 [1] => b [2] => e2 ) [47] => Array ( [0] => a2 [1] => b [2] => e3 ) [48] => Array ( [0] => a1 [1] => b [2] => d1 ) [49] => Array ( [0] => a1 [1] => b [2] => d2 ) [50] => Array ( [0] => a1 [1] => b [2] => d3 ) [51] => Array ( [0] => a2 [1] => b [2] => d1 ) [52] => Array ( [0] => a2 [1] => b [2] => d2 ) [53] => Array ( [0] => a2 [1] => b [2] => d3 ) [54] => Array ( [0] => a1 [1] => b [2] => c1 ) [55] => Array ( [0] => a1 [1] => b [2] => c2 ) [56] => Array ( [0] => a2 [1] => b [2] => c1 ) [57] => Array ( [0] => a2 [1] => b [2] => c2 ))
