经典的笛卡尔乘积代码，要怎么修改它？
笛卡尔乘积代码：
//////////////////////////////////////
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
)
......
）
------解决方案--------------------
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);
php codearray( [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 ))