PHP手册中对它的语法定义如下:
for (expr1; expr2; expr3)
statement
下面说说for语句几种有用的变型。
1、无限循环
首先是人尽皆知的无限循环(亦可称"死循环")。由于空表达式null在语法上是有效的,所以我们可以把for语句的三个表达式留空,这样就会产生不断执行for嵌套语句的效果。
for (;;) {
//放置需要不断执行的语句
}
?>
虽然有一些任务会使用到无限循环,但是大多数程序任务,特别是PHP所能涉及的领域,在使用无限循环时都会添加一些终止循环的条件。
for (;;) {
//如果是公元2199年,则跳出循环
if (date('Y') == '2199') {
break;
}
}
?>
2、使用空表达式
接下来就是说说在初始化语句expr1中使用null语法,留空expr1最常见的作用就是完成更为复杂的初始化工作。
复制代码 代码如下:
if (isset($i)) {
unset($i);
if ((int) date('') $i = 0;
} else {
$i = 1;
}
} else {
$i =3;
}
for (;$i echo $i;
}
?>
同样道理,迭代表达式expr3也可能留空,也可以利用这点编写更为复杂的迭代式,比如说根据不同的条件调用不同的迭代式。
而for语句中的条件语句expr2留空则是上面所说的无限循环,当然也可以添加一些更为复杂的条件去判断什么时候跳出循环,在此不在重复。
3、多重循环
使用多重循环来控制多个变量也是在for语句中使经常被忽略的一个特性。如下面的例子,在一般的任务中用到的一般会是双重循环,三重以上的循环一般意义不大。
复制代码 代码如下:
for ($i = 0, $j = 10;$i echo "$i $j = 10\r\n";
}
?>
以上代码将输出:
0 10 = 10
1 9 = 10
2 8 = 10
3 7 = 10
4 6 = 10
5 5 = 10
6 4 = 10
7 3 = 10
8 2 = 10
9 1 = 10
10 0 = 10
4、更为复杂的表达式
如果把for语句的三个表达式写得复杂一些,则可以用于优化算法。甚至可以使用没有循环体的for语句来完成一些任务。比如计算累加或阶乘:
复制代码 代码如下:
//计算1-5的累加结果,斌值到$j
for ($i = 1,$j = 0; $i echo $j;
//计算1-5的阶乘结果,斌值到$j
for ($i = 1,$j = 1; $i echo $j;
?>
PHP借助了C语言的语法,一定程度上也会拥有C的特性,比如说强大的for循环语句就是一个典型的例子。

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