nested 循环

循环可以嵌套在另一个循环中。 Perl允许嵌套所有类型的循环嵌套。

语法 (Syntax)

Perl中nested for loop语句的语法如下 -

for ( init; condition; increment ) {
   for ( init; condition; increment ) {
      statement(s);
   }
   statement(s);
}

Perl中nested while loop语句的语法如下 -

while(condition) {
   while(condition) {
      statement(s);
   }
   statement(s);
}

Perl中nested do...while loop语句的语法如下 -

do{
   statement(s);
   do{
      statement(s);
   }while( condition );
}while( condition );

Perl中nested until loop语句的语法如下 -

until(condition) {
   until(condition) {
      statement(s);
   }
   statement(s);
}

Perl中nested foreach loop语句的语法如下 -

foreach $a (@listA) {
   foreach $b (@listB) {
      statement(s);
   }
   statement(s);
}

例子 (Example)

以下程序使用嵌套的while循环来显示用法 -

#/usr/local/bin/perl
$a = 0;
$b = 0;
# outer while loop
while($a < 3) {
   $b = 0;
   # inner while loop
   while( $b < 3 ) {
      print "value of a = $a, b = $b\n";
      $b = $b + 1;
   }
   $a = $a + 1;
   print "Value of a = $a\n\n";
}

这会产生以下结果 -

value of a = 0, b = 0
value of a = 0, b = 1
value of a = 0, b = 2
Value of a = 1
value of a = 1, b = 0
value of a = 1, b = 1
value of a = 1, b = 2
Value of a = 2
value of a = 2, b = 0
value of a = 2, b = 1
value of a = 2, b = 2
Value of a = 3