§12 循环101－while循环
While和for具有一定的可替换性。语法如下：
- while test body
continue终止当次循环，break退出整个循环。
注意while之后要用大括号来括住，用引号的话会导致只进行一次替换，导致无限循环。
实例：
set x 1
# This is a normal way to write a Tcl while loop.
while {$x < 5} {
puts "x is $x"
set x [expr {$x + 1}]
}
puts "exited first loop with X equal to $x "
# The next example shows the difference between ".." and {...}
# How many times does the following loop run? Why does it not
# print on each pass?
set x 0
while "$x < 5" {
set x [expr {$x + 1}]
if {$x > 7} break
if "$x > 3" continue
puts "x is $x"
}
puts "exited second loop with X equal to $x"
执行结果：
x is 1
x is 2
x is 3
x is 4
exited first loop with X equal to 5
x is 1
x is 2
x is 3
exited second loop with X equal to 8
存在问题：while为什么要用大括号括住 的解释还没有明白。