Tcl之looping

1　　While loop

while  test  body

　　The while command evaluates test as an expression. If test is true, the code in body is executed. After body has been executed, test is evaluated again.

Example:

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"

2　　For and incr

for  start  test  next  body

　　During evaluation of the for command, the start code is evaluated once, before any other arguments are evaluated. After the start code has been evaluated, the test is evaluated. If the test evaluates to true, then the body is evaluated, and finally, the next argument is evaluated. After evaluating the next argument, the interpreter loops back to the test, and repeats the process. If the test evaluates as false, then the loop will exit immediately.

incr  varName ?increment?

　　This command adds the value in the second argument to the variable named in the first argument. If no value is given for the second argument, it defaults to 1.

Example:

for {set i 0} {$i < 10} {incr i} {
    puts "I inside first loop: $i"
}

for {set i 3} {$i < 2} {incr i} {
    puts "I inside second loop: $i"
}

puts "Start"
set i 0
while {$i < 10} {
    puts "I inside third loop: $i"
    incr i
    puts "I after incr: $i"
}

set i 0
incr i
# This is equivalent to:
set i [expr {$i + 1}]