python - 你怎么知道一个while循环是否不会终止？

Question

你怎么知道它是否不会终止？有没有让它继续运行的功能？如果我想在一定次数后停止循环，我该怎么做？

Answer 1

Generally, it's impossible to know ahead of time whether a program will loop forever or eventually stop. This is called the Halting problem. Of course, in practice you can probably make a reasonable guess just by looking at the condition.

a while loop will keep going as long as its condition is true. You do not need a function to make it keep going.

while True:
    print "hello, world!"
    #no functions required here!

If you want something to loop a certain number of times, it's preferable to use a for loop:

for i in range(10):
    print "hello, world!"
    #prints ten times

although you still can use a while loop if you really want.

count = 0
while count < 10:
    print "hello, world!"
    count += 1

Answer 2

A while loop is terminated

• if the condition it uses is false at the time it gets evaluated.

  Example:

  x = 10
  while x > 5:
      x -= 7
      print x
      x += 6
      print x
  

  successively will print the numbers 3, 9, 2, 8, 1, 7, 0, 6, -1, 5 and only then terminate.

  x becomes <= 5 during execution, but only the state at the time where the loop is restarted is relevant.

• if it is left meanwhile with break:

  x = 10
  while x > 5:
      print x
      x -= 1
      break
  

  only prints 10, because it is left "forcefully" afterwards.

A loop which runs a certain number of times would be done

x = 0
while x < n:
    do_stuff()
    x += 1

or better with for:

for x in range(n):
    do_stuff()