给变量赋值的差异

Question

In Python, why are the below 2 functions different, ie how is this function:

def year (rate):
    money=100
    count=0
    while money<(2*money):
        money+=money*rate
        print ("{0:.2f}".format(money))
        count+=1
    return (count)

(which results in count having the value 10423 ) different from this other function?

def year (rate):
    money=100
    count=0
    while money<200:
        money+=money*rate
        print ("{0:.2f}".format(money))
        count+=1
    return (count)

(which results in count having the value 11 )

If we assume that x=100, isn't x<200 the same as x<(x+x). Also, isn't x+x=200?

Answer 1

money < 2 * money will ALWAYS be true.

Because as money will increase, so will 2 * money .

Answer 2

You are changing the value of money during your loop with money += money*rate so it looks like you are creating an endless loop. If you did something like this you should be start to see the results you expect:

def year (rate):
    money=100
    count=0
    end_limit = 2*money
    while money<(end_limit):
        money+=money*rate
        print ("{0:.2f}".format(money))
        count+=1
    return (count)

Answer 3

In the first function, if you want to compare with the original money , you can do something like this:

def year (rate):
  money=100
  orig_money = money
  count=0
  while money < (2 * orig_money):
    money+=money*rate
    print ("{0:.2f}".format(money))
    count+=1
  return (count)
  # count=10423

Answer 4

As mentioned by others, your expression money<(2*money) changes and is not constant. All expressions should retain their condition in most cases to prevent an endless/infinite loop.

Try and not compare a variable to the same variable as this results in the left and right hand side of the expression both changing.

By using money < 200 , the 200 is a constant . Every iteration of the loop will check against this constant and then determine to continue or break out of the loop.

Hope I have made this situation clear. :)