Python - mystery infinite loop

Question

I'm taking this Python course online and am trying to figure out why this loop is infinite when the x value is 3

def mystery(x):
  a = [0, 4, 0, 3, 2]
  while x > 0:
    x = a[x]
  return "Done"

mystery(3) runs infinitely.

Is it because it keeps trying to set x to 3 when the list value is already 3?

Answer 1

Remember array indices start from 0, so if

a = [0, 4, 0, 3, 2]

then a[3] == 3

so this line

x = a[x] 

never sets x to anything other than 3!

Answer 2

"Is it because it keeps trying to set x to 3 when the list value is already 3?"

Yes. a[3] points to 3 in that list. So x is just being assigned to 3 repeatedly.

Answer 3

是的，x始终为3。最初x是3，索引3是list的值，即a [x]也是3.因此无限循环。

Answer 4

Remember that list indices start at zero, so a[3]=3. Then, try unrolling the loop manually:

1. x = 3

   is x=3 > 0, YES

2. x = a[x] = a[3] = 3

   is x=3 > 0, YES

3. x = a[x] = a[3] = 3

   is x=3 > 0, YES

and so on.

Answer 5

def mystery(x):               # Here x = 3
  a = [0, 4, 0, 3, 2]         
  while x > 0:                # Since x = 3 the program enters the loop
    x = a[x]                  # a[3] = 3 and hence x is assigned 3. Again x = 3 and therefore 
                              # the program will get into an infinite loop in the while 
                              # statement.
  return "Done"