I have a code:

x = 6

for y in range(x):
    print(y)
    x -= 2

It gives: 0, 1, 2, 3, 4, 5

I wrongly predicted that it would give either of these 2 results:

1. 0, 0, 0 Since x goes from 6 to 4 to 2 to 0, there will only be 3 y printed. Also, to my understanding, after each loop, it goes back to the for loop statement, thus resets the range() completely and now every y is 0. I ran the code on PythonTutor and the pointer also seemed to go back to the loop statement after each loop.

2. 0, 1, 2 Since x goes from 6 to 4 to 2 to 0, there will only be 3 y printed. I thought it might be possible that while y takes on the value of the original range (i.e. 6), it would be limited by each new x and thus only have 3 y printed.

Possibility 1 was the most intuitive (albeit wrong) for me and I am not sure how to go about understanding why the answer is as such.

CodePudding user response：

The instance of range produced by range(x) uses the value of x at the time range is called. It does not repeatedly check the value of x each time you need a new value from the range. Your code is effectively the same as

x = 6

for y in [0, 1, 2, 3, 4, 5]:
   print(y)
   x -= 2

Nothing you do to x has any effect on the range object being iterated.

CodePudding user response：

The range() call creates a range object from x at the start of the loop. Changing x after this has no effect on the range object that is being used to iterate.

If you want to be able to change how many iterations you make in a loop, you probably want to look at using while.

CodePudding user response：

Try this:

# Firstly you assign 6 to the variable x
x = 6 

# 'y' will now be assigned to the numbers [0, 1, 2, 3, 4, 5, 6] because the range is 'x' which is 6.

# by printing 'x' it will execute [0, 1, 2, 3, 4, 5, 6] but since x is 'x -2' which is the same as  '6 - 2'  it will print out the number 6 and -2 until it gets to the range which is -6. the output will be 6 numbers [6, 4, 2, 0, -2, -4] 


for y in range(x):
    print(x)
    x = x - 2

# i am showing you a way of making it give you your expected output [6, 4, 2, 0, -2, -4]