Python代码逐步了解代码

Question

Hi everyone I am currently practicing interpreting code and writing down the process of it every step of the way! This is what I have currently come up with.

x = 4
y = 19
finished = False
while x <= y and not finished:
    subtotal = 0
    for z in range(0, x, 4):
        print(x)
        subtotal += x
        print("This is subtotal", subtotal)
        if y // x <= 1:
            finished = True
        else:
            x += x
            print("New x value:", x)

1. x = 4, y = 19, finished = false, subtotal =4, z = 0
2. x = 8, y = 19, finished = false, subtotal =8, z = 0
3. x = 16, y = 19, finished = True, subtotal =24, z = 0

I believe what I did here is correct but I am not sure how the subtotal are going to 4 to 8 to 24? If someone could explain this to me that would be great.

I understand that range is exclusive so when the x value is 4 it only goes through the for loop once hence why the subtotal is = 4. However when the x value is 8 it goes through the for loop to my under standing 2 times so this is the part where I get lost.

My last question is each time it goes through this loop does the subtotal get reset everytime the x value is changed? Would this be the reason I can not get the correct subtotals?

If someone could visually show this to me or explain it that would be awesome thank you very much!

Answer 1

That is because in the first loop subtotal is 0. The for loop iterates just once because it then looks like this for z in range(0, 4, 4) . Then x and subtotal become 4. NOW subtotal is brought back to 0 and the for loop becomes for z in range(0, 8, 4) so this time the for loop will iterate twice as there are two possible numbers in that range (which are 0 and 4), subtotal gets added to x which is 8 and x becomes 16, the for loop iterates ( NOTE the subtotal doesn't get brought back to 0 as the subtotal = 0 statement isn't inside the for loop) again making subtotal now 8 + 16. Which is 24.

Answer 2

Just examining the changes to the variables:

Start: x = 4, y = 19, finished = False
1.      subtotal = 0
2.       z = 0
3.       subtotal += x (0+4) = 4
4.       x += x (4+4) = 8
5.      subtotal = 0
6.       z = 0
7.       subtotal += x (0+8) = 8
8.       x += x (8+8) = 16
9.       z = 4
10.      subtotal += x (8+16) = 24
11.      finished = True
End:   x = 16, y = 19, finished = True, z = 4, subtotal = 24

subtotal only gets reset to 0 when the inner loop exits, as x gets large the inner loop repeats more times, 1 the first time, 2 the second.