Deleting Objects

[Mark Schaedel]

I sometimes have trouble removing objects from a list and also deleting them from memory. Here is a quick demo of how I am trying to do this. Can someone tell me what is wrong with my approach?

https://www.glowscript.org/#/user/schaedelm/folder/FunStuff/program/CollisionTest

delAllButton = button(text = "Delete All", bind = delAll)
def delAll():
    for thisBall in ballList:
        thisBall.visible = False
        ballList.remove(thisBall)
        del thisBall

[Bruce Sherwood]

Using list.remove(object) does indeed remove the object from the list.

However, from the documentation at https://www.glowscript.org/docs/VPythonDocs/delete.html:

Deleting an Object

To hide a VPython object just make it invisible: ball.visible = False. This does not delete the object from computer memory, and you can make it visible again later.

Currently it is not possible to use del ball to delete a Web VPython object.

Bruce

P.S. In practice, the failure of Web VPython to delete a 3D object from memory isn't a problem, because a 3D object takes up only 40 bytes of memory.

[Bruce Sherwood]

I misspoke; a 3D object takes up about 120 bytes.

Bruce

[Mark Schaedel]

Why does my code only remove half(ish) of the balls and not all of them?

[Bruce Sherwood]

That's tricky, because repeated executions of "for thisBall in ballList:" are working with a changing ballList. This isn't a Web VPython problem; it's also a problem with installed Python.

Here is a way to do this that does remove all the elements:

L = [10, 20, 30, 40, 50, 60]
print('>', L)
N = len(L)
for i in range(N):
    L.remove(L[0]) # remove what is now the first element
    print('>', L)
print(L)

However, in your case the simple thing to do is simply to execute ballList = []

Bruce

[Bruce Sherwood]

Here's a slightly different version:

L = [10, 20, 30, 40, 50, 60]
print('>', L)

while len(L) > 0:

    L.remove(L[0]) # remove what is now the first element
    print('>', L)
print(L)

However, getting back to your actual case, it could look like this:

for o in ballList:

    o.visible = False

ballList = []

Bruce

[陳冠榮]

You may try the following codes:

 

delAllButton = button(text = "Delete All", bind = delAll)

def delAll():

    for thisBall in ballList:

        thisBall.visible = False

        del thisBall

    ballList.clear()

 

The way of for-loop you use is wrong. I used to have the same problem. You can trace the ballList, and find what is going on.

 

CKJ

 

從 Windows 的郵件傳送

 

寄件者: Mark Schaedel
傳送時間: 2022年2月8日 22:05
收件者: Glowscript Users
主旨: [glowscript-users] Deleting Objects

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[Mark Schaedel]

Thank you Bruce and CKJ for your responses!