next(dataset.cam0) doesn't work

[jinchenglee]

It always returns the first frame data no matter how many times I've called it.

Which version of python does it require? Python 2 or 3? I use Python3.

[leeclemnet]

dataset.cam0 is a property that creates a new generator each time you access it, so calling next(dataset.cam0) will only ever return the first frame. This is just syntactic sugar to avoid explicitly rewinding the dataset every time you want to iterate through it. The usual thing you'll need to do is something like this:

for image in dataset.cam0:
     do_something(image)

which works as you'd expect.

If you need to access arbitrary frames, you have a few options:

1. Pass a frames argument to the constructor to load only specific frames like I do here. You can pass an arbitrary list instead of a range if need be.
2. Use the itertools.islice method like I do here.
3. Create a list from the generator (this can use up a lot of memory):

   cam0_frames = list(dataset.cam0)
   image01 = cam0_frames[1]

4. Create another variable to point to a specific instance of the generator, and use next like you were trying to do:

   cam0_generator = dataset.cam0
   image00 = next(cam0_generator)
   image01 = next(cam0_generator)

[jinchenglee]

Thanks for the thorough explanation.

It would be helpful you can add these to README.md so others won't ask again.

2017-06-27 6:04 GMT-07:00 Lee Clement notifications@github.com:

dataset.cam0 is a property that creates a new generator each time you access it, so calling next(dataset.cam0) will only ever return the first frame. This is just syntactic sugar to avoid explicitly rewinding the dataset every time you want to iterate through it. The usual thing you'll need to do is something like this:

for image in dataset.cam0: do_something(image)

which works as you'd expect.

If you need to access arbitrary frames, you have a few options:

1. Pass a frames argument to the constructor to load only specific frames like I do here https://github.com/utiasSTARS/pykitti/blob/master/demos/demo_raw.py#L23. You can pass an arbitrary list instead of a range if need be.
2. Use the itertools.islice method like I do here https://github.com/utiasSTARS/pykitti/blob/master/demos/demo_raw.py#L34 .
3. Create a list from the generator (this can use up a lot of memory):

cam0_frames = list(dataset.cam0) image01 = cam0_frames[1]

1. Create another variable to point to a specific instance of the generator, and use next like you were trying to do:

cam0_generator = dataset.cam0 image00 = next(cam0_generator) image01 = next(cam0_generator)

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[leeclemnet]

Good idea. I updated the example in the README.md to hopefully make the usage more clear. See: https://github.com/utiasSTARS/pykitti/commit/87035d0c3bef3bd8ce3535a7de63506f8a6ed4a8