snapshots = [
np.genfromtxt('data/velocity0.{}.csv'.format(i), delimiter=',', skip_header=1)[:, 0]
for i in range(20, 40)
]
pts = np.genfromtxt('data/velocity0.20.csv', delimiter=',', skip_header=1)[:, -3:-1]
这段代码中的[:, 0]与[:, -3:-1]代表什么意思呀