python - What does [:, :] mean on NumPy arrays

Question

Sorry for the stupid question. I'm programming on PHP but found some nice code on Python and want to "recreate" it on PHP. But I'm quite frustrated about the line

self.h = -0.1    
self.activity = numpy.zeros((512, 512)) + self.h
self.activity[:, :] = self.h

But I don't understand what does

[:, :]

mean.

Besides I wasn't able to "Google It".

Full code

import math
import numpy
import pygame
from scipy.misc import imsave
from scipy.ndimage.filters import gaussian_filter


class AmariModel(object):

    def __init__(self, size):
        self.h = -0.1
        self.k = 0.05
        self.K = 0.125
        self.m = 0.025
        self.M = 0.065

        self.stimulus = -self.h * numpy.random.random(size)
        self.activity = numpy.zeros(size) + self.h
        self.excitement = numpy.zeros(size)
        self.inhibition = numpy.zeros(size)

    def stimulate(self):
        self.activity[:, :] = self.activity > 0

        sigma = 1 / math.sqrt(2 * self.k)
        gaussian_filter(self.activity, sigma, 0, self.excitement, "wrap")
        self.excitement *= self.K * math.pi / self.k

        sigma = 1 / math.sqrt(2 * self.m)
        gaussian_filter(self.activity, sigma, 0, self.inhibition, "wrap")
        self.inhibition *= self.M * math.pi / self.m

        self.activity[:, :] = self.h
        self.activity[:, :] += self.excitement
        self.activity[:, :] -= self.inhibition
        self.activity[:, :] += self.stimulus


class AmariMazeGenerator(object):

    def __init__(self, size):
        self.model = AmariModel(size)

        pygame.init()
        self.display = pygame.display.set_mode(size, 0)
        pygame.display.set_caption("Amari Maze Generator")

    def run(self):
        pixels = pygame.surfarray.pixels3d(self.display)

        index = 0
        running = True
        while running:
            self.model.stimulate()

            pixels[:, :, :] = (255 * (self.model.activity > 0))[:, :, None]
            pygame.display.flip()

            for event in pygame.event.get():
                if event.type == pygame.QUIT:
                    running = False
                elif event.type == pygame.KEYDOWN:
                    if event.key == pygame.K_ESCAPE:
                        running = False
                    elif event.key == pygame.K_s:
                        imsave("{0:04d}.png".format(index), pixels[:, :, 0])
                        index = index + 1
                elif event.type == pygame.MOUSEBUTTONDOWN:
                    position = pygame.mouse.get_pos()
                    self.model.activity[position] = 1

        pygame.quit()


def main():
    generator = AmariMazeGenerator((512, 512))
    generator.run()


if __name__ == "__main__":
    main()

question from:https://stackoverflow.com/questions/16815928/what-does-mean-on-numpy-arrays

Answer 1

The [:, :] stands for everything from the beginning to the end just like for lists. The difference is that the first : stands for first and the second : for the second dimension.

a = numpy.zeros((3, 3))

In [132]: a
Out[132]: 
array([[ 0.,  0.,  0.],
       [ 0.,  0.,  0.],
       [ 0.,  0.,  0.]])

Assigning to second row:

In [133]: a[1, :] = 3

In [134]: a
Out[134]: 
array([[ 0.,  0.,  0.],
       [ 3.,  3.,  3.],
       [ 0.,  0.,  0.]])

Assigning to second column:

In [135]: a[:, 1] = 4

In [136]: a
Out[136]: 
array([[ 0.,  4.,  0.],
       [ 3.,  4.,  3.],
       [ 0.,  4.,  0.]])

Assigning to all:

In [137]: a[:] = 10

In [138]: a
Out[138]: 
array([[ 10.,  10.,  10.],
       [ 10.,  10.,  10.],
       [ 10.,  10.,  10.]])