How to make a function that outputs an array with a predefined size?

Question

I want to make a function that outputs an array of predefined size. The function is dependent on variables x, y, rx, and ry, and d. Variables x and y are directly related to the cartesian values. rx and ry are the radius of the blobs generated from the function.

I have already converted array indiies from the traditional 0, 0 on the upper left corner to the middle most pixel. This array should always be odd in herms of length and width.

The blob below gives me one array at a time. I need a stack of arrays I can add together. When I change x and y, the size of the array changes, but I need the array to be the same size or zeroed out at on the borders.

I have some code that follows:

def equation(x, y):
    return 1 * np.exp(-(x**2 + y**2))


def make_matrix(xs, ys, x_min=nxs, y_min=nys): #makes cartesian values
    out = [] #outputs array
    for i in range(x_min, xs - center_x):
        row = []
        for j in range(y_min, ys - center_y):
            row.append(equation(i, j))
        out.append(row)
    return out

blob = np.asarray(list(np.float_(make_matrix(x, y))))

Your prose does not match the code at all. I have provided an answer that matches your code, and hopefully shows you the tools that you need. I am unlikely to be able to match the prose portion without a lot of additional information. — Mad Physicist
– Mad Physicist, Commented Jul 15, 2019 at 19:13

Mad Physicist · Accepted Answer · 2019-07-15 18:59:16Z

1

Vectorization is your friend here. No need to compute lists element-by element when your function can be evaluated element-wise for an entire array.

def make_matrix(xs, ys, x_min=nxs, y_min=nys):
    x = np.arange(x_min, xs - center_x).reshape(1, -1)
    y = np.arange(y_min, ys - center_y).reshape(-1, 1)
    out = np.exp(-(x**2 + y**2))
    return out

The reshape operations create a 1xN row vector for x and a Nx1 column vector for y. These shapes ensure that broadcasting in the add (+) operation expands x**2 + y**2 to an array of the final size you want.

answered Jul 15, 2019 at 18:59

Mad Physicist

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