Pattern in Python

I want to draw a diamond pattern (using 1's) in the zeros matrix 80x80. The first half is going really well but I get nothing but 0s in the second half.

img = np.zeros((80,80))

def draw_pic(image):
    for i in range(len(image)):
        for j in range(len(image[i])):
            print(int(image[i][j]), end = '')
        print()

def gen_diamond(image):
    ret = np.copy(image)
    for i in range(len(image)):
        for j in range(len(image[i])):
            if (i < len(image)/2 and j >= len(image[i])/2 - (i + 1) and j <= len(image[i])/2 + i):
                ret[i][j] = 1
            if (i > len(image)/2 and j >= len(image[i])/2 - (i + 1)and j <= len(image[i])/2 - i):
                ret[i][j] = 1


    return ret

draw_pic(gen_diamond(img))

Solution

Your error is in the range checks in the lower half. Let's look at the arithmetic for row 42 ...

        if (i > len(image)/2 and
            j >= len(image[i])/2 - (i + 1) and
            j <= len(image[i])/2 - i):

Substituting the proper values, we have:

        if (42 > 40 and
            j >= 40 - (42 + 1) and
            j <= 40 - 42):

That last condition cannot happen: you need to subtract the row number from the midpoint and take the absolute value. Simpler yet, just set your loop values directly to the ranges you need:

row_mid = len(image) // 2
col_mid = len(image[0]) // 2

for row in range(row_mid):
    for col in range(col_mid-row, col_mid+row):
        print(row, col)
        ret[row, col] = 1

for tmp in range(row_mid):
    row = len(image) - tmp    # Work from the bottom up
    for col in range(col_mid-tmp, col_mid+tmp):
        ret[row, col] = 1

Output for a 10x10 array:

I'll trust you to adjust the boundary conditions. :-)