I've tried solving the following HackerRank question:

Write a function blur_image() that takes as a parameter an image in the form of a nested list A and blurs it by averaging each pixel value with four of its neighboring pixels (Top, Bottom, Right, and Left). Note: not all of the neighbors are available in boundary cases. You have to write suitable conditions accordingly.

My Solution:

import ast
A = input()
A = ast.literal_eval(A)

def blur_image(a):
    result = []
    for i in range(len(a)):
        row = []
        for j in range(len(a[i])):
            total, count = a[i][j], 1
            if i + 1 < len(a): total, count = total + a[i+1][j], count + 1 
            if j + 1 < len(a[i]): total, count = total + a[i][j+1], count + 1
            if i - 1 > -1: total, count = total + a[i-1][j], count + 1 
            if j - 1 > -1: total, count = total + a[i][j-1], count + 1
            row.append(round(total/count, 2))
    return result


I would appreciate any suggestions and advice you can give me to improve this solution. Please note that my focus is to solve this without using any modules.

  • \$\begingroup\$ Do you care for speed at the expense of longer code? \$\endgroup\$ – Cris Luengo Jan 17 at 2:04
  • \$\begingroup\$ If you want speed, look up summed area tables. \$\endgroup\$ – user1118321 Jan 17 at 2:18

First, I think this code arguably abuses tuple destructuring to declare and reassign variables. Declaring multiple variables on a line generally hurts readability, just to save a line/some keystrokes. I would write everything out fully, even if that comes with the cost of verbosity. I'd also space it out a bit:

def my_blur_image1(a):
    result = []
    for i in range(len(a)):
        row = []
        for j in range(len(a[i])):
            total = a[i][j]
            count = 1
            if i + 1 < len(a):
                total += a[i+1][j]
                count += 1

            if j + 1 < len(a[i]):
                total += a[i][j+1]
                count += 1

            if i - 1 > -1:
                total += a[i-1][j]
                count += 1

            if j - 1 > -1:
                total += a[i][j-1]
                count += 1

            row.append(round(total/count, 2))


    return result

Twice, you have something along the lines of

lst = [] # Create a list
for i in range(len(a)):
    res = # Calculate some result

I think the logic could be broken up, and could make use of some list comprehensions. This is basically the scenario that list comprehensions (and map) are intended for. Iterating over a list to produce a new list is a very common operation.

I'm not necessarily recommending this way, but it does show an alternative, more functional way of approaching the problem. I'll say that my way ended up a fair bit slower than yours. On my machine, yours takes roughly 14 seconds for a 2000x2000 matrix, while my version takes 25 seconds unfortunately. You didn't tag performance though :D

# Returns whether or not i,j is inbound for a given a matrix
def inbounds(i, j, a):
    return 0 <= i < len(a) and \
           0 <= j < len(a[i])

# Returns the inbound pixel values on and surrounding the given point
def inbound_pixels_around(i, j, a):
    # I figured it was best to hard-code the indices instead of using several "if"s like you had done
    # That way, I can make use of looping to reduce some duplication
    # If diagonal pixels were included, this could be generated by another comprehension instead
    indices = [(i, j), (i - 1, j), (i + 1, j), (i, j - 1), (i, j + 1)]

    # Iterate over the indices.
    # Remove the ones that are  out of bounds, and use the inbound ones to index the list
    return [a[i][j] for (i, j) in indices if inbounds(i, j, a)]

def my_blur_image2(a):
    # A 3D-array. Each pixel has been replaced by a list of inbound neighbor values
    inbound_neigh_rows = [[inbound_pixels_around(i, j, a) for j in range(len(a[i]))]
                          for i in range(len(a))]

    # Then iterate ever each set of neighbors, and divide the sum of the neighbors by their length
    # This does away with needing an explicit "count" variable
    return [[sum(neighs) / len(neighs) for neighs in row]
            for row in inbound_neigh_rows]

I'm making fairly extensive use of list comprehensions here. I'm using them to filter out non-inbound cells in inbound_pixels_around using inbounds, and to generate neighbors and their average values in my_blur_image2.

test_data = [[1, 2, 3],
             [4, 5, 6],
             [7, 8 ,9]]


[[2.33, 2.75, 3.67], [4.25, 5.0, 5.75], [6.33, 7.25, 7.67]]
[[2.33, 2.75, 3.67], [4.25, 5.0, 5.75], [6.33, 7.25, 7.67]]
[[2.3333333333333335, 2.75, 3.6666666666666665], [4.25, 5.0, 5.75], [6.333333333333333, 7.25, 7.666666666666667]]

Updated to make use of comparison chaining. Thanks @Mathias.

  • \$\begingroup\$ You can simplify inbounds by using extended comparison syntax: 0 <= i < len(a) and 0 <= j < len(a[i]). \$\endgroup\$ – Mathias Ettinger Jan 17 at 8:12
  • \$\begingroup\$ @MathiasEttinger Oh yes, thanks. Forgot Python had that. I'll need to update that on a bit. \$\endgroup\$ – Carcigenicate Jan 17 at 14:19

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