trjhtr

I am solving interview questions from here.

Problem : Print concentric rectangular pattern in a 2d matrix. The outermost rectangle is formed by A, then the next outermost is formed by A-1 and so on. You will be given number as an argument to the function you need to implement, and you need to return a 2D array.

Example 1: Input: A = 4.

 Output: 

 4 4 4 4 4 4 4 
 4 3 3 3 3 3 4 
 4 3 2 2 2 3 4 
 4 3 2 1 2 3 4 
 4 3 2 2 2 3 4 
 4 3 3 3 3 3 4 
 4 4 4 4 4 4 4 

How can I make this code better?

def pretty_print(num):

 m = n = 2*num -1 ## m x n matrix , length of each row and column
 k = 0 # row start counter
 l = 0 # column start counter
 i = 0 # iterator

 matrix = [[0 for _ in range(n)] for _ in range(m)]

 while k < m and l < n : 
 #insert the first row
 for i in range(l, n) : 
 if matrix[k][i] == 0:
 matrix[k][i] = num # row index constt, change values in columns

 k += 1 # first row printed, so increment row start index

 #insert the last column
 for i in range(k, m) : 
 if matrix[i][n-1]==0:
 matrix[i][n-1] = num # column index constt, change values in rows
 n -= 1 # last column printed, so decrement num of columns

 #insert the last row
 if (k<m): # if row index less than number of rows remaining
 for i in range(n-1, l-1, -1):
 if matrix[m-1][i] == 0:
 matrix[m-1][i] = num # row index constt, insert in columns
 m -= 1 # last row printed, so decrement num of rows

 #insert the first column
 if (l<n): # if column index less than number of columns remaining
 for i in range(m-1, k-1, -1):
 if matrix[i][l] == 0:
 matrix[i][l] = num # column index constt, insert in rows
 l += 1 # first column printed, so increment column start index

 num -= 1 # all elements of value A inserted , so decrement

 return matrix

print pretty_print(6)

Simplify

I find the logic a bit complicated. It could be simpler:

• Loop from -A to +A, let's call the loop variable n
  


• Take the absolute value of n
  


• Generate the values in the row:
  
  
  • Loop from -A to +A, let's call the loop variable m
    
  
  
  • Use the maximum of abs(n) + 1 and abs(m) + 1
    
  
  

Like this:

def pretty_print(num):
 def print_line(n):
 low = abs(n)
 print(' '.join(str(max(low + 1, abs(i) + 1)) for i in range(-num + 1, num)))

 for i in range(-num + 1, num):
 print_line(i)

Testing

Doctests are awesome, I recommend to use them.
Here's the complete solution with doctests,
you can run this with python -mdoctest pretty_print.py:

#!/usr/bin/env python


def pretty_print(num):
 """
 >>> pretty_print(1)
 1

 >>> pretty_print(2)
 2 2 2
 2 1 2
 2 2 2

 >>> pretty_print(6)
 6 6 6 6 6 6 6 6 6 6 6
 6 5 5 5 5 5 5 5 5 5 6
 6 5 4 4 4 4 4 4 4 5 6
 6 5 4 3 3 3 3 3 4 5 6
 6 5 4 3 2 2 2 3 4 5 6
 6 5 4 3 2 1 2 3 4 5 6
 6 5 4 3 2 2 2 3 4 5 6
 6 5 4 3 3 3 3 3 4 5 6
 6 5 4 4 4 4 4 4 4 5 6
 6 5 5 5 5 5 5 5 5 5 6
 6 6 6 6 6 6 6 6 6 6 6
 """

 def print_line(n):
 low = abs(n)
 print(' '.join(str(max(low + 1, abs(i) + 1)) for i in range(-num + 1, num)))

 for i in range(-num + 1, num):
 print_line(i)

pretty_print(6)

Style

There are some minor style issues with the posted code.
I suggest to follow the PEP8 guidelines.