Printing a diamond shape in ASCII

Question

import math

num_rows = int(input())
if num_rows % 2 != 0:
    upper_mid = math.ceil(num_rows/2)
    lower_mid = num_rows//2
    
    #prints upper normal triangle
    for row in range(0, upper_mid):                 
        for col in range(upper_mid-1, row, -1):     
            print(end=" ")                         
        for col in range(row+1):                  
            print('*', end="")
        for col in range(row):                   
            print('*', end="")
        print()
    
    # prints lower inverted triangle 
    for row in range(0, lower_mid):                
        for col in range(row+1):                    
            print(end=" ")
        for col in range(lower_mid - row):         
            print('*', end="")
        for col in range(lower_mid - row-1):    
            print('*', end="")
        print()
else:
    print("Only odd numbers!")

Input:

9

Ouput:

    *
   ***
  *****
 *******
*********
 *******
  *****
   ***
    *

This code works 100%, and prints a diamond shape star pattern. I'm a beginner in Python, and wrote this by myself, so I might have written something unnecessary and the code could be simplified, or something else. Feedback is appreciated.

Answer 1

Calling print is very slow, you should focus on building strings then have one print. Fortunately Python has some really nice string manipulation options.

for col in range(upper_mid-1, row, -1):
    print(end=" ")

• You should rename col to _, which is standard nomenclature to say the variable is thrown away.

• Since the print isn't based on col walking up (step=1) would be easier to understand.

  for _ in range(row, upper_mid - 1):
  

• We can instead use * with a string and an integer to duplicate the string that amount of times.

  output = " " * (upper_mid - 1 - row)
  

Lets convert your code to remove the ranges, and a number of prints.
I'm also remove the whitespace at the end of lines, something which you should never have.

num_rows = int(input())
if num_rows % 2 != 0:
    upper_mid = math.ceil(num_rows/2)
    lower_mid = num_rows//2

    #prints upper normal triangle
    for row in range(0, upper_mid):
        print(
            " " * (upper_mid - 1 - row)
            + "*" * (row + 1)
            + "*" * row
        )

    # prints lower inverted triangle 
    for row in range(0, lower_mid):
        print(
            " " * (row + 1)
            + "*" * (lower_mid - row)
            + "*" * (lower_mid - 1 - row)
        )
else:
    print("Only odd numbers!")

• We can see the two loops for the upper and lower triangle are very similar. If we change the lower triangle to walk from lower_mid - 1 down, then the inner parts would be the same.

• We can also see we can merge the two lines creating the stars with row * 2 + 1

num_rows = int(input())
if num_rows % 2 != 0:
    upper_mid = math.ceil(num_rows/2)
    lower_mid = num_rows//2
    for row in range(0, upper_mid):
        print(
            " " * (upper_mid - 1 - row)
            + "*" * (2 * row + 1)
        )
    for row in reversed(range(0, lower_mid)):
        print(
            " " * (upper_mid - 1 - row)
            + "*" * (2 * row + 1)
        )
else:
    print("Only odd numbers!")

I wouldn't expect a beginner to know these. But there are some more advanced improvements we can make:

• Making a function, to increase reusability of the code.
• Using itertools.chain to let us not write the same body for the for loops twice.
• Changing num_rows % 2 != 0 to a guard clause to reduce the arrow anti-pattern.
• We know lower_mid = upper_mid - 1, so we can replace upper_mid - 1 with lower_mid and remove the need for using math.ceil.
• Use "\n".join and a generator expression to return just a string to be printed.
• Use typehints to document the types the function takes and returns.

import itertools


def diamond(size: int) -> str:
    if size % 2 == 0:
        raise ValueError("only odd numbers are allowed")
    mid = size // 2
    return "\n".join(
        " " * (mid - half_width) + "*" * (2 * half_width + 1)
        for half_width in itertools.chain(
            range(0, mid + 1),
            reversed(range(0, mid)),
        )
    )


print(diamond(int(input("Size of diamond: "))))