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Python

Arnav Goyal
Arnav Goyal
1,167 Points
Posted by Arnav Goyal
Arnav Goyal
1,167 Points

List comprehension problem

The purpose is to print out all possible coordinates of a given cuboid with specified dimensions(x,y,z) to form a multidimensional list. However the sum of coordinates should not be equal to a given number 'n'. Note: Can be done using for loops but trying to learn list comprehension better:) x,y,z,n are standard inputs Can you help with the error in this code

coordinates = print([[x,y,z] for x in range(x+1) for y in range(y+1) for z in range(z+1)if x + y + z != n])

1 Answer

Chris Freeman
MOD
Chris Freeman
Treehouse Moderator 68,453 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,453 Points

Hey Arnav Goyal, neat challenge! There are two errors in your code.

  • print returns None after output a value, so coordinates would always be None
  • the comprehension does not seem to like implied nested loops where the loop variable i is the same name as range variable. [I need to dig a bit more to find out why]
# why does this work:
>>> x = y = 3
>>> x , y
(3, 3) 
>>> [[x] for x in range(x+1)]
[[0], [1], [2], [3]]

# but this does not work:
>>> [[x,y] for x in range(x+1) for y in range(y+1)]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 1, in <listcomp>
UnboundLocalError: local variable 'y' referenced before assignment

To solve your problem, change the loop variables:

x = y = z = 3
n = 6

coordinates = [[x0 ,y0 ,z0]
               for x0 in range(x+1)
               for y0 in range(y+1)
               for z0 in range(z+1)
               if x0 + y0 + z0 != n]

print(coordinates)

# outputs
# [[0, 0, 0], [0, 0, 1], [0, 0, 2], [0, 0, 3], [0, 1, 0], [0, 1, 1], [0, 1, 2], [0, 1, 3], [0, 2, 0], [0, 2, 1], [0, 2, 2], [0, 2, 3], [0, 3, 0], [0, 3, 1], [0, 3, 2], [1, 0, 0], [1, 0, 1], [1, 0, 2], [1, 0, 3], [1, 1, 0], [1, 1, 1], [1, 1, 2], [1, 1, 3], [1, 2, 0], [1, 2, 1], [1, 2, 2], [1, 3, 0], [1, 3, 1], [1, 3, 3], [2, 0, 0], [2, 0, 1], [2, 0, 2], [2, 0, 3], [2, 1, 0], [2, 1, 1], [2, 1, 2], [2, 2, 0], [2, 2, 1], [2, 2, 3], [2, 3, 0], [2, 3, 2], [2, 3, 3], [3, 0, 0], [3, 0, 1], [3, 0, 2], [3, 1, 0], [3, 1, 1], [3, 1, 3], [3, 2, 0], [3, 2, 2], [3, 2, 3], [3, 3, 1], [3, 3, 2], [3, 3, 3]]

Post back if you need more help. Good luck!!!