Dyalog APL, 31 29 bytes
Takes number of lines as input from user, prints hollow square
∘.{' #'[1+∨/(∊∘1 n)¨⍺ ⍵]}⍨⍳n←⎕
Output (for user input 10)
##########
# #
# #
# #
# #
# #
# #
# #
# #
##########
I recently answered the above Code Golf question (to print a hollow square, user input is number of lines) with this code ∘.{' #'[1+∨/(∊∘1 n)¨⍺ ⍵]}⍨⍳n←⎕
Any suggestions for improvement / character reduction?
@WolfgangTS Painstakingly. Dyalog APL comes with ability to make basic tree diagrams of tacit functions. Try it online! I started with that... — AdámMar 27 '17 at 20:23
@nathanrogers I have the line drawing chars on my keyboard layout.