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Ruby 1.9, 108 characters

f=->z{l=[*[r=1]*17,2]*17;c=152;z.times{c+=r=(r*r>1?r/18:-r*18)*l[c]*=-1};l[c]=0;puts l.map{|a|"@_
#"[a]}*""}

Input via function argument.

• (146 -> 142) Inlined m
• (142 -> 140) Check for r*r>1 instead of r.abs>1
• (142 -> 128) Use String#scan to generate the output. Changed a == to >
• (128 -> 125) Removed obsolete variable
• (125 -> 122) Replace String#tr with a conditional
• (122 -> 122) Now generates the same output as the updated examples
• (122 -> 111) Use ints instead of chars when generating the ant's path.
• (111 -> 109) Reorder some expressions to save parentheses
• (109 -> 108) Code is now a function

Ruby 1.9, 104 characters

f=->z{l=[*[r=1]*17,2]*17;c=152;z.times{c+=r=(r*r>1?r/18:-r*18)*l[c]*=-1};l[c]=0;l.map{|a|putc"@_
#"[a]}}

Input via function argument.

• (146 -> 142) Inlined m
• (142 -> 140) Check for r*r>1 instead of r.abs>1
• (142 -> 128) Use String#scan to generate the output. Changed a == to >
• (128 -> 125) Removed obsolete variable
• (125 -> 122) Replace String#tr with a conditional
• (122 -> 122) Now generates the same output as the updated examples
• (122 -> 111) Use ints instead of chars when generating the ant's path.
• (111 -> 109) Reorder some expressions to save parentheses
• (109 -> 108) Code is now a function
• (108 -> 104) Print every character individually

Ruby 1.9, 109 characters

l=[*[r=1]*17,2]*17;c=152;gets.to_i.times{c+=r=(r*r>1?r/18:-r*18)*l[c]*=-1};l[c]=0;puts l.map{|a|"@_
#"[a]}*""

Input via stdin. A function would be 6 characters shorter (103):

f=->z{l=[*[r=1]*17,2]*17;c=152;z.times{c+=r=(r*r>1?r/18:-r*18)*l[c]*=-1};l[c]=0;l.map{|a|"@_
#"[a]}*""}

• (146 -> 142) Inlined m
• (142 -> 140) Check for r*r>1 instead of r.abs>1
• (142 -> 128) Use String#scan to generate the output. Changed a == to >
• (128 -> 125) Removed obsolete variable
• (125 -> 122) Replace String#tr with a conditional
• (122 -> 122) Now generates the same output as the updated examples
• (122 -> 111) Use ints instead of chars when generating the ant's path.
• (111 -> 109) Reorder some expressions to save parentheses

Ruby 1.9, 108 characters

f=->z{l=[*[r=1]*17,2]*17;c=152;z.times{c+=r=(r*r>1?r/18:-r*18)*l[c]*=-1};l[c]=0;puts l.map{|a|"@_
#"[a]}*""}

Input via function argument.

• (146 -> 142) Inlined m
• (142 -> 140) Check for r*r>1 instead of r.abs>1
• (142 -> 128) Use String#scan to generate the output. Changed a == to >
• (128 -> 125) Removed obsolete variable
• (125 -> 122) Replace String#tr with a conditional
• (122 -> 122) Now generates the same output as the updated examples
• (122 -> 111) Use ints instead of chars when generating the ant's path.
• (111 -> 109) Reorder some expressions to save parentheses
• (109 -> 108) Code is now a function

Ruby 1.9, 109 characters

l=[*[r=1]*17,2]*17;c=152;gets.to_i.times{c+=r=(r*r>1?r/18:-r*18)*l[c]*=-1};l[c]=0;puts l.map{|a|"@_
#"[a]}*""

Input via stdin. A function would be 1 character shorter:

f=->a{l=[*[r=1]*17,2]*17;c=152;gets.to_i.times{c+=r=(r*r>1?r/18:-r*18)*l[c]*=-1};l[c]=0;puts l.map{|a|"@_
#"[a]}*""}

• (146 -> 142) Inlined m
• (142 -> 140) Check for r*r>1 instead of r.abs>1
• (142 -> 128) Use String#scan to generate the output. Changed a == to >
• (128 -> 125) Removed obsolete variable
• (125 -> 122) Replace String#tr with a conditional
• (122 -> 122) Now generates the same output as the updated examples
• (122 -> 111) Use ints instead of chars when generating the ant's path.
• (111 -> 109) Reorder some expressions to save parentheses

Ruby 1.9, 109 characters

l=[*[r=1]*17,2]*17;c=152;gets.to_i.times{c+=r=(r*r>1?r/18:-r*18)*l[c]*=-1};l[c]=0;puts l.map{|a|"@_
#"[a]}*""

Input via stdin. A function would be 6 characters shorter (103):

f=->z{l=[*[r=1]*17,2]*17;c=152;z.times{c+=r=(r*r>1?r/18:-r*18)*l[c]*=-1};l[c]=0;l.map{|a|"@_
#"[a]}*""}

• (146 -> 142) Inlined m
• (142 -> 140) Check for r*r>1 instead of r.abs>1
• (142 -> 128) Use String#scan to generate the output. Changed a == to >
• (128 -> 125) Removed obsolete variable
• (125 -> 122) Replace String#tr with a conditional
• (122 -> 122) Now generates the same output as the updated examples
• (122 -> 111) Use ints instead of chars when generating the ant's path.
• (111 -> 109) Reorder some expressions to save parentheses