Reading in multidigit command line parameter - argv

Question

I'm learning J and have modified a tutorial into a jconsole script invoked by ./knight.j N to return as output the Knight's tour for an NxN board.
#!/usr/local/bin/j
kmoves=: 3 : 0
t=. (>,{;~i.y) +"1/ _2]\2 1 2 _1 1 2 1 _2 _1 2 _1 _2 _2 1 _2 _1
(*./"1 t e. i.y) <##"1 y#.t
)
ktour=: 3 : 0
M=. >kmoves y
p=. k=. 0
b=. 1 $~ *:y
for. i.<:*:y do.
b=. 0 k}b
p=. p,k=. ((i.<./) +/"1 b{~j{M){j=. ({&b # ]) k{M
end.
assert. ~:p
(,~y)$/:p
)
echo ktour 0".>2}.ARGV
exit''
However, I'm having difficulty in handling ARGV for numbers greater than 9. The script works correctly with single digit input:
$ ./knight.j 8
0 25 14 23 28 49 12 31
15 22 27 50 13 30 63 48
26 1 24 29 62 59 32 11
21 16 51 58 43 56 47 60
2 41 20 55 52 61 10 33
17 38 53 42 57 44 7 46
40 3 36 19 54 5 34 9
37 18 39 4 35 8 45 6
But fails on double digit input:
$ ./knight.j 10
|length error: kmoves
| (*./"1 t e.i.y)<##"1 y #.t
ARGV
┌─────────────────┬──────────┬──┐
│/Users/v64/.bin/j│./knight.j│10│
└─────────────────┴──────────┴──┘
It works if I separate the digits of the parameter into different arguments:
$ ./knight.j 1 0
0 17 96 67 14 19 84 35 12 21
99 64 15 18 97 68 13 20 37 34
16 1 98 95 66 85 36 83 22 11
63 92 65 86 81 94 69 72 33 38
2 87 90 93 76 71 82 39 10 23
91 62 53 78 89 80 75 70 73 32
44 3 88 61 52 77 40 59 24 9
47 50 45 54 79 60 27 74 31 58
4 43 48 51 6 41 56 29 8 25
49 46 5 42 55 28 7 26 57 30
ARGV
┌─────────────────┬──────────┬─┬─┐
│/Users/v64/.bin/j│./knight.j│1│0│
└─────────────────┴──────────┴─┴─┘
I understand conceptually why this works, but I can't figure out how to modify the script to accept "10" as a single argument.

Answer 1

Thanks for the additional information on ARGV.I think the issue is that 0 ". > 2}. ARGV is a list of length 1 when '10' is the third box and an atom with shape empty when '9' is in the third box.
ARGV=: '/Users/v64/.bin/j';'./knight.j';'10'
ARGV
┌─────────────────┬──────────┬──┐
│/Users/v64/.bin/j│./knight.j│10│
└─────────────────┴──────────┴──┘
$ 0 ".>2}. ARGV NB. 1 item list
1
0 ".>2}. ARGV
10
ARGV=: '/Users/v64/.bin/j';'./knight.j';'9'
$ 0 ".>2}. ARGV NB. atom with empty shape
0 ".>2}. ARGV
9
You can change the shape of the '10' result by using {. on the length 1 list to make it an atom and I think you will find that your verb now works for double digits.
ARGV=: '/Users/v64/.bin/j';'./knight.j';'10'
ARGV
┌─────────────────┬──────────┬──┐
│/Users/v64/.bin/j│./knight.j│10│
└─────────────────┴──────────┴──┘
$ {. 0 ".>2}. ARGV NB. Atom with empty shape
{. 0 ".>2}. ARGV
10
I don't imagine this was the reason that you expected, but it does happen from time to time that results that look like atoms are actually 1 item lists which can result in length errors.
Hope this helps.