```
; simple sudoku solver based on the Python example from computerphile:
; https://www.youtube.com/watch?v=G_UYXzGuqvM&list=WL&index=228
grid: [
5 3 0 0 7 0 0 0 0
6 0 0 1 9 5 0 0 0
0 9 8 0 0 0 0 6 0
8 0 0 0 6 0 0 0 3
4 0 0 8 0 3 0 0 1
7 0 0 0 2 0 0 0 6
0 6 0 0 0 0 2 8 0
0 0 0 4 1 9 0 0 5
0 0 0 0 8 0 0 7 9
]
print-grid: func [<with> grid <local> output][
output: copy ""
count-up i 81 [
append output grid/:i
either i mod 9 = 0 [
append output newline
if all [ i mod 27 = 0
i < 81 ][
append output "------+-------+------^/"
]
][
append output either any [ i mod 9 = 3
i mod 9 = 6 ][ " | " ][ " " ]
]
]
print output
]
possible: func [
y
x
n
<with> grid
<local> x0 y0
][
count-up i 9 [
if n = grid/(9 * (y - 1) + i) [
return false
]
]
count-up i 9 [
if n = grid/(9 * (i - 1) + x) [
return false
]
]
x0: ((to integer! (x - 1) / 3)) * 3 + 1
y0: ((to integer! (y - 1) / 3)) * 3 + 1
count-up i 3 [
count-up j 3 [
if n = grid/(9 * (y0 + (i - 1) - 1) + (x0 + (j - 1))) [
return false
]
]
]
return true
]
solve: func [<with> grid][
count-up y 9 [
count-up x 9 [
if 0 = grid/(9 * (y - 1) + x) [
count-up n 9 [
if possible y x n [
grid/(9 * (y - 1) + x): n
solve
grid/(9 * (y - 1) + x): 0 ; backtracking
]
] return _
]
]
]
print-grid
]
```

The result of running it:

```
>> solve
5 3 4 | 6 7 8 | 9 1 2
6 7 2 | 1 9 5 | 3 4 8
1 9 8 | 3 4 2 | 5 6 7
------+-------+------
8 5 9 | 7 6 1 | 4 2 3
4 2 6 | 8 5 3 | 7 9 1
7 1 3 | 9 2 4 | 8 5 6
------+-------+------
9 6 1 | 5 3 7 | 2 8 4
2 8 7 | 4 1 9 | 6 3 5
3 4 5 | 2 8 6 | 1 7 9
== _
```