Recursive Backtracking Sudoku Solver Problems, c++ - c++

Question

It's my first time dealing with recursion as an assignment in a low level course. I've looked around the internet and I can't seem to find anybody using a method similar to the one I've come up with (which probably says something about why this isn't working). The error is a segmentation fault in std::__copy_move... which I'm assuming is something in the c++ STL.
Anywho, my code is as follows:
bool sudoku::valid(int x, int y, int value)
{
if (x < 0) {cerr << "No valid values exist./n";}
if (binary_search(row(x).begin(), row(x).end(), value))
{return false;} //if found in row x, exit, otherwise:
else if (binary_search(col(y).begin(), col(y).end(), value))
{return false;} //if found in col y, exit, otherwise:
else if (binary_search(box((x/3), (y/3)).begin(), box((x/3), (y/3)).end(), value))
{return false;} //if found in box x,y, exit, otherwise:
else
{return true;} //the value is valid at this index
}
int sudoku::setval(int x, int y, int val)
{
if (y < 0 && x > 0) {x--; y = 9;} //if y gets decremented past 0 go to previous row.
if (y > 8) {y %= 9; x++;} //if y get incremented past 8 go to next row.
if (x == 9) {return 0;} //base case, puzzle done.
else {
if (valid(x,y,val)){ //if the input is valid
matrix[x][y] = val; //set the element equal to val
setval(x,y++,val); //go to next element
}
else {
setval(x,y,val++); //otherwise increment val
if(val > 9) {val = value(x,y--); setval(x,y--,val++); }
} //if val gets above 9, set val to prev element,
} //and increment the last element until valid and start over
}
I've been trying to wrap my head around this thing for a while and I can't seem to figure out what's going wrong. Any suggestions are highly appreciated! :)

Answer 1

sudoku::setval is supposed to return an int but there are at least two paths where it returns nothing at all. You should figure out what it needs to return in those other paths because otherwise you'll be getting random undefined behavior.

Answer 2

Without more information, it's impossible to tell. Things like the data
structures involved, and what row and col return, for example.
Still, there are a number of obvious problems:
In sudoku::valid, you check for what is apparently an error
condition (x < 0), but you don't return; you still continue your
tests, using the negative value of x.
Also in sudoku:valid: do row and col really return references to
sorted values? If the values aren't sorted, then binary_search will
have undefined behavior (and if they are, the names are somewhat
misleading). And if they return values (copies of something), rather
than a reference to the same object, then the begin() and end()
functions will refer to different objects—again, undefined
behavior.
Finally, I don't see any backtracking in your algorithm, and I don't
see how it progresses to a solution.
FWIW: when I wrote something similar, I used a simple array of 81
elements for the board, then created static arrays which mapped the
index (0–80) to the appropriate row, column and box. And for each of
the nine rows, columns and boxes, I kept a set of used values (a
bitmap); this made checking for legality very trivial, and it meant that
I could increment to the next square to test just by incrementing the
index. The resulting code was extremely simple.
Independently of the data representation used, you'll need: some
"global" (probably a member of sudoku) means of knowing whether you've
found the solution or not; a loop somewhere trying each of the nine
possible values for a square (stopping when the solution has been
found), and the recursion. If you're not using a simple array for the
board, as I did, I'd suggest a class or a struct for the index, with a
function which takes care of the incrementation once and for all.

Answer 3

All of the following is for Unix not Windows.
std::__copy_move... is STL alright. But STL doesn't do anything by itself, some function call from your code would've invoked it with wrong arguments or in wrong state. You need to figure that out.
If you have a core dump from teh seg-fault then just do a pstack <core file name>, you will see the full call stack of the crash. Then just see which part of your code was involved in it and start debugging (add traces/couts/...) from there.
Usually you'll get this core file with nice readable names, but in case you don't you can use nm or c++filt etc to dismangle the names.
Finally, pstack is just a small cmd line utility, you can always load the binary (that produced the core) and the core file into a debugger like gdb, Sun Studio or debugger built into your IDE and see the same thing along with lots of other info and options.
HTH

Answer 4

It seems like your algorithm is a bit "brute forcy". This is generally not a good tactic with Constraint Satisfaction Problems (CSPs). I wrote a sudoku solver a while back (wish I still had the source code, it was before I discovered github) and the fastest algorithm that I could find was Simulated Annealing:
http://en.wikipedia.org/wiki/Simulated_annealing
It's probabilistic, but it was generally orders of magnitude faster than other methods for this problem IIRC.
HTH!

Answer 5

segmentation fault may (and will) happen if you enter a function recursively too many times.
I noted one scenario which lead to it. But I'm pretty sure there are more.
Tip: write in your words the purpose of any function - if it is too complicated to write - the function should probably be split...