Character / int equality evaluating incorrectly - c++

Question

Background info: I'm writing a C++ program to solve sudoku puzzles, and I've hit a major road block. The general flow of the program is like this:
Iterate through the grid and check if a 0 can be replaced with a number. (blanks are represented by 0s)
Check in 3 dimensions (vertical, horizontal, and in the box) which numbers are already there.
Determine if we can narrow that number down to one possibility, replace it, then move on.
Repeat until grid has no zeros
I'm running into issues at the second step, I'll post the whole program at the bottom, but only relevant code here.
int* check_v(string g, size_t x, size_t y){
int *ans = new int[9]; //array of possible ints
memset(ans, 0, sizeof(ans)); //set array to all 0s
size_t size = 0;
for(int i = 1; i < 10; i++){ //iterate through 1-10
//check if i is in the col, if it isn't then add i to ans
bool placeable = true;
for(size_t j = 0; j < 9; j++){ //iterate through ints in the col
size_t r = (j + x) % 9; //the string is a 9x9 grid of numbers
cout << get(g,r,y) << " == " << i << " is " << (get(g,r,y) - 0 == i - 0);
//this is my debug statement, because the if below isn't working.
if(get(g,r,y) - 0 == i - 0){ //if i is equal to a num in the grid,
placeable = false;//we know it can't be that number
}
}
if(placeable) ans[size++] = i; //only add i if we didn't find it in the grid
}
return ans;
}
This is one of the methods that checks the column for each number to see what numbers are/aren't there yet.
Here's the relevant get() method:
char get(string g, size_t x, size_t y){
return g.at(x * 9 + y);
}
Where g is a string of numbers 0-9 81 letters long. It's a 9x9 grid, but put into one long string.
So the get(g,r,y) returns a char like '6', and i is an int. I do '6' - 0 to make them both ints, and compare them. However, it's always false! Even when i = 6 and get(g,r,y) = '6'. Am I doing my comparisons wrong? I must have a typo somewhere and I just don't see it. Here's some sample output from that cout call, and I'll post the whole file for context.
//output
0 == 1 is 0
3 == 1 is 0
8 == 1 is 0
2 == 1 is 0
5 == 1 is 0
7 == 1 is 0
4 == 1 is 0
9 == 1 is 0
6 == 1 is 0 //this is all right, there aren't any 1s in the col
0 == 2 is 0
3 == 2 is 0
8 == 2 is 0
2 == 2 is 0 //but this is wrong! why isn't this true?
5 == 2 is 0
7 == 2 is 0
4 == 2 is 0
9 == 2 is 0
6 == 2 is 0
Now here's the entire file to give you the whole picture.
using namespace std;
#include <iostream>
#include <cstring>
void print(string g){
for(size_t i = 0; i < 9; i++){
for(size_t j = 0; j < 9; j++){
cout << g.at(i * 9 + j);
}
cout << endl;
}
}
void set(string & g, size_t x, size_t y, char z){
size_t i = x * 9 + y;
string beg = g.substr(0,i);
string end = g.substr(i+1,g.length());
g = beg + z + end;
}
char get(string g, size_t x, size_t y){
return g.at(x * 9 + y);
}
int* check_v(string g, size_t x, size_t y){
int *ans = new int[9];
memset(ans, 0, sizeof(ans));
size_t size = 0;
for(int i = 1; i < 10; i++){
bool placeable = true;
for(size_t j = 0; j < 9; j++){
size_t r = (j + x) % 9;
cout << get(g,r,y) << " == " << i << " is " << (get(g,r,y) - 0 == i - 0) << endl;
if(get(g,r,y) - 0 == i - 0){
placeable = false;
}
}
if(placeable) ans[size++] = i;
}
return ans;
}
int* check_b(string g, size_t x, size_t y){
int *ans = new int[9];
memset(ans, 0, sizeof(ans));
size_t size = 0;
x = x / 3 * 3;
y = y / 3 * 3;
for(size_t i = 0; i < 3; i++){
bool placeable = true;
for(size_t j = 0; j < 3; j++)
if(get(g,x + i, y + j) == static_cast<char>(i))
placeable = false;
if(placeable) ans[size++] = i;
}
return ans;
}
int* check_h(string g, size_t x, size_t y){
int *ans = new int[9];
memset(ans, 0, sizeof(ans));
size_t size;
for(size_t i = 1; i < 10; i++){
bool placeable = true;
for(size_t j = 0; j < 9; j++){
cout << get(g,x,(j + y) % 9) << " == " << i << endl;
if(get(g,x,(y + j) % 9) == static_cast<char>(i)){
placeable = false;
}
}
if(placeable) ans[size++] = i;
}
return ans;
}
void check(string g, size_t x, size_t y){
int *n_v = check_v(g, x, y);
int *n_h = check_h(g, x, y);
int *n_b = check_b(g, x, y);
int n_y[9] = {0};
int n;
size_t size = 0;
cout << "vert: ";
for (int i = 0; i < 9; i++)
cout << n_v[i];
cout << endl << "hor: ";
for (int i = 0; i < 9; i++)
cout << n_h[i];
cout << endl << "box: ";
for (int i = 0; i < 9; i++)
cout << n_b[i];
cout << endl;
if(n_v[0] == 0 || n_h[0] == 0 || n_b[0] == 0)
cout << "Error, no number works in slot " << x << ", " << y << endl;
else{
if(n_v[1] == 0)
n = n_v[0];
else if(n_h[1] == 0)
n = n_h[0];
else if(n_b[1] == 0)
n = n_b[0];
}
for(size_t i = 0; i < 9; i++){
bool possible = true;
for(size_t j = 0; possible && j < 9; j++){
if(n_h[j] != n_v[i])
possible = false;
}
for(size_t j = 0; possible && j < 9; j++){
if(n_b[j] != n_v[i])
possible = false;
}
if(possible)
n_y[size++] = n_v[i];
}
if(n_y[1] == 0)
n = n_y[0];
if(n != 0){
char c = n;
set(g,x,y,c);
}
}
int main(){
//initializations
size_t dim = 9;
string data = "";
string one_row;
for (size_t r = 0; r < dim ; r = r + 1) {
cin >> one_row;
data += one_row;
}
//start solving
bool cont = true;
while(cont){
cont = false;
for(size_t i = 0; i < data.length(); i ++){
if(data.at(i) == '0'){
cont = true;
cout << "Checking at point " << i / 9 << ", " << i % 9 << endl;
string old = data;
check(data, i / 9, i % 9);
if(old.compare(data) != 0)
print(data);
}
}
}
print(data);
}

Answer 1

This:
if(get(g,r,y) - 0 == i - 0)
Doesn't work. If get returns a char, it's a character from a string representing the CODE of the number. You're subtracting an integer 0 from that, which is not subtracting anything. What you want is either
if(get(g,r,y) - '0' == i)
Or
if(get(g,r,y) == i + '0')
Which assumes ascii (not unicode or something else).
What you're attempting to do is "convert" between an ascii CHARACTER and an integer. You do that conversion to one or the other, not both. The difference between 0 and '0' is that the first is an integer zero, the second is a character of 0 as would be found in a string.

Answer 2

(get(g,r,y) - 0 == i - 0);
This is wrong. Use it instead.
(get(g,r,y) - '0' == i - 0);