C++ trying multiple conditions in if statement with loop - c++

Question

I'm currently trying to create a sudoku solver in C++, but I have run into a slight problem. Instead of starting out with a 9x9 sudoku, I've started with a 4x4 one. For every square that can contain a number I am trying to get the possible numbers that square can hold into an array, e.g. if the possibilities in square 3 are 1 and 4,
for ( int i; i < 4; i++ ) {
cout << candidates[3][i];
}
would give me 1004. It is easily accessible when I need to code hard solving methods or a brute force solving method.
Here is the problem: I need to check whether a number occurs in any row, column or (2x2 or 3x3) box, and if it does not, add it to the candidates array. Here's what I thought of:
for ( int i = 0; i < 16; i++ ) {
for ( int j = 1; j < 5; j++ ) {
if ( for ( int k = 0; k < 4; k++ ) { allRows[getRow(i)][k] != j || allCols[getCol(i)] [k] != j || allBoxs[getBox(i)][k] != j}) {
candidates[i][j-1] = j;
}
}
}
The arrays allRows, allCols, allBoxs all contain the sudoku, only in such an order that they are easily accessible when they are needed. The functions getRow(), getCol() and getBox() return the number of the row, column or box (respectively, of course) the input number is in.
I tried to make it loop though everything in the same row, column or box as the number, to get all twelve checked by the if, and if they all passed, then I would add the number to the candidates array.
It came to me as no surprise that it returned an error when I tried to compile this. But I really don't want to write out twelve AND conditions in the if statement, or 27 when I upgrade to 9x9 sudoku, because:
I don't learn anything and
Where's the fun in that?
Can anyone help me / suggest another idea to do this without typing everything out?
I apologize if my inability to be concise hinders an answer to this question, I was only trying to provide all the details I thought maybe were necessary...

Answer 1

Condition of if should be an expression convertible to bool. it is not the case of for-statement.
You might do instead (assuming I correctly understood your condition):
for ( int i = 0; i < 16; i++ ) {
for ( int j = 1; j < 5; j++ ) {
bool b = false;
for ( int k = 0; k < 4; k++ ) {
if (allRows[getRow(i)][k] != j || allCols[getCol(i)][k] != j || allBoxs[getBox(i)][k] != j) {
b = true;
break;
}
}
if (b) {
candidates[i][j-1] = j;
}
}
}
or with helper function
template <typename Range, typename T>
bool contains(const Range& range, constT& value)
{
using std::begin;
using std::end;
return std::find(begin(range), end(range), value) != end(range);
}
// ...
for ( int i = 0; i < 16; i++ ) {
for ( int j = 1; j < 5; j++ ) {
if (!contains(allRows[getRow(i)], j)
|| !contains(allCols[getCol(i)], j)
|| !contains(allBoxs[getBox(i)], j)) {
candidates[i][j-1] = j;
}
}
}

Answer 2

Like you already noticed, adding an for-statement inside of the condition of the if doesn't work. There are several reasons for it, however, let me point out a few that cause confusion:
What if the for-loop wouldn't iterate? Would this result in true or false?
What if you have multiple iterations? Would we && or || the boolean values into it? And how about short cutting of those operators?
Long story short: Even if this would be considered to ever add to the standard, I expect it to be blocked thanks to the many edge cases.
So what does this mean? Is this impossible? Off course not, it's C++. Like already mentioned in the answer of #Jarod42 you can calculate a boolean and than do the if-statement. Or like both of us mentioned (me in the comments), you could move this into a function.
Personally, I've stepped away from remembering state about my for-loop like with this boolean. The main reason: This is hard to read. Every single time you read that code, you need to check what's actually happening.
Hence, I suggest to move this to a function. A function has a clear name and could even be tested separately to ensure it's behavior is correct.
So in this case, I would write something like:
template<typename T1, typename T2, typename T3>
auto checkForExistanceOf(int shouldExist, int locator, T1 &&allRows, T2 &&allCols, T3 &&allBoxs)
{
for ( int k = 0; k < 4; k++ )
{
if (allRows[getRow(locator)][k] != shouldExist)
return true;
if (allCols[getCol(locator)] [k] != shouldExist)
return true;
if (allBoxs[getBox(locator)][k] != shouldExist)
return true;
}
return false;
}
The code on caller side than becomes:
for ( int i = 0; i < 16; i++ )
{
for ( int j = 1; j < 5; j++ )
{
if (checkForExistanceOf(j, i, allRows, allCols, allBoxs))
{
candidates[i][j-1] = j;
}
}
}
For sure, my names ain't that good, as I don't know the exact purpose, nor would I suggest using a template as you know the actual types, though, this is the idea behind it.