How to compare int with char? - c++

Question

Ok basically, I have a file containing a 9x9 grid (sudoku puzzle). Now this sudoku puzzle is only partially completed, the empty spaces are replaced with "_" (underscores). So here is how the file looks.
5 _ _ _ _ _ 1 7 _
1 _ 6 5 _ 9 _ 4 _
4 7 2 1 _ 6 _ _ _
9 _ _ _ _ _ 5 _ _
_ 1 8 _ 9 5 4 _ _
6 _ _ 4 _ 2 3 8 9
_ 4 _ _ _ _ 9 3 _
_ 9 _ 7 _ 3 _ 5 _
2 6 3 9 5 8 7 1 4
And here is my code:
for(int row=0;row<9;row++)
{
for(int column=0;column<9;column++)
{
fin >> num;
if(num == '\95')
sudokuPuzzle[row][column] = 0;
else
sudokuPuzzle[row][column] = num;
cout << sudokuPuzzle[row][column] << " ";
}
cout << endl;
}
The problem I'm having is on this line:
if(num == '\95')
I looked in an ASCII table for the decimal value of underscore and that was what I found. However, whenever the cout statement in my loop executes, it shows that my array is filled with just the first number in the file, 5. How do I make a proper comparison between integers and characters?
FYI: essentially what I am attempting to do is replace all underscores with 0.

Answer 1

Use
if (num=='_')
It really is that simple.
But I think the problem is in another part of the code - when you read
fin >> num;
You will not get a satisfactory conversion from underscore to int: you need to read things into an intermediate string variable:
fin >> myStringVariable;
Then see whether the character is a number or an underscore, and act on it.
EDIT complete program:
#include <iostream>
#include <fstream>
int main(void)
{
std::ifstream fin ("sudoku.txt");
std::string line;
int pos;
int sudokuPuzzle[9][9];
for(int row=0;row<9;row++)
{
getline(fin, line);
std::cout << "as read in: " << line << std::endl;
while((pos = line.find("_", 0))!=std::string::npos)
{
line[pos] = '0';
}
// show that it worked:
std::cout << "after substitution: " << line << std::endl;
// make a copy of line to process with strtok:
char *dup = strdup(line.c_str());
// and now numerically:
sudokuPuzzle[row][0] = atoi(strtok(dup, " "));
std::cout << "after conversion: " << sudokuPuzzle[row][0] << " ";
for(int column=1;column<9;column++)
{
sudokuPuzzle[row][column] = atoi(strtok(NULL, " "));
std::cout << sudokuPuzzle[row][column] << " ";
}
std::cout << std::endl;
std::cout << std::endl;
free(dup);
}
}
Produces the following output. I think this is self-explanatory.
as read in: 5 _ _ _ _ _ 1 7 _
after substitution: 5 0 0 0 0 0 1 7 0
after conversion: 5 0 0 0 0 0 1 7 0
as read in: 1 _ 6 5 _ 9 _ 4 _
after substitution: 1 0 6 5 0 9 0 4 0
after conversion: 1 0 6 5 0 9 0 4 0
as read in: 4 7 2 1 _ 6 _ _ _
after substitution: 4 7 2 1 0 6 0 0 0
after conversion: 4 7 2 1 0 6 0 0 0
as read in: 9 _ _ _ _ _ 5 _ _
after substitution: 9 0 0 0 0 0 5 0 0
after conversion: 9 0 0 0 0 0 5 0 0
as read in: _ 1 8 _ 9 5 4 _ _
after substitution: 0 1 8 0 9 5 4 0 0
after conversion: 0 1 8 0 9 5 4 0 0
as read in: 6 _ _ 4 _ 2 3 8 9
after substitution: 6 0 0 4 0 2 3 8 9
after conversion: 6 0 0 4 0 2 3 8 9
as read in: _ 4 _ _ _ _ 9 3 _
after substitution: 0 4 0 0 0 0 9 3 0
after conversion: 0 4 0 0 0 0 9 3 0
as read in: _ 9 _ 7 _ 3 _ 5 _
after substitution: 0 9 0 7 0 3 0 5 0
after conversion: 0 9 0 7 0 3 0 5 0
as read in: 2 6 3 9 5 8 7 1 4
after substitution: 2 6 3 9 5 8 7 1 4
after conversion: 2 6 3 9 5 8 7 1 4
BONUS
5 3 9 8 2 4 1 7 6
1 8 6 5 7 9 2 4 3
4 7 2 1 3 6 8 9 5
9 2 4 3 8 7 5 6 1
3 1 8 6 9 5 4 2 7
6 5 7 4 1 2 3 8 9
7 4 5 2 6 1 9 3 8
8 9 1 7 4 3 6 5 2
2 6 3 9 5 8 7 1 4

Answer 2

Subtracting 48 from character will give you numeric equivalent of it.
e.g suppose if you have
char num = '0';
to convert this char '0' into numeric 0 you can use
int num1= num - 48;
You can just define variable 'num' of type 'char' and use following
sudokuPuzzle[row][column] = num - 48;
char num;
for(int row=0;row<9;row++)
{
for(int column=0;column<9;
{
fin >> num;
if(num == '_')
sudokuPuzzle[row][column] = 0;
else
sudokuPuzzle[row][column] = num - 48;
cout << sudokuPuzzle[row][column] << " ";
}
cout << endl;
}

Answer 3

In \nnn, the nnn is in octal. 9 is not an octal digit. I'm pretty sure -Wall would have told you that, but I could be wrong about that.
However, if num is an int, then:
fin >> num;
is unlikely to do what you want, since _ is not an integer. I would think that the streaming input operator would set the error bit on fin in that case.

Answer 4

You say the file looks like this:
5 _ _ _ _ _ 1 7 _
1 _ 6 5 _ 9 _ 4 _
4 7 2 1 _ 6 _ _ _
9 _ _ _ _ _ 5 _ _
_ 1 8 _ 9 5 4 _ _
6 _ _ 4 _ 2 3 8 9
_ 4 _ _ _ _ 9 3 _
_ 9 _ 7 _ 3 _ 5 _
2 6 3 9 5 8 7 1 4
That means that the first line of the file could be expressed as follows:
char line[] = { '5', '_', '_', '_', '_', '_', '1', '7', '_' };
Although this is a char array, it's not a string because I didn't add the trailing nul.
What does this array contain?
The first char, line[0] represents the ASCII character '5'. That's not the same as the integer value, 5, it is the ASCII code for '5'. Looking at an ascii table we can see that '5' is ASCII character 53.
So, line[0], if printed as an ASCII character, will display '5', but if printed as an integer value will display '53'.
#include <iostream>
int main() {
char fourtyTwoPlus11 = 53;
std::cout << fourtyTwoPlus11 << "\n";
std::cout << static_cast<int>(fourtyTwoPlus11) << "\n";
return 0;
}
Live demo: http://ideone.com/JEUcLO
Your file appears to already contain ASCII characters rather than integer values. So for each character, you actually need to convert to numeric values. You could do something like the following:
enum { NumColumns = 9, UnknownValue = -1 };
char line[NumColumns] = { '5', '_', '_', '_', '_', '_', '1', '7', '_' };
int values[NumColumns];
for (size_t i = 0; i < NumColumns; ++i) {
char col = line[i];
if (col == '_')
values[i] = UnknownValue;
else
values[i] = col - '0';
}
The integer values are in the range 0-9, which we obtain using our knowledge that the ASCII table stores digits in the sequence 0, 1, 2 ... 9. So '0' - '0' == 0, and '9' - '0' = 9.
I chose to use -1 to represent the Unknown Value but you could just as easily use 0 here.

Answer 5

For this kind of situation (get an input of either a char or an int), I usually use this trick:
char c;
int n;
cin >> c;
if(c != '_'){
cin.putback(c);
cin >> n;
}
fin is similar.