[c++] 'was not declared in this scope' error

So I was writing this simple program to calculate the day of any date using the Gaussian algorithm found here.

#include <iostream>
using namespace std;

//Using the Gaussian algorithm
int dayofweek(int date, int month, int year ){
    int d=date;
    if (month==1||month==2)
        {int y=((year-1)%100);int c=(year-1)/100;}
    else
        {int y=year%100;int c=year/100;}
    int m=(month+9)%12+1;
    int product=(d+(2.6*m-0.2)+y+y/4+c/4-2*c);
    return product%7;
}

int main(){
    cout<<dayofweek(19,1,2054);
    return 0;
}

It's a very simple program and what's even more puzzling is the output.

:In function  dayofweek(int, int, int)’:
:19: warning:  unused variable ‘y’
:19: warning: unused variable ‘c’
:21: warning: unused variable ‘y’
:21: warning: unused variable ‘c’
:23: error: ‘y’ was not declared in this scope
:25: error: ‘c’ was not declared in this scope

It says that my variable is unused but then says that it isn't declared? Could anyone please tell me whats wrong.

#include <iostream>
using namespace std;
class matrix
{
    int a[10][10],b[10][10],c[10][10],x,y,i,j;
    public :
        void degerler();
        void ters();
};
void matrix::degerler()
{
    cout << "Satirlari giriniz: "; cin >> x;
    cout << "Sütunlari giriniz: "; cin >> y;
    cout << "Ilk matris elamanlarini giriniz:\n\n";
    for (i=1; i<=x; i++)
    {
        for (j=1; j<=y; j++)
        {
            cin >> a[i][j];
        }
    }
    cout << "Ikinci matris elamanlarini giriniz:\n\n";
    for (i=1; i<=x; i++)
    {
        for (j=1; j<=y; j++)
        {
            cin >> b[i][j];
        }
    }
}

void matrix::ters()
{
    cout << "matrisin tersi\n";
    for (i=1; i<=x; i++)
    {
        for (j=1; j<=y; j++)
        {
    if(i==j)
    {
    b[i][j]=1;
    }
    else
    b[i][j]=0;
    }
}
float d,k;
    for (i=1; i<=x; i++)
    {
    d=a[i][j];
        for (j=1; j<=y; j++)
        {
    a[i][j]=a[i][j]/d;
            b[i][j]=b[i][j]/d;
    }
        for (int h=0; h<x; h++)
        {
            if(h!=i)
    {
       k=a[h][j];
               for (j=1; j<=y; j++)
               {
                    a[h][j]=a[h][j]-(a[i][j]*k);
                    b[h][j]=b[h][j]-(b[i][j]*k);
               }
    }
    count << a[i][j] << "";
    }
    count << endl;
}  
}
int main()
{
    int secim;
    char ch;    
    matrix m;
    m.degerler();
    do
     {
    cout << "seçiminizi giriniz\n";
    cout << " 1. matrisin tersi\n";
    cin >> secim;
    switch (secim)
    {
        case 1:
            m.ters();
            break;
    }
    cout << "\nBaska bir sey yap/n?";
    cin >> ch;
    }
    while (ch!= 'n');
    cout << "\n";
    return 0;
}

Here

{int y=((year-1)%100);int c=(year-1)/100;}

you declare and initialize the variables y, c, but you don't used them at all before they run out of scope. That's why you get the unused message.

Later in the function, y, c are undeclared, because the declarations you made only hold inside the block they were made in (the block between the braces {...}).

You need to declare y and c outside the scope of the if/else statement. A variable is only valid inside the scope it is declared (and a scope is marked with { })

#include <iostream> 
using namespace std; 
//Using the Gaussian algorithm 
int dayofweek(int date, int month, int year ){ 
int d=date; 
int y, c;
if (month==1||month==2) 
        {y=((year-1)%100);c=(year-1)/100;} 
else 
        {y=year%100;c=year/100;} 
int m=(month+9)%12+1; 
int product=(d+(2.6*m-0.2)+y+y/4+c/4-2*c); 
return product%7; 
} 

int main(){ 
cout<<dayofweek(19,1,2054); 
return 0; 
} 

Here's a simplified example based on of your problem:

if (test) 
{//begin scope 1
    int y = 1; 
}//end scope 1
else 
{//begin scope 2
    int y = 2;//error, y is not in scope
}//end scope 2
int x = y;//error, y is not in scope

In the above version you have a variable called y that is confined to scope 1, and another different variable called y that is confined to scope 2. You then try to refer to a variable named y after the end of the if, and not such variable y can be seen because no such variable exists in that scope.

You solve the problem by placing y in the outermost scope which contains all references to it:

int y;
if (test) 
{
    y = 1; 
}
else 
{
    y = 2;
}
int x = y;

I've written the example with simplified made up code to make it clearer for you to understand the issue. You should now be able to apply the principle to your code.