2 deleted 88 characters in body
source | link

C# (10194 characters)

string I(string i){int s=0,j=0,f;forj=0;for(;j<12;)s+=(i[j]-48)*(++j%2>0j++%2<1?3:1:3);f=10-s%10;return;return i+(f>9?0:f(10-s%10)%10);}

With linebreaks/whitespace for readability:

string I(string i) 
        { 
            int s = 0, j = 0, f;0;
            for (; j < 12; )
                s += (i[j] - 48) * (++jj++ % 2 >< 01 ? 31 : 13); 
            f = 10 - s % 10;
            return i + (f(10 >- 9s ?% 010) :% f10); 
        }

Tested on several ISBNs from books on my shelf, so I know it's working!

C# (101 characters)

string I(string i){int s=0,j=0,f;for(;j<12;)s+=(i[j]-48)*(++j%2>0?3:1);f=10-s%10;return i+(f>9?0:f);}

With linebreaks/whitespace for readability:

string I(string i) 
        { 
            int s = 0, j = 0, f;
            for (; j < 12; )
                s += (i[j] - 48) * (++j % 2 > 0 ? 3 : 1); 
            f = 10 - s % 10;
            return i + (f > 9 ? 0 : f); 
        }

Tested on several ISBNs from books on my shelf, so I know it's working!

C# (94 characters)

string I(string i){int s=0,j=0;for(;j<12;)s+=(i[j]-48)*(j++%2<1?1:3);return i+((10-s%10)%10);}

With linebreaks/whitespace for readability:

string I(string i) 
{ 
    int s = 0, j = 0;
    for (; j < 12; )
        s += (i[j] - 48) * (j++ % 2 < 1 ? 1 : 3); 
    return i + ((10 - s % 10) % 10); 
}

Tested on several ISBNs from books on my shelf, so I know it's working!

1
source | link

C# (101 characters)

string I(string i){int s=0,j=0,f;for(;j<12;)s+=(i[j]-48)*(++j%2>0?3:1);f=10-s%10;return i+(f>9?0:f);}

With linebreaks/whitespace for readability:

string I(string i) 
        { 
            int s = 0, j = 0, f;
            for (; j < 12; )
                s += (i[j] - 48) * (++j % 2 > 0 ? 3 : 1); 
            f = 10 - s % 10;
            return i + (f > 9 ? 0 : f); 
        }

Tested on several ISBNs from books on my shelf, so I know it's working!