Why is it bad to put stuff in global scope

[EwCjE3v7]

Hi there I was just wondering why is it bad to declare variables in global scope like it can make code shorter for ex

Int a = 0, b = 0;

int add()
{
   Cout ......;
   cin >> a >> b;
   Cout .... a + b;
}

int subtract()
{
   Cout ......;
   cin >> a >> b;
   Cout .... a - b;
}

int devide()
{
   ........
}
int multiply()
{
   ........
}

Int main ()
{
   char choice = 'a'
   Cout .....;
   Cin >> choice;

   If ....
   If ....
   If ....
   If ....
}

Sorry for code, I wrote it on ipad so I couldn't write all. So that saved me a few lines. Why is it bad? And leave the capitals out.

[keskiverto]

http://bytes.com/topic/c/insights/737451-case-against-global-variables

[moorecm]

The article also left out the disaster relating to external dependencies that hurt modularity (making unit testing more difficult, for example).

[EwCjE3v7]

Thanks I'm nearly finished reading it.thanks guys, I got it.