simple calc

/* Changes relative to original post marked with comments
 * Original version at http://pastebin.com/95GwtFtP
 */
#include <iostream>
/* don't import whole namespace with 'using namespace std;'
 * just select the parts you need
 */
using std::cin;
using std::cout;
using std::endl;
/* include string header because string is used
 * was previously included indirectly via iostream
 */
#include <string>
using std::string;
/* new dependency. See calc */
#include <cctype>
using std::tolower;

/* got rid of all global variables */

int calc (string op, int x, int y) {
  /* convert string to lower case to simplify string matching */
  for(int i = 0; i < op.length(); ++i)
    op[i] = tolower(op[i]);
  int d = 0;
  if (op == "mult")
    d = x * y;
  else if (op == "div")
    d = x / y;
  else if (op == "add")
    d = x + y;
  else if (op == "sub")
    d = x - y;
  else
    cout << "Error";

  return d;
}

void printanswer(string op, int x, int y) {
  int answer = calc(op, x, y);
  cout << "Answer: " << answer;
  /* add missing whitespaces between values */
  cout << " Parameters: " << op << " " << x << " " << y;
  /* add a line break */
  cout << endl;
}

int main() {
  string op;
  int x, y;
  cout << "What Operation? (Mult, Div, Sub, Add)";
  cin >> op;
  cout << "First Number?";
  cin >> x;
  cout << "Second Number?";
  cin >> y;
  printanswer(op, x, y);
  /* add return statement. Value 0 signals normal termination */
  return 0;
}