title: I need to read all integers, mathematical operators, and characters in a string
body: <p>I have a string which may contain either single integers between <code>0-9</code> or mathematical operators <code>(+, -, *, /)</code>.</p>
  
  <p>Basically, I need to read in all characters / numbers. I am checking if the character is either +,-,* or /. If not, then I know it is either a number or an invalid character. I am using atoi to convert it to an integer. atoi will return 0 in both cases: if the integer is a 0 OR if it was an invalid character.</p>
  
  <p>How else can I make this distinction?</p>