8. Simple Calculator
Write a program that will evaluate simple expressions such as 17 + 3 and 3.14159 * 4.7. The expressions are to be typed in by the user. The input always consist of a number, followed by an operator, followed by another number. The operators that are allowed are +, -, *, and /. Your program should read an expression, print its value, read another expression, print its value, and so on. The program should end when the user enters 0 as the first number on the line.