Exercise 5-10: expr: a reverse Polish expression from the command line

Write the program expr, which evaluates a reverse Polish expression from the command line, where each operator or operand is a separate argument. For example,

expr 2 3 4 + *

evaluates 2 × (3+4).

Handle +, -, * and /; an operand is a number such as 2, -1.5 or .5. Print the value with printf("%.8g\n", ...). Print error instead if the expression is invalid: an operator without two operands, an argument that's neither an operator nor a number, division by zero, or anything other than exactly one value left at the end. The stack never needs more than 100 values.