CalcEntryDemo

# Simple Calculator program to demonstrate use of functions for sanitized user input
# Written on 04/18/2014 by Darryl Medley for Coding 101 on the TWiT network

# Function that only accepts and returns a floating-point number. Optionally prints
# a built in error message for bad values if the showErrMsg parameter is True
def getFloat(prompt, showErrMsg):
    while True:
        try:
            return float(raw_input(prompt))
        except:
            if showErrMsg:
                print "(invalid number)"
# end function getFloat

# Function that only allows a character that's in the passed string of legal
# characters to be entered. Accepts a blank / null entry if nullOk is True
def getChar(prompt, legalValues, nullOk):
    while True:
        ch = raw_input(prompt).strip()   # get entry and remove any leading and trailing whitespace
        if ch == "":
            if nullOk:
                return ch   # return empty string
        else:   # Something was entered.
            ch = ch[0]   # [0] grabs just the first character in case a complete word is entered
            if ch in legalValues:   # test to see if the character is valid
                return ch
# end function getChar


print "Simple Calculator"

# Main loop
# Since our functions are so powerful, the main routine is really simple
while True:
    print
    fnbr1 = getFloat("Enter operand 1: ", True)
    oprnd = getChar("Enter the operator (+,-,*,/): ","+-*/", False)
    fnbr2 = getFloat("Enter operand 2: ", True)

    if (oprnd == "/") and (fnbr2 == 0.0):
        print "Division by Zero in undefined"
    else:
        # Calculate and print the result. Here we cheat and use Python's
        # eval() function that interprets a string as an expression.
        print fnbr1, oprnd, fnbr2, "=", eval(str(fnbr1)+" "+oprnd+" "+str(fnbr2))

    optn = getChar("Calculate another value (Y/No)?","YyNn",True)
    if (optn == "") or (optn not in "Yy"):
        break   # exit program if not Yes (N, n or nothing entered)

# end main loop