Advanced Polish Notation File Calc

# Advanced Polish Notation file calculation program
# Written on 04/28/2014 by Darryl Medley for Coding 101 on the TWiT network
# This advanced version calculates expressions of as many terms as memory allows

# This program reads a text file called pnequation_list.txt that is in the same folder
# as the .py program file. The file must contain one expression per line and be in
# Polish (aka prefix) notation. Example: +, 12.5, 24.3
# See end of the program for a sample file


# Polish Notation Calculation function - based on the sample code in Wikipedia
# Parameters:
#   termsList = list containing the expression to evaluate, 1 term per list element
#   pncErrMsg = error message list. Pass in an empty list with the same name
# returns the result if calc is successful, 0 otherwise
def PNCalc(termsList, pncErrMsg):
    oprnd1 = 0.0
    oprnd2 = 0.0
    stk = []   # Stack list

    # evaluate in reverse order so we can use a simpler code
    for oper in reversed(termsList):
        if oper in "+-*/%":
            # if an operator is found the stack should have a least 2 operands to calc
            if len(stk) < 2:
                pncErrMsg.append("Invalid Polish Notation Expression")
                return 0

            try:
                oprnd1 = float(stk.pop())   # pop operands off of the stack
                oprnd2 = float(stk.pop())
                if (oper in "/%") and (oprnd2 == 0.0):
                    pncErrMsg.append("Division by Zero")
                    return 0
                else:
                    # calculate and put result on the stack
                    stk.append(eval(str(oprnd1)+" "+oper+" "+str(oprnd2)))
            except:
                pncErrMsg.append("Invalid Polish Notation Expression")
                return 0

        else:   # must be an operand (number)
            stk.append(oper)   # put on the stack

    return stk.pop()   # the final result is the only thing left on the stack


# Begin main program
print "Advanced Polish (prefix) Notation Equation File Processor"
fname = "./pnequation_list.txt"
try:
    eqlstFile = open(fname, "r")  # open the text file
except:
    print "Error: File",fname,"does not exist"
else:
    # The first thing we do is read the file into a Python list. Each line in
    # the file becomes a string element in the list. .splitlines() removes the
    # \n (newline) character from the end of each line.
    eqList = eqlstFile.read().splitlines()   # read file into list and strip newline characters
    eqlstFile.close()   # We're done with the file now that the lines are in a list

    # Process each equation in the list
    for eqLine in eqList:
        if eqLine.strip():   # skip blank lines - True if eqLine is not empty
            # Parse the equation string into operator and operands by creating a little list using split()
            opList = [s.strip() for s in eqLine.split(',')] # strip blanks and parse sting into a list

            pncErrMsg = []
            calcResult = PNCalc(opList, pncErrMsg)

            if not pncErrMsg:   # pncErrMsg is empty. No errors were found with the equation line
                print eqLine,"=",calcResult
            else:
                print eqLine,"= Error:",pncErrMsg[0]

print
raw_input("(press Enter to close)")

# Sample Test File (save as pnequation_list.txt):
# +, 12.5, 24.3
# x, 1, 2
# -, 23, zz
# +, 1
# *, 7, 19
#
# /, 33.0, 5.0
# /, 33, 0
# %, 5, 3
# -, *, /, 15, -, 7, +, 1, 1, 3, +, 2, +, 1, 1