Polynomial Evaluator

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#!/usr/bin/python

#
# This program evaluates polynomials.  It first asks for the coefficients
# of a polynomial, which must be entered on one line, highest-order first.
# It then requests values of x and will compute the value of the poly for
# each x.  It will repeatly ask for x values, unless you the user enters
# a blank line.  It that case, it will ask for another polynomial.  If the
# user types quit for either input, the program immediately exits.
#

# Need some string services, and some standard system services.
import string, sys

#
# Function to evaluate a polynomial at x.  The polynomial is given
# as a list of coefficients, from the greatest to the least.
def polyval(x, coef):
    '''Evaluate at x the polynomial with coefficients given in coef.
    The value p(x) is returned.'''

    sum = 0
    while 1:
        sum = sum + coef[0]     # Add the next coef.
        coef = coef[1:]         # Done with that one.
        if not coef: break      # If no more, done entirely.
        sum = sum * x           # Mult by x (each coef gets x right num times)

    return sum

#
# Function to read a line containing a list of integers and return
# them as a list of integers.  If the string conversion fails, it
# returns the empty list.  If the input line is the word 'quit', then
# it exits the program (which is actually accomplished with an exception).
# Other exceptions are passed on.
def readints(prompt):
    '''Read a line of integers and return the list of integers.'''

    # Read a line
    line = raw_input(prompt)
    if line == 'quit': sys.exit(0)
            
    # Go through each item on the line, converting each one and adding it
    # to retval.
    retval = [ ];
    for str in string.split(line):
        try:
            retval.append(int(str))
        except ValueError:
            # On conversion failure, whine and return empty list.
            print 'Conversion of', str, 'failed.'
            return []

    return retval

#
# Create a string of the polynomial in sort-of-readable form.
def polystr(p):
    # Get the exponent of first coefficient, plus 1.
    exp = len(p)

    # Go through the coefs and turn them into terms.
    retval = ''
    while p:
        # Adjust exponent.  Done here so continue will run it.
        exp = exp - 1

        # Strip first coefficient
        coef = p[0]
        p = p[1:]

        # If zero, leave it out.
        if coef == 0: continue

        # If adding, need a + or -.
        if retval:
            if coef >= 0:
                retval = retval + ' + '
            else:
                coef = -coef
                retval = retval + ' - '

        # Add the coefficient, if needed.
        if coef != 1 or exp == 0:
            retval = retval + str(coef)
            if exp != 0: retval = retval + '*'

        # Put the x, if we need it.
        if exp != 0:
            retval = retval + 'x'
            if exp != 1: retval = retval + '^' + str(exp)

    # For zero, say that.
    if not retval: retval = '0'

    return retval
        
#
# Run until some kind of endfile.
try:
    # Repeat until an exception or quit gets us out.
    while 1:
        # Read a poly until it works.  An EOF will except out of the
        # program.
        while 1:
            poly = readints('Enter a polynomial coefficients: ')
            if poly: break
            print 'Try again.'

        # Read and evaluate x values until the user types a blank line.
        # Again, and EOF will except out of the pgm.
        while 1:
            resp = raw_input('Enter x value or blank line: ')
            if resp == 'quit': sys.exit(0)
            if not resp: break
            try:
                x = int(resp)
            except ValueError:
                print "That doesn't look like an integer.  Please try again."
            else:
                print 'p(x) =', polystr(poly)
                print 'p(' + str(x) + ') =', polyval(x, poly)

except (EOFError, KeyboardInterrupt):
    # Exit without error for EOF or ^C.  Print a blank line to clear after
    # any prompt.
    print