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# Code from http://sandbox.mc.edu/~bennet/ruby/code/poly_rb.html

#

# 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.

#

​

#

# 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)

    sum = 0

    coef = coef.clone           # Don't want to destroy the original

    while true

        sum += coef.shift       # Add and remove the next coef

        break if coef.empty?    # If no more, done entirely.

        sum *= x                # This happens the right number of times.

    end

    return sum

end

​

#

# Function to read a line containing a list of integers and return

# them as an array of integers.  If the string conversion fails, it

# throws TypeError.  If the input line is the word 'quit', then it

# converts it to an end-of-file exception

def readints(prompt)

    # Read a line

    print prompt

    line = readline.chomp

    raise EOFError.new if line == 'quit' # You can also use a real EOF.

    # Go through each item on the line, converting each one and adding it

    # to retval.

    retval = [ ]

    for str in line.split(/\s+/)

        if str =~ /^\-?\d+$/

            retval.push(str.to_i)

        else

            raise TypeError.new

        end

    end

​

    return retval

end

​

#

# Take a coeff and an exponent and return the string representation, ignoring

# the sign of the coefficient.

def term_to_str(coef, exp)

    ret = ""

​

    # Show coeff, unless it's 1 or at the right

    coef = coef.abs

    ret = coef.to_s     unless coef == 1 && exp > 0

    ret += "x" if exp > 0                               # x if exponent not 0

    ret += "^" + exp.to_s if exp > 1                    # ^exponent, if > 1.

​

    return ret

end

​

#

# Create a string of the polynomial in sort-of-readable form.

def polystr(p)

    # Get the exponent of first coefficient, plus 1.

    exp = p.length

​

    # Assign exponents to each term, making pairs of coeff and exponent,

    # Then get rid of the zero terms.

    p = (p.map { |c| exp -= 1; [ c, exp ] }).select { |p| p[0] != 0 }

​

    # If there's nothing left, it's a zero

    return "0" if p.empty?

​

    # *** Now p is a non-empty list of [ coef, exponent ] pairs. ***

​

    # Convert the first term, preceded by a "-" if it's negative.

    result = (if p[0][0] < 0 then "-" else "" end) + term_to_str(*p[0])

​

    # Convert the rest of the terms, in each case adding the appropriate

    # + or - separating them.  

    for term in p[1...p.length]

        # Add the separator then the rep. of the term.

        result += (if term[0] < 0 then " - " else " + " end) + 

                term_to_str(*term)

    end

​

    return result

end

#

# Run until some kind of endfile.

begin

    # Repeat until an exception or quit gets us out.

    while true

        # Read a poly until it works.  An EOF will except out of the

        # program.

        print "\n"

        begin

            poly = readints("Enter a polynomial coefficients: ")

        rescue TypeError

            print "Try again.\n"

            retry

        end

        break if poly.empty?

​

        # Read and evaluate x values until the user types a blank line.

        # Again, an EOF will except out of the pgm.

        while true

            # Request an integer.

            print "Enter x value or blank line: "

            x = readline.chomp

            break if x == ''

            raise EOFError.new if x == 'quit'

​

            # If it looks bad, let's try again.

            if x !~ /^\-?\d+$/

                print "That doesn't look like an integer.  Please try again.\n"

                next

            end

​

            # Convert to an integer and print the result.

            x = x.to_i

            print "p(x) = ", polystr(poly), "\n"

            print "p(", x, ") = ", polyval(x, poly), "\n"

        end

    end

rescue EOFError

    print "\n=== EOF ===\n"

rescue Interrupt, SignalException

    print "\n=== Interrupted ===\n"

else

    print "--- Bye ---\n"

end

​