###
# caesar.py
#
# author: Kristina Striegnitz
#
# version: 1/27/2010
#
###

def caesar_encipher(instring, num):
    """
    The given string <instring> is encoded by rotating the letters by
    <num> positions in the alphabet. For example, if <num> is 2, then
    'a' becomes 'c', 'b' becomes 'd', 'z' becomes 'b', and so
    on. Capitalization is preserved. All non-alphabetic characters -
    numbers, punctuation - are also left as they are.
    """
    outstring = ""
    for inchar in instring:
        if inchar < "A" or inchar > "z":
            outchar = inchar
        else:
            incode = ord(inchar)
            if incode >= ord("a"):
                offset = ord("a")
            else:
                offset = ord("A")
            outcode = ((incode - offset + num) % 26) + offset
            outchar = chr(outcode)
        outstring += outchar

    return outstring