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#!/opt/local/bin/python
#
# See ../READ.md
#
#
# Run as python ch-1.py -s SHIFT < input-file
#
import fileinput
import sys
import getopt
NR_OF_LETTERS = 26
ORD_A = ord ("A")
#
# Parse and validate options
#
shift = -1
opts, args = getopt . getopt (sys . argv [1:], 's:')
for opt, val in opts:
if opt == "-s":
shift = int (val) % NR_OF_LETTERS
sys . argv [1:] = []
if shift < 0:
sys . stderr . write ("Argument -s SHIFT is required\n")
sys . exit (1)
#
# Iterate over the input. Shift capital letters
#
for line in fileinput . input ():
out = ""
for i in range (len (line)):
if "A" <= line [i] <= "Z":
n = ord (line [i]) - shift
if n < ORD_A:
n = n + NR_OF_LETTERS
out += chr (n)
else:
out += line [i]
sys . stdout . write (out)
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