# caesar.py - Let's practice the Caesar cipher.
# We need to read an input file one character at a time.
# For each letter, let's add 3.
# We need to make use of built-in functions bytearray() and chr()
# in order to change character values.  For example,
# >>> chr(bytearray("pie")[0]+3) produces 's'
# >>> chr(bytearray("pie")[1]+3) produces 'l'
# >>> chr(bytearray("pie")[2]+3) produces 'h'

inFile = open("plain.txt", "r")
outFile = open("cipher.txt", "w")

# The built-in read function can put the text of an entire file
# into a variable.  There is no need to read the file one 
# character at a time with read(1).
text = inFile.read()

# Loop to modify each character one by one.
for i in range(0, len(text)):
    c = chr(bytearray(text)[i]+3)
    outFile.write(c)

# Finally, tell Python we are done with the files.
inFile.close()
outFile.close()