4.3.STRINGSANDSECRETCODES 45

Finally, weneedto converteachcharacterto a number. Thesimplestapproachis to usetheASCIInumber
(providedbyord) foreachcharacterinthemessage.
Hereis thefinalprogramforencodingthemessage:

text2numbers.py

A program to converta textual message into a sequence of

numbers,utlilizing the underlying ASCII encoding.

def main():
print "This programconverts a textual message into a sequence"
print "of numbersrepresenting the ASCII encoding of the message."
print

# Get the messageto encode

message = raw_input("Please enter the message to encode: ")

print

print "Here are theASCII codes:"

# Loop throughthe message and print out the ASCII values

for ch in message:

print ord(ch), # use comma to print all on one line.

print

main()

We canusetheprogramtoencodeimportantmessages.

This program convertsa textual message into a sequence
of numbers representingthe ASCII encoding of the message.

Please enter the messageto encode: What a Sourpuss!

Here are the ASCIIcodes:
87 104 97 116 32 97 32 83 111 117 114 112 117 115 115 33

Onethingtonoticeaboutthisresultis thateventhespacecharacterhasa correspondingASCIIcode.It is
representedbythevalue32.

4.3.3 Programminga Decoder

Nowthatwehave a programtoturna messageintoa sequenceofnumbers,it wouldbeniceif ourfriend
ontheotherendhada similarprogramtoturnthenumbersbackintoa readablemessage.Let’s solve that
problemnext.Ourdecoderprogramwillprompttheuserfora sequenceofnumbersrepresentingASCII
codesandthenprintoutthetextmessagecorrespondingtothosecodes. Thisprogrampresentsuswitha
coupleofchallenges;we’ll addresstheseaswegoalong.
Theoveralloutlineofthedecoderprogramlooksverysimilartotheencoderprogram. Onechangein
structureis thatthedecodingversionwillcollectthecharactersofthemessageina stringandprintoutthe
entiremessageattheendoftheprogram.To dothis,weneedtouseanaccumulatorvariable,a patternwe
saw inthefactorialprogramfromthepreviouschapter. Hereis thedecodingalgorithm:

get the sequence of numbersto decode
message = ""
for each number in theinput:
convert the numberto the appropriate character