4.3.STRINGSANDSECRETCODES 43

month number position

Jan 1 0

Feb 2 3

Mar 3 6

Apr 4 9

Ofcourse,thepositionsallturnouttobemultiplesof3. To getthecorrectmultiple,wejustsubtract 1
fromthemonthnumberandthenmultiplyby3.Sofor1 weget

^1 ^1
^3 ^0 ^3 0 andfor^12 wehave
12  1
3 11 3 33
Now we’re readytocodetheprogram.Again,thefinalresultis shortandsweet;thecommentsdocument
thealgorithmwe’ve developed.

month.py

A program to printthe abbreviation of a month, given its number

def main():

months is usedas a lookup table

months = "JanFebMarAprMayJunJulAugSepOctNovDec"

n = input("Entera month number (1-12): ")

# compute startingposition of month n in months

pos = (n-1) * 3

# Grab the appropriateslice from months

monthAbbrev = months[pos:pos+3]

# print the result

print "The monthabbreviation is", monthAbbrev + "."

main()

Noticethelastlineofthisprogramusesstringconcatenationtoputa periodat theendofthemonthabbrevi-
ation.
Hereis a sampleofprogramoutput.

Enter a month number(1-12): 4
The month abbreviationis Apr.

4.3 StringsandSecretCodes

4.3.1 StringRepresentation.

Hopefully, youarestartingtogetthehangofcomputingwithtextual(string)data.However, youmightstill
bewonderinghow computersactuallymanipulatestrings.Inthepreviouschapter, yousawhow computers
storenumbersinbinarynotation(sequencesofzerosandones);thecomputerCPUcontainscircuitrytodo
arithmeticwiththeserepresentations.Textualinformationis representedinexactlythesameway. Under-
neath,whenthecomputeris manipulatingtext,it is reallynodifferentfromnumbercrunching.
To understandthis,youmightthinkintermsofmessagesandsecretcodes.Considertheage-oldgrade
schooldilemma.Youaresittinginclassandwanttopassa notetoa friendacrosstheroom.Unfortunately,
thenotemustpassthroughthehands,andinfrontofthecuriouseyes,ofmany classmatesbeforeit reaches
itsfinaldestination.And,ofcourse,thereis alwaystheriskthatthenotecouldfallintoenemyhands(the
teacher’s).Soyouandyourfriendneedtodesigna schemeforencodingthecontentsofyourmessage.
Oneapproachis tosimplyturnthemessageintoa sequenceofnumbers. Youcouldchoosea number
tocorrespondtoeachletterofthealphabetandusethenumbersinplaceofletters. Withouttoomuch