20 CHAPTER2. WRITINGSIMPLEPROGRAMS
Becausetheassignmentis simultaneous,it avoidswipingoutoneoftheoriginalvalues.
Simultaneousassignmentcanalsobeusedtogetmultiplevaluesfromtheuserina singleinput. Con-
siderthisprogramforaveragingexamscores:
avg2.py
A simple programto average two exam scores
Illustrates use of multipleinput
def main():
print "This programcomputes the average of two exam scores."
score1, score2= input("Enter two scores separated by a comma:")
average = (score1+ score2) / 2.0print "The averageof the scores is:", averagemain()
Theprogrampromptsfortwo scoresseparatedbya comma.Supposetheusertypes86, 92. Theeffectof
theinputstatementis thenthesameasif wehaddonethisassignment:
score1, score2 = 86, 92
We have gottena valueforeachofthevariablesinonefellswoop.Thisexampleusedjusttwo values,butit
couldbegeneralizedtoany numberofinputs.
Ofcourse,wecouldhave justgottentheinputfromtheuserusingseparateinputstatements.
score1 = input("Enterthe first score: ")
score2 = input("Enterthe second score: ")
Insomewaysthismaybebetter, astheseparatepromptsaremoreinformative fortheuser. Inthisexample
thedecisionastowhichapproachtotake is largelya matteroftaste.Sometimesgettingmultiplevaluesina
singleinputprovidesa moreintuitive userinterface,soit is nicetechniquetohave inyourtoolkit.
2.6 DefiniteLoops
Youalreadyknow thatprogrammersuseloopsto executea sequenceofstatementsseveraltimesin succession.
Thesimplestkindofloopis calledadefiniteloop. Thisis a loopthatwillexecutea definitenumberoftimes.
Thatis,atthepointintheprogramwhentheloopbegins,Pythonknowshowmany timestogoaround
(oriterate) thebodyoftheloop.Forexample,theChaosprogramfromChapter1 useda loopthatalways
executedexactlytentimes.
for i in range(10):
x = 3.9 * x * (1 - x)
print xThisparticularlooppatternis calledacountedloop, andit is builtusinga Pythonforstatement.Before
consideringthisexampleindetail,let’s take a lookat whatforloopsareallabout.
A Pythonforloophasthisgeneralform.
for <var> in <sequence>:
<body>Thebodyoftheloopcanbeany sequenceofPythonstatements.Thestartandendofthebodyis indicated
byitsindentationundertheloopheading(thefor in
Themeaningoftheforstatementis a bitawkwardtoexplaininwords,butis veryeasytounderstand,
onceyougetthehangofit. Thevariableafterthekeywordforiscalledtheloopindex. It takeson