1.7.INSIDEA PYTHONPROGRAM 9

File: chaos.py

A simple programillustrating chaotic behavior.

print "This programillustrates a chaotic function"
x = input("Enter a numberbetween 0 and 1: ")
for i in range(10):
x = 3.9 x (1 - x)
print x

Thisversionis a bitshorter, butit is customarytoplacetheinstructionsthatcomprisea programinsideofa
functioncalledmain. Oneimmediatebenefitofthisapproachwasillustratedabove; it allowsusto(re)run
theprogrambysimplyinvokingchaos.main(). We don’t have toreloadthemodulefromthefileinorder
torunit again,whichwouldbenecessaryinthemain-lesscase.
Thefirstlineinsideofmainis reallythebeginningofourprogram.

print "This programillustrates a chaotic function"

ThislinecausesPythontoprinta messageintroducingtheprogramwhenit runs.
Take a lookat thenextlineoftheprogram.

x = input("Enter a numberbetween 0 and 1: ")

Herexis anexampleofavariable. A variableis usedtogive a nametoa valuesothatwecanrefertoit at
otherpointsintheprogram.Theentirelineis aninputstatement.WhenPythongetstothisstatement,it
displaysthequotedmessageEnter a number between0 and 1:andthenpauses,waitingforthe
usertotypesomethingonthekeyboardandpressthekey. Thevaluethattheusertypesis then
storedasthevariablex. Inthefirstexampleshownabove, theuserentered.25, whichbecomesthevalueof
x.
Thenextstatementis anexampleofaloop.

for i in range(10):

Aloopis a devicethattellsPythontodothesamethingoverandoveragain.Thisparticularloopsaysto
dosomething 10 times.Thelinesindentedunderneaththeloopheadingarethestatementsthataredone 10
times.Theseformthebodyoftheloop.

x = 3.9 x (1 - x)
print x

Theeffectoftheloopis exactlythesameasif wehadwrittenthebodyoftheloop 10 times:

x = 3.9 x (1 - x)
print x
x = 3.9 x (1 - x)
print x
x = 3.9 x (1 - x)
print x
x = 3.9 x (1 - x)
print x
x = 3.9 x (1 - x)
print x
x = 3.9 x (1 - x)
print x
x = 3.9 x (1 - x)
print x
x = 3.9 x (1 - x)
print x
x = 3.9 x (1 - x)