7.2.TWO-WAY DECISIONS 105

if name == ’main’:
main()

Thisguaranteesthatmainwillautomaticallyrunwhentheprogramis invokeddirectly, butit willnotrun
if themoduleis imported.Youwillseea lineofcodesimilartothisat thebottomofvirtuallyeveryPython
program.

7.2 Two-WayDecisions.

Nowthatwehave a waytoselectivelyexecutecertainstatementsina programusingdecisions,it’s timeto
gobackandspruceupthequadraticequationsolverfromChapter3.Hereis theprogramasweleftit:

quadratic.py

A program thatcomputes the real roots of a quadratic equation.

Illustrates use of the mathlibrary.

Note: this programcrashes if the equation has no real roots.

import math # Makesthe math library available.

def main():
print "This programfinds the real solutions to a quadratic"
print

a, b, c = input("Please enter the coefficients (a, b, c): ")

discRoot = math.sqrt(b* b - 4 * a * c)

root1 = (-b + discRoot)/ (2 * a)

root2 = (-b - discRoot)/ (2 * a)

print

print "The solutionsare:", root1, root2

main()

Asnotedinthecomments,thisprogramcrasheswhenit is givencoefficientsofa quadraticequationthat
hasnorealroots.Theproblemwiththiscodeis thatwhenb^2  4 acis lessthan0,theprogramattemptsto
take thesquarerootofa negative number. Sincenegative numbersdonothave realroots,themathlibrary
reportsanerror. Here’s anexample.

import quadratic
This program findsthe real solutions to a quadratic

Please enter the coefficients(a, b, c): 1,2,3
Traceback (innermostlast):
File "", line1, in?
File "quadratic.py",line 21, in?
main()
File "quadratic.py",line 14, in main
discRoot = math.sqrt(b b - 4 a * c)
OverflowError: mathrange error

We canusea decisiontocheckforthissituationandmake surethattheprogramcan’t crash.Here’s a
firstattempt:

quadratic2.py

import math