6.5.FUNCTIONSTHAT RETURNVALUES 93
Theneteffectis asif thefunctionbodyhadbeenprefacedwiththreeassignmentstatements.
window = win
year = 0
height = principal
Youmustalwaysbecarefulwhencallinga functionthatyougettheactualparametersinthecorrectorderto
matchthefunctiondefinition.
6.5 FunctionsthatReturnValues.
Youhave seenthatparameterpassingprovidesa mechanismforinitializingthevariablesina function.Ina
way, parametersactasinputstoa function.We cancalla functionmany timesandgetdifferentresultsby
changingtheinputparameters.
Sometimeswealsowanttogetinformationbackoutofa function. Thisisaccomplishedbyhaving
functionsreturna valuetothecaller. Youhave alreadyseennumerousexamplesofthistypeoffunction.For
example,considerthiscalltothesqrtfunctionfromthemathlibrary.
discRt = math.sqrt(bb- 4a*c)
Herethevalueofbb - 4a*cistheactualparameterofmath.sqrt. Thisfunctioncalloccurson
therightsideofanassignmentstatement;thatmeansit is anexpression.Themath.sqrtfunctionmust
somehowproducea valuethatis thenassignedtothevariablediscRt. Technically, wesaythatsqrt
returnsthesquarerootofitsargument.
It’s veryeasytowritefunctionsthatreturnvalues.Here’s anexamplevalue-returningfunctionthatdoes
theoppositeofsqrt; it returnsthesquareofitsargument.
def square(x):
return x * x
Thebodyofthisfunctionconsistsofa singlereturnstatement.WhenPythonencountersreturn, it exits
thefunctionandreturnscontroltothepointwherethefunctionwascalled.Inaddition,thevalue(s)provided
inthereturnstatementaresentbacktothecallerasanexpressionresult.
We canuseoursquarefunctionany placethatanexpressionwouldbelegal.Herearesomeinteractive
examples.
square(3)
9
print square(4)
16
x = 5
y = square(x)
print y
25
print square(x)+ square(3)
34
Let’s usethesquarefunctiontowriteanotherfunctionthatfindsthedistancebetweentwo points.Given
two points
x 1 y 1 and
x 2 y 2 , thedistancebetweenthemis calculatedfromthePythagoreanTheoremas
x 2 x 1 ^2
y 2 y 1 ^2. Hereis a PythonfunctiontocomputethedistancebetweentwoPointobjects.def distance(p1, p2):
dist = math.sqrt(square(p2.getX() - p1.getX())
- square(p2.getY()- p1.getY())
return dist
Usingthedistancefunction,wecanaugmenttheinteractive triangleprogramfromlastchapterto
calculatetheperimeterofthetriangle.Here’s thecompleteprogram: