6.3.FUTUREVALUEWITHA FUNCTION 89

sing("Lucy")

print

sing("Elmer")

6.3 FutureValuewitha Function.

Nowthatyou’ve seenhowdefiningfunctionscanhelpsolve thecodeduplicationproblem,let’s returnto
thefuturevaluegraph.Recalltheproblemis thatbarsofthegraphareprintedat two differentplacesinthe
program.
Thecodejustbeforethelooplookslike this.

Draw bar for initialprincipal

bar = Rectangle(Point(0, 0), Point(1, principal))
bar.setFill("green")
bar.setWidth(2)
bar.draw(win)

Andthecodeinsideoftheloopis asfollows.

bar = Rectangle(Point(year, 0), Point(year+1, principal))
bar.setFill("green")
bar.setWidth(2)
bar.draw(win)

Let’s trytocombinethesetwo intoa singlefunctionthatdrawsa baronthescreen.
Inordertodraw thebar, weneedsomeinformation.Specifically, weneedtoknow whatyearthebarwill
befor, how tallthebarwillbe,andwhatwindow thebarwillbedrawnin.Thesethreevalueswillbesupplied
asparametersforthefunction.Here’s thefunctiondefinition.

def drawBar(window,year, height):

Draw a bar in windowfor given year with given height

bar = Rectangle(Point(year, 0), Point(year+1, height))
bar.setFill("green")
bar.setWidth(2)
bar.draw(window)

To usethisfunction,wejustneedtosupplyvaluesforthethreeparameters. Forexample,ifwinisa
GraphWin, wecandraw a barforyear0 anda principalof$2,000byinvokingdrawBarlike this.

drawBar(win, 0, 2000)

IncorporatingthedrawBarfunction,hereis thelatestversionofourfuturevalueprogram.

futval_graph3.py

from graphics import*

def drawBar(window,year, height):

Draw a bar in windowstarting at year with given height

bar = Rectangle(Point(year, 0), Point(year+1, height))
bar.setFill("green")
bar.setWidth(2)
bar.draw(window)

def main():

Introduction

print "This programplots the growth of a 10-year investment."