Python Programming: An Introduction to Computer Science

11.3. STATISTICSWITHLISTS 181

We have seenhow listscangrow byappendingnew items.Listscanalsoshrinkwhenitemsaredeleted.
Individualitemsorentireslicescanberemovedfroma listusingthedeloperator.

myList
[34, 26, 0, 10]
del myList[1]
myList
[34, 0, 10]
del myList[1:3]
myList
[34]

Asyoucansee,Pythonlistsprovidea veryflexiblemechanismforhandlingindefinitelylargesequences
ofdata.Usinglistsis easyif youkeepthesebasicprinciplesinmind:

 A listis a sequenceofitemsstoredasa singleobject.

 Itemsina listcanbeaccessedbyindexing,andsublistscanbeaccessedbyslicing.

 Listsaremutable;individualitemsorentireslicescanbereplacedthroughassignmentstatements.

 Listswillgrow andshrinkasneeded.

11.3 StatisticswithLists

Nowthatyouknowmoreaboutlists,wearereadytosolve ourlittlestatisticsproblem.Recallthatweare
tryingtodevelopa programthatcancomputethemean,medianandstandarddeviationofa sequenceof
numbersenteredbytheuser. Oneobviouswaytoapproachthisproblemis tostorethenumbersina list.We
canwritea seriesoffunctions—mean,stdDevandmedian—thattake a listofnumbersandcalculatethe
correspondingstatistics.
Let’s startbyusingliststorewriteouroriginalprogramthatonlycomputesthemean.First,weneeda
functionthatgetsthenumbersfromtheuser. Let’s callitgetNumbers. Thisfunctionwillimplementthe
basicsentinelloopfromouroriginalprogramtoinputa sequenceofnumbers.We willuseaninitiallyempty
listasanaccumulatortocollectthenumbers.Thelistwillthenbereturnedfromthefunction.
Here’s thecodeforgetNumbers:

def getNumbers():
nums = [] # startwith an empty list

# sentinel loopto get numbers

xStr = raw_input("Entera number (<Enter> to quit) >> ")

while xStr != "":

x = eval(xStr)

nums.append(x) # add this value to the list

xStr = raw_input("Entera number (<Enter> to quit) >> ")

return nums

Usingthisfunction,wecangeta listofnumbersfromtheuserwitha singlelineofcode.

data = getNumbers()

Next,let’s implementa functionthatcomputesthemeanofthenumbersina list.Thisfunctiontakesa
listofnumbersasa parameterandreturnsthemean.We willusea looptogothroughthelistandcompute
thesum.

def mean(nums):
sum = 0.0