Py4inf 09 Dictionaries

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Python DictionariesChapter 9

Python for Informatics: Exploring Information

www.pythonlearn.com


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Unless otherwise noted, the content of this course material is licensed under a Crea

Commons Attribution 3.0 License.

http://creativecommons.org/licenses/b/3.0/.

Copright !0"0# Charles $everance


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What is a Collection?

•A collection is nice eca!se we can p!t more tha"al!e in them an# carry them all aro!n# in onecon"enient pac$age.

•We ha"e a !nch of "al!es in a single %"ariale&

•We #o this y ha"ing more than one place %in& the"ariale.

•We ha"e ways of %n#ing the #i&erent places in th"ariale


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What is not a %Collectio

•'ost of o!r "ariales ha"e one "al!e in them ( wp!t a new "al!e in the "ariale ( the ol# "al!e iswritten

) python

Python *.+.* ,r*+*:-9//0 1e ** *20 3:67CC 8../ ,Apple Comp!ter0 Inc. !il# +4-4 x ; * x ; 8 print x

8


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A ist

•A linear collection of "al!es that stay in or#er

•Dictionary

•A %ag& of "al!es0 each with its own lael


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Dictionarie

s

money

calc!lator

perf!me

can#y

http:en.wi$ipe#ia.orgwi$iAssociati"e@arra


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Dictionaries

•Dictionaries are Python's most powerf!l #ata collecti•Dictionaries allow !s to #o fast #ataase(li$e operati

Python

•Dictionaries ha"e #i&erent names in #i&erent lang!a

•Associati"e Arrays ( Perl Php

•Properties or 'ap or ash'ap ( Ba"a

•Property ag ( C .ethttp:en.wi$ipe#ia.orgwi$iAssociati"e@ar


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Dictionaries

•>ists in#ex their entriesase# on the positionin the list

•Dictionaries are li$eags ( no or#er

•


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Comparing >ists an#Dictionaries

•Dictionaries are li$e >ists except that they !se $instea# of n!mers to loo$ !p "al!es

lst ; list,5

lst.appen#,*/5 lst.appen#,/245 print lst6*/0 /24 lst6 ; *4 print lst6*40 /24

### ; #ict, ###6FageF ;

###6Fco!rse print ###GFco!rseF: /2*0 Fa ###6FageF ; print ###

GFco!rseF: /2*0 Fa


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lst ; list,5 lst.appen#,*/5 lst.appen#,/245 print lst

6*/0 /24 lst6 ; *4 print lst6*40 /24

### ; #ict,5

###6FageF ; */ ###6Fco!rseF ; /2* print ###GFco!rseF: /2*0 FageF: */H ###6FageF ; *4 print ###

GFco!rseF: /2*0 FageF: *4H

6 */

6/ /2

Key L

6Fco!rseF /

6FageF *

Key

>ist

Diction


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Dictionary >iterals,Constants5

•Dictionary literals !se c!rly races an# ha"e a list of $ey : "

• Mo! can ma$e an empty #ictionary !sing empty c!rly races

NNN ; G Fch!c$F : / 0 Ffre#F : 8*0 F NanF print NNNGF NanF: /0 Fch!c$F: /0 Ffre#F: 8*H ooo ; G H print oooGH


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'ost Common ame

cse"Ohen Ohen

mar!ar#Ohen

cwe

mar!ar#Ohen

mar!ar#cse" c

Ohen


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'ost Common ame


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'ost Common ame

cse"Ohen Ohen

mar!ar#Ohen

cwe

mar!ar#Ohen

mar!ar#cse" cOhen


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'any Co!nters with aDictionary

•Qne common !se of #ictionary isco!nting how often we %see& something

Key

ccc ; #ict,5 ccc6Fcse"F ; / ccc6FcwenF ; / print cccGFcse"F: /0 FcwenF: /H ccc6FcwenF ; ccc6FcwenF / print cccGFcse"F: /0 FcwenF: *H


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Dictionary =raceac$

• It is an error to reference a $ey which is not in the #ict

•We can !se the in operator to see if a $ey is in the #ict ccc ; #ict,5 print ccc6Fcse"F =raceac$ ,most recent call last5: 1ile RSst#inR0 line /0 in Smo#!leKeyError: Fcse"F print Fcse"F in ccc1alse


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When we see a new na

•When we enco!nter a new name0 we nee# to a## a new ent#ictionary an# if this the secon# or later time we ha"e seen we simply a## one to the co!nt in the #ictionary !n#er that

counts = dict()

names = ['csev', 'cwen', 'csev', 'zqian', 'cwen']

for name in names : if name not in counts:

counts[name] = 1

else :

counts[name] = counts[name] + 1

print counts

GFcse"F: *0 FOia


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=he get metho# for#ictionaries

• =his pattern of chec$ingto see if a $ey is alrea#yin a #ictionary an#ass!ming a #efa!lt"al!e if the $ey is notthere is so common0that there is a metho# calle# get,5 that #oesthis for !s

if name in co! x ; co!nts6n else : x ;

x ; co!nts.get

Defa!lt "al!e if $ey#oes not exist ,an# no

=raceac$5.

GFcse"F: *0 FOianF:


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http:www.yo!t!e.comwatch?";EB9!Mx+>

counts = dict()

names = ['csev', 'cwen', 'csev', 'zqian', 'cwen']

for name in names :

counts[name] = counts.get(name, 0) + 1

print counts


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Writing programs ,or programming5 is a "ery creati"e an#acti"ity. Mo! can write programs for many reasons rangma$ing yo!r li"ing to sol"ing a #iTc!lt #ata analysis prha"ing f!n to helping someone else sol"e a prolem. =

ass!mes that e"eryone nee#s to $now how to program an

yo! $now how to program0 yo! will %g!re o!t what yo! wwith yo!r newfo!n# s$ills.We are s!rro!n#e# in o!r #aily li"es with comp!ters ran

laptops to cell phones. We can thin$ of these comp!teUUpersonal assistantsFF who can ta$e care of many thin

ehalf. =he har#ware in o!r c!rrent(#ay comp!ters is essto contin!o!sly as$ !s the !estion0 UUWhat wo!l# yo! li

next?FF.Q!r comp!ters are fast an# ha"e "asts amo!nts of memoe "ery helpf!l to !s if we only $new the lang!age to speato the comp!ter what we wo!l# li$e it to UU#o nextFF. If wlang!age we co!l# tell the comp!ter to #o tas$s on o!r were reptiti"e. Interestingly0 the $in#s of things comp!t

est are often the $in#s of things that we h!mans %n# min#(n!min .


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the clown ran after the car an# the car ran intotent an# the tent fell #own on the clown an# the


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Co!nting Pattern

counts = dict()

print 'Enter a line of text:

'line = raw_input('')

words = line.split()

print 'Words:', words

print 'Counting...’

for word in words:

counts[word] = counts.get(word,0) + 1

print 'Counts', counts

=he general pco!nt the wor#stext is to split thwor#s0 then loo

the wor#s an#ictionary to tracof each wor# in#


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Co!nting Wor#spython wor#co!nt.py Enter a line of text:the clown ran after the car an# the car ran intothe tent an# the tent fell #own on the clownan# the carWor#s: 6FtheF0 FclownF0 FranF0 FafterF0 FtheF0 FcarF0Fan#F0 FtheF0 FcarF0 FranF0 FintoF0 FtheF0 FtentF0Fan#F0 FtheF0 FtentF0 FfellF0 F#ownF0 FonF0 FtheF0

FclownF0 Fan#F0 FtheF0 FcarFCo!nting...Co!nts GFan#F: 40 FonF: /0 FranF: *0 FcarF: 40FintoF: /0 FafterF: /0 FclownF: *0 F#ownF: /0 FfellF:/0 FtheF: 30 FtentF: *H

http:www.Vic$r.comphotos3/+


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counts = dict()

print 'Enter a line of text:'line = raw_input('')

words = line.split()

print 'Words:', words

print 'Counting...’

for word in words:

counts[word] = counts.get(word,0) + 1print 'Counts', counts

python wor#co!nt.pEnter a line of text:after the car an# ththe tent an# the ten

on the clown an# th

Wor#s: 6FtheF0 FclowFafterF0 FtheF0 FcarF0 FaFcarF0 FranF0 FintoF0 FthFan#F0 FtheF0 FtentF0 Ff

FonF0 FtheF0 FclownF0 FFcarFCo!nting...

Co!nts GFan#F: 40 FoFcarF: 40 FintoF: /0 FafFclownF: *0 F#ownF: /

FtheF: 30 FtentF: *H

D % it > #


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De%nite >oops an#Dictionaries

•E"en tho!gh #ictionaries are not store# in or#er0 wewrite a for loop that goes thro!gh all the entries in #ictionary ( act!ally it goes thro!gh all of the $eys i#ictionary an# loo$s !p the "al!es

co!nts ; G Fch!c$F : / 0 Ffre#F : 8*0 FNanF: /H for $ey in co!nts:... print $ey0 co!nts6$ey...

Nan /ch!c$ /fre# 8*

X t i i li t f K


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Xetrie"ing lists of Keys aLal!es

• Mo! can get a list of$eys0 "al!es or items,oth5 from a #ictionary

NNN ; G Fch!c$F : / print list,NNN56FNanF0 Fch!c$F0 Ffre#F print NNN.$eys,56FNanF0 Fch!c$F0 Ffre#F print NNN."al!es,56/0 /0 8* print NNN.items,56,

What is a Ft!pleF?

= It ti


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on!s: =wo IterationLarialesY

•We loop thro!gh the$ey("al!e pairs in a#ictionary !sing ZtwoZiteration "ariales

•Each iteration0 the%rst "ariale is the$ey an# the thesecon# "ariale is thecorresponding "al!efor the $ey

NNN ; G Fch!c$F : / for aaa0 in NNN.i... print aaa0 ... Nan /ch!c$ /fre# 8*

6c

6


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Py4inf 09 Dictionaries

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