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1.13. Exercises
17
>>> primt
'
Hello world!
'
File "<stdin>", , line 1
primt
'
Hello world!
'
ˆ
SyntaxError: invalid syntax
>>>
Exercise1.6 Whereinthecomputerisavariablesuchas”X”storedafterthe
followingPythonlinefinishes?
x = 123
a)Centralprocessingunit
b)MainMemory
c)SecondaryMemory
d)InputDevices
e)OutputDevices
Exercise1.7 Whatwillthefollowingprogramprintout:
x = 43
x = x x + + 1
print x
a)43
b)44
c)x+1
d)Errorbecausex=x+1isnotpossiblemathematically
Exercise1.8 Explaineachofthefollowingusinganexampleofahumancapa-
bility:(1)Centralprocessingunit,(2)MainMemory,(3)SecondaryMemory,(4)
InputDevice,and(5)OutputDevice.Forexample,”Whatisthehumanequivalent
toaCentralProcessingUnit”?
Exercise1.9 Howdoyoufixa”SyntaxError”?
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18
Chapter1. Whyshouldyoulearntowriteprograms?
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Chapter2
Variables,expressionsand
statements
2.1 Valuesandtypes
Avalueisoneofthebasicthingsaprogramworkswith,likealetteroranumber.
Thevalueswehaveseensofarare
1
,
2
,and'
Hello, World!
'.
Thesevaluesbelongtodifferenttypes:
2
isaninteger,and'
Hello, World!
'isa
string,so-calledbecauseitcontainsa“string”ofletters.You(andtheinterpreter)
canidentifystringsbecausetheyareenclosedinquotationmarks.
The
print
statementalsoworksforintegers. Weusethe
python
commandto
starttheinterpreter.
python
>>> print 4
4
Ifyouarenotsurewhattypeavaluehas,theinterpretercantellyou.
>>> type(
'
Hello, World!
'
)
<type
'
str
'
>
>>> type(17)
<type
'
int
'
>
Notsurprisingly,stringsbelongtothetype
str
andintegersbelongtothetype
int
.Lessobviously,numberswithadecimalpointbelongtoatypecalled
float
,
becausethesenumbersarerepresentedinaformatcalledfloating-point.
>>> type(3.2)
<type
'
float
'
>
Whataboutvalueslike'
17
'and'
3.2
'?Theylooklikenumbers,buttheyarein
quotationmarkslikestrings.
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20
Chapter2. Variables,expressionsandstatements
>>> type(
'
17
'
)
<type
'
str
'
>
>>> type(
'
3.2
'
)
<type
'
str
'
>
They’restrings.
Whenyoutypealargeinteger, youmightbetemptedtousecommasbetween
groupsofthreedigits,asin
1,000,000
.ThisisnotalegalintegerinPython,but
itislegal:
>>> print 1,000,000
1 0 0
Well,that’snotwhatweexpectedatall!Pythoninterprets
1,000,000
asacomma-
separatedsequenceofintegers,whichitprintswithspacesbetween.
Thisisthefirstexamplewehaveseenofasemanticerror:thecoderunswithout
producinganerrormessage,butitdoesn’tdothe“right”thing.
2.2 Variables
Oneofthemostpowerfulfeaturesofaprogramminglanguageistheabilityto
manipulatevariables.Avariableisanamethatreferstoavalue.
Anassignmentstatementcreatesnewvariablesandgivesthemvalues:
>>> message =
'
And now for something g completely y different
'
>>> n n = = 17
>>> pi i = = 3.1415926535897931
Thisexamplemakesthreeassignments. Thefirstassignsastringtoanewvari-
ablenamed
message
;thesecondassignstheinteger
17
to
n
;thethirdassignsthe
(approximate)valueofπto
pi
.
Todisplaythevalueofavariable,youcanuseaprintstatement:
>>> print n
17
>>> print pi
3.14159265359
Thetypeofavariableisthetypeofthevalueitrefersto.
>>> type(message)
<type
'
str
'
>
>>> type(n)
<type
'
int
'
>
>>> type(pi)
<type
'
float
'
>
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2.3. Variablenamesandkeywords
21
2.3 Variablenamesandkeywords
Programmersgenerallychoosenamesfortheirvariablesthataremeaningful—
theydocumentwhatthevariableisusedfor.
Variablenamescanbearbitrarilylong.Theycancontainbothlettersandnumbers,
buttheyhavetobeginwithaletter. Itislegaltouseuppercaseletters,butitisa
goodideatobeginvariablenameswithalowercaseletter(you’llseewhylater).
Theunderscorecharacter(
_
)canappearinaname.Itisoftenusedinnameswith
multiplewords,suchas
my_name
or
airspeed_of_unladen_swallow
.
Ifyougiveavariableanillegalname,yougetasyntaxerror:
>>> 76trombones s =
'
big parade
'
SyntaxError: invalid syntax
>>> more@ = 1000000
SyntaxError: invalid syntax
>>> class =
'
Advanced Theoretical Zymurgy
'
SyntaxError: invalid syntax
76trombones
isillegalbecauseitdoesnotbeginwithaletter.
more@
isillegal
becauseitcontainsanillegalcharacter,
@
.Butwhat’swrongwith
class
?
Itturnsoutthat
class
isoneofPython’skeywords.Theinterpreteruseskeywords
torecognizethestructureoftheprogram, andtheycannotbeusedasvariable
names.
Pythonreserves31keywords
1
foritsuse:
and
del
from
not
while
as
elif
global
or
with
assert
else
if
pass
yield
break
except
import
print
class
exec
in
raise
continue
finally
is
return
def
for
lambda
try
Youmightwanttokeepthislisthandy. Iftheinterpretercomplainsaboutoneof
yourvariablenamesandyoudon’tknowwhy,seeifitisonthislist.
2.4 Statements
AstatementisaunitofcodethatthePythoninterpretercanexecute. Wehave
seentwokindsofstatements:printandassignment.
Whenyoutypeastatementininteractivemode, theinterpreterexecutesitand
displaystheresult,ifthereisone.
1
InPython3.0,
exec
isnolongerakeyword,but
nonlocal
is.
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22
Chapter2. Variables,expressionsandstatements
Ascriptusuallycontainsasequenceofstatements. Ifthereis s morethanone
statement,theresultsappearoneatatimeasthestatementsexecute.
Forexample,thescript
print 1
x = 2
print x
producestheoutput
1
2
Theassignmentstatementproducesnooutput.
2.5 Operatorsandoperands
Operatorsarespecialsymbolsthatrepresentcomputationslikeadditionandmul-
tiplication.Thevaluestheoperatorisappliedtoarecalledoperands.
Theoperators
+
,
-
,
*
,
/
and
**
performaddition,subtraction,multiplication,divi-
sionandexponentiation,asinthefollowingexamples:
20+32
hour-1
hour*60+minute
minute/60
5**2
(5+9)*(15-7)
Thedivisionoperatormightnotdowhatyouexpect:
>>> minute = 59
>>> minute/60
0
Thevalueof
minute
is59,andinconventionalarithmetic59dividedby60is
0.98333,not0.ThereasonforthediscrepancyisthatPythonisperformingfloor
division
2
.
Whenbothoftheoperandsareintegers,theresultisalsoaninteger;floordivision
chopsoffthefractionpart,sointhisexampleitroundsdowntozero.
Ifeitheroftheoperandsisafloating-pointnumber,Pythonperformsfloating-point
division,andtheresultisa
float
:
>>> minute/60.0
0.98333333333333328
2
InPython3.0,theresultofthisdivisionisa
float
.InPython3.0,thenewoperator
//
performs
integerdivision.
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2.6. Expressions
23
2.6 Expressions
Anexpressionisacombinationofvalues,variables,andoperators. Avalueall
byitselfisconsideredanexpression,andsoisavariable,sothefollowingareall
legalexpressions(assumingthatthevariable
x
hasbeenassignedavalue):
17
x
x + 17
Ifyoutypeanexpressionininteractive mode, theinterpreterevaluatesitand
displaystheresult:
>>> 1 1 + + 1
2
Butinascript,anexpressionallbyitselfdoesn’tdoanything!Thisisacommon
sourceofconfusionforbeginners.
Exercise2.1 TypethefollowingstatementsinthePythoninterpretertoseewhat
theydo:
5
x = 5
x + 1
2.7 Orderofoperations
Whenmorethanoneoperatorappearsinanexpression,theorderofevaluation
dependsontherulesofprecedence.Formathematicaloperators,Pythonfollows
mathematicalconvention. TheacronymPEMDASisausefulwaytoremember
therules:
• Parentheseshavethehighestprecedenceandcanbeusedtoforceanexpres-
siontoevaluateintheorderyouwant.Sinceexpressionsinparenthesesare
evaluatedfirst,
2 * (3-1)
is4,and
(1+1)**(5-2)
is8.Youcanalsouse
parenthesestomakeanexpressioneasiertoread,asin
(minute * 100) /
60
,evenifitdoesn’tchangetheresult.
• Exponentiationhasthenexthighestprecedence,so
2**1+1
is3,not4,and
3*1**3
is3,not27.
• MultiplicationandDivisionhavethesameprecedence,whichishigherthan
AdditionandSubtraction,whichalsohavethesameprecedence.So
2*3-1
is5,not4,and
6+4/2
is8,not5.
• Operatorswiththesameprecedenceareevaluatedfromlefttoright.Soin
theexpression
5-3-1
is1,not3becausethe
5-3
happensfirstandthen
1
is
subtractedfrom
2
.
24
Chapter2. Variables,expressionsandstatements
Whenindoubtalwaysputparenthesesinyourexpressionstomakesurethecom-
putationsareperformedintheorderyouintend.
2.8 Modulusoperator
Themodulusoperatorworksonintegersandyieldstheremainderwhenthefirst
operandisdividedbythesecond. InPython,themodulusoperatorisapercent
sign(
%
).Thesyntaxisthesameasforotheroperators:
>>> quotient = 7 7 / / 3
>>> print quotient
2
>>> remainder = = 7 7 % 3
>>> print remainder
1
So7dividedby3is2with1leftover.
Themodulusoperatorturnsouttobesurprisinglyuseful. Forexample,youcan
checkwhetheronenumberisdivisiblebyanother—if
x % % y
iszero,then
x
is
divisibleby
y
.
Also,youcanextracttheright-mostdigitordigitsfromanumber. Forexample,
x % 10
yieldstheright-mostdigitof
x
(inbase10).Similarly
x % % 100
yieldsthe
lasttwodigits.
2.9 Stringoperations
The
+
operatorworkswithstrings,butitisnotadditioninthemathematicalsense.
Insteaditperformsconcatenation,whichmeansjoiningthestringsbylinking
themend-to-end.Forexample:
>>> first = 10
>>> second = 15
>>> print first+second
25
>>> first =
'
100
'
>>> second =
'
150
'
>>> print first t + + second
100150
Theoutputofthisprogramis
throatwarbler
.
2.10 Askingtheuserforinput
Sometimeswewouldliketotakethevalueforavariablefromtheuserviatheir
keyboard. Pythonprovidesabuilt-infunctioncalled
raw_input
thatgetsinput
2.11. Comments
25
fromthekeyboard
3
.Whenthisfunctioniscalled,theprogramstopsandwaitsfor
theusertotypesomething. Whentheuserpresses
Return
or
Enter
,the program
resumes and
raw_input
returns what the user typed as a string.
>>> input = raw_input()
Some silly stuff
>>> print input
Some silly stuff
Before getting input from the user, it is a good idea to print a prompt telling the
user what to input. You can pass a string to
raw_input
to be displayed to the user
before pausing for input:
>>> name = raw_input(
'
What is your name?\n
'
)
What is your name?
Chuck
>>> print name
Chuck
The sequence
\n
at the endof the prompt represents a newline, which is a special
character that causes a line break. That’s why the user’s input appears below the
prompt.
If you expect the user to type an integer, you can try to convert the return value to
int
using the
int()
function:
>>> prompt =
'
What...is the airspeed velocity of an unladen swallow?\n
'
>>> speed = raw_input(prompt)
What...is the airspeed velocity of an unladen swallow?
17
>>> int(speed)
17
>>> int(speed) + 5
22
But if the user types something other than a string of digits, you get an error:
>>> speed = raw_input(prompt)
What...is the airspeed velocity of an unladen swallow?
What do you mean, an African or a European swallow?
>>> int(speed)
ValueError: invalid literal for int()
We will see how to handle this kind of error later.
2.11 Comments
As programs get bigger and more complicated, they get more difficult to read.
Formal languages are dense, and it is often difficult to look at a piece of code and
figure out what it is doing, or why.
3
InPython 3.0, this function isnamed
input
.
26
Chapter 2. Variables, expressions and statements
For this reason, it is agood idea toaddnotes to your programsto explainin natural
language what the program is doing. These notes are called comments, and they
start with the
#
symbol:
# compute the percentage of the hour that has elapsed
percentage = (minute * 100) / 60
In this case, the comment appears on a line by itself. You can also put comments
at the end of a line:
percentage = (minute * 100) / 60
# percentage of an hour
Everything from the
#
to the end of the line is ignored—it has no effect on the
program.
Comments aremost useful when they document non-obvious features of the code.
It is reasonable to assume that the reader can figure out what the code does; it is
much more useful to explain why.
This comment is redundant with the code and useless:
v = 5
# assign 5 to v
This comment contains useful information that is not in the code:
v = 5
# velocity in meters/second.
Goodvariable namescan reduce theneedfor comments, but longnames can make
complex expressions hard to read, so there is a tradeoff.
2.12 Choosing mnemonic variable names
As long as you follow the simple rules of variable naming, and avoid reserved
words, you have a lot of choice when you name your variables. In the beginning,
this choice can be confusing both when you read a program and when you write
your own programs. For example, the following three programs are identical in
terms of what they accomplish, but very different when you read them and try to
understandthem.
a = 35.0
b = 12.50
c = a * b
print c
hours = 35.0
rate = 12.50
pay = hours * rate
print pay
x1q3z9ahd = 35.0
x1q3z9afd = 12.50
x1q3p9afd = x1q3z9ahd * x1q3z9afd
print x1q3p9afd
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