lesson 2.3 looping

8/19/2019 Lesson 2.3 Looping

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CONTROL STATEMENTS AND FUNCTIONS

Unit 2


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LOOPING

Lesson 2.3


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Objectives:

At the end of the lesson, you should be able to:

1. Create Loops usin !or"#e$t %tate&ents and

2. Control the nu&ber of iterations in a loop

usin %tep.


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Loops

At instances 'hen 'e 'ant the co&puter to doa tas( repeatedly for a certain nu&ber of ti&es,loopin beco&es necessary. #ote that this is not

the sa&e as havin a user re"enter a value until a valid value is iven )li(e our %* prora& inLesson 2.2+. n such a case it is the user doin therepeated tas(, not the co&puter. -he loop 'asfirst conceptualied by Ada /yron )recall Lesson1.1+.


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-he !or"#e$t Loop

One 0/ state&ent that allo's loopin is calledthe !or"#e$t state&ent or loop.

!or variable start value -o end value %tep incre&ent%tate&ents

#e$t


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-he !or"#e$t Loop

-his loop repeats a set of state&ents a certainnu&ber of ti&es. -hat nu&ber is deter&ined bythree values"the start value, the end value, and

the incre&ent. #ote that the state&ents to berepeated are enclosed by !or and #e$t. #e$t isthe end &ar(er of the loop just as nd %ub is theend &ar(er for a subroutine. -he variable used ina !or"#e$t state&ent &ay be used in thestate&ents bet'een !or and #e$t.

Continuation


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-he !or"#e$t Loop

n order to understand this state&ent better, 'e shall conduct a little e$peri&ent.

Continuation


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-ry this on your co&puter


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#a&e the te$t bo$es )fro& top to botto&+ ast$t%tart, t$tnd and t$tncr. -hen, na&e, too,the co&&and button as c&dO( and the list bo$

on the riht as lstLoop. 4eclare $, start5val,end5val, incr as variables of type %inle. !inally,assin the follo'in codes to c&dO(.

Continuation


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6rivate %ub c&dO(5Clic( ) +

lstLoop.Clear

start5val 0al )t$t%tart.-e$t+

end5val 0al )t$tnd.-e$t+

incr 0al )t$tncr.-e$t+

!or $ start5val to end5val %tep incr

lstLoop.Addte& )789+

#e$t

nd %ub

Continuation


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%ave the prora& then, run it. -a(e note of thenu&ber of 8 that appears as you chane the values in the te$t bo$es.

Continuation


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-he !or"#e$t Loop

#ote that the object 'hich loo(s li(e a huete$t bo$ is actually a list bo$.

A list bo$ is different fro& a te$t bo$, for you

can add ite&s to it via the state&entList/o$5na&e. Addte& )ite&s to be added+. nour prora&, 'e shall add 8 to the list bo$. -his

'ill be repeated several ti&es dependin on the values 'e enter in the te$t bo$es. -he state&ent,1stLoop.Clear, is used to clear the contents of thelist bo$ prior to loadin it 'ith 8;s.

Continuation


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-he !or"#e$t Loop

n the course of runnin the prora&, you &ayhave co&e across the follo'in outputs.

Continuation

ForStatement

Number of

*’s

For x = 1To 10Step 1

10

For x = 1To 6 Step2

3

For x = 3To 5 Step

1


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-he !or"#e$t Loop

f you;ve tried enterin a ero incre&ent, you&ay have noticed so&ethin unusual < 0/haned=

Continuation

Warning! A ero incre&ent 'ill result in an infinite loop < a

loop that never ter&inates.

>hy this happens 'ill beco&e evident once 'e understand ho' the values affect the nu&berof iterations or loops.


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-he !or"#e$t Loop

/elo' is a flo'chart that illustrates 'hat ta(esplace in our !or"#e$t loop.

Continuation

$ start5val

add an 8 to 1stLoop

$ $ ? incr

$ @ end5val

end

es#o


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-he !or"#e$t Loop

>hat happens at first is that our du&&y variable $ ta(es on the value of start5val. t )$+ isthen chec(ed if it is reater than end5val. >hile

it )$+ is not reater than end5val, t'o thinshappen: An 8 is added to the list bo$ and incr isadded to the value of $ via the state&ent $ $ ?incr.

Continuation


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-he !or"#e$t Loop

!or e$a&ple, consider !or $ 1 -o B %tep 3.nitially $ is assined the value of 1. %ince it is not yet reater than B, an iteration is &ade and 3 is

added to it &a(in it . %ince it is not yet reaterthan B, another iteration is &ade and 3 is addedto it, &a(in it D. #o', that it is reater than B,the iterations cease. A total of t'o iterations 'ere&ade thus, 'e see t'o 8;s

Continuation


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-he !or"#e$t Loop

>hen the incre&ent is ero, $ 'ill never reachor surpass the end value hence, 'e have aninfinite loop.

Continuation


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Usin the 4u&&y 0ariable

#o' that 'e understand ho' !or"#e$t 'or(s,there are other thins 'e should learn about it.One is that !or $ 1 -o 1E %tep 1 is eFuivalent to

!or $ 1 -o 1E. >hat it &eans is that if theincre&ent is 1, 'e do not have to add %tep 1. -heother is that the du&&y variable can be used.


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n the Loop -ester prora&, chane thestate&ent 1stLoop.Addte& )789+ to1stLoop.Addte& )$+ and run the prora&,

ta(in note of the outputs as you chane the values in the te$t bo$es.

We can now see the values x holds in a loop. Keep this in mind as we tackle another program.


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-he 6ri&e valuator

!or e$a&ple, to test if H is pri&e, 'e need to testthe nu&bers 2 to B aainst it. Of course, 'e shallfind that H is perfectly divisible by 2 and 3 hence,

H is not pri&e. -he Fuestion is this: Io' do 'echec( for divisibilityG -he ans'er lies in t'o&athe&atical operators < division )J+ andinteer division )K+.

Continuation


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-he 6ri&e valuator

#ote that H is divisible by 3 because HJ3 is eFualto HK3 )since HJ3 and HK3 are both eFual to 2+.On the other hand, H is not divisible by since

HJ is not eFual to HK )since HJ 1.B and HK 1+. -hus, 'e can say that # is divisible by if andonly if, #J #K. #o', all 'e need is for to vary fro& 2 to #"1.

Continuation


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-he 6ri&e valuator

Let us assu&e that 'e have a for& 'ith aco&&and button c&dO(, a te$t bo$ t$t#, and alabel lblMesult. >hat should happen is that the

user supplies a nu&ber via t$t#, clic(s c&dO(,and vie's the result of 'hether the nu&ber 'aspri&e or not in lblMesult.

Continuation


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-he 6ri&e valuator

>e also need t'o variables: # to held thenu&ber supplied by the user and to hold thenu&bers to be tested aainst # for divisibility.

Continuation

n code,4i& #, As nteer

6rivate %ub c&dO(5Clic() +# 0al )t$t#.-e$t+

nd %ub


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-he 6ri&e valuator

#o', 'e need to test if perfectly divides #, because if it does, # is not pri&e.

Continuation

n code,

4i& #, As nteer

6rivate %ub c&dO(5Clic() +# 0al )t$t#.-e$t+

f #J #K -henlblMesult.Caption 7#ot 6ri&e9nd f

nd %ub


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-he 6ri&e valuator

#otice that for this to 'or(, &ust vary in valuefro& 2 to #"1. -his is 'here the !or"#e$t loop co&esin.

Continuation

n code,

4i& #, As nteer

6rivate %ub c&dO(5Clic() +# 0al )t$t#.-e$t+!or 2 -o #"1

f #J #K -henlblMesult.Caption 7#ot 6ri&e9

nd f #e$t

nd %ub


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-he 6ri&e valuator

n hindsiht, it see&s a 'aste of co&putinenery to test all ;s 'hen 'e have alreadyencountered an 'hich perfectly divides #.

!or e$a&ple, the co&puter has found that 2perfectly divides H, so it does not need to chec( if

3, , or B also perfectly divides H. >e need to finda 'ay to step out of the loop once this happens.!ortunately, there is a state&ent that allo's that.

Continuation


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-he 6ri&e valuatorContinuation

4i& #, As nteer

6rivate %ub c&dO(5Clic() +

# 0al )t$t#.-e$t+

!or 2 -o #"1

f #J #K -hen

lblMesult.Caption 7#ot 6ri&e9

$it !ornd f

#e$t

nd %ub


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Construct the 6ri&e valuator prora& 'e;ve just discussed. 0erify if it 'or(s.


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-he 6ri&e valuatorContinuation

ou probably noticed that the prora& does 'or( 'hen # is not pri&e. >hen # is pri&e,ho'ever, it doesn;t say so. 'ill leave it up to you

to e$peri&ent 'here in the code this line should be placed.

lblMesult.Caption 76ri&e=9


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#estin Loops

n the previous lesson, 'e found that 'e can nestf state&ents. >e can do the sa&e for !or"#e$t.-he rules of nestin still apply, of course.

C i i


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#estin Loops

Iere is an e$a&ple:!or A 1 -o B

!or / 1 to 1E

)%tate&ents+#e$t

#e$t

Continuation

In this example, for every loop of A, B has made tenloops. Since A will loop five times, the statementsounded y the B loop would have een repeated fifty times!


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Other Loops

Aside fro& !or"#e$t, there are other loopinstate&ents that can be used. -he three sets ofcode outlined belo' are essentially the sa&e.

!or * B to 1E %tep 2

)Mepeated %tate&ents+

#e$t

C ti ti


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Other Loops

* B >hile * N 1E


* * ? 2 >end

Continuation

C ti ti


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Other Loops

* B4o >hile * N 1E


* * ? 2Loop

Continuation

"he last two sets of code are more literaltranslations of the flowchart that was shownearlier.

lesson 2.3 looping

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