CS1150 Principles of Computer Science Loops

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1 CS1150 Principles f Cmputer Science Lps Yanyan Zhuang Department f Cmputer Science CS1150 UC. Clrad Springs

2 Annuncement HW1 graded HW2 due tnight HW3 will be psted sn Due next Mnday July 3 Midterm July 5 in class N assignment next week, but practice questins will be psted Extra ffice hurs: 10am -12pm July 5 (n ffice hur July 6) CS1150 UC. Clrad Springs

3 Feedback n HW1 if there are multiple lines f cmments /*... */ Recmmend: indenting them as it visually makes them easier t pick ut, e.g. /* this is a multi-line cmment fr an example. */ CS1150 UC. Clrad Springs

4 Feedback n HW1 'whitespace' vs. tabs Editrs tday in Windws/Mac will let yu use tabs t indent This causes readability / indenting prblems when yur cde is viewed in a different envirnment (a different OS, editr, etc.) Recmmendatin (but nt enfrced): change Eclipse t use spaces instead f tabs CS1150 UC. Clrad Springs

5 Review Blean variables Assume x=3, true r false?!(x<2) y>3 If statement Be careful: multiple/nested if else By default: else is mathced with if? Switch statement Be careful: where t use break CS4500/5500 UC. Clrad Springs

6 Overview While lp D while lp Fr lp CS1150 UC. Clrad Springs

7 Opening Prblem: Why Lps? Prblem: 100 times System.ut.println("Welcme t Java!"); System.ut.println("Welcme t Java!"); System.ut.println("Welcme t Java!"); System.ut.println("Welcme t Java!"); System.ut.println("Welcme t Java!"); System.ut.println("Welcme t Java!"); System.ut.println("Welcme t Java!"); System.ut.println("Welcme t Java!"); System.ut.println("Welcme t Java!"); 7

8 Intrducing while Lps int cunt = 0; while (cunt < 100) { System.ut.println("Welcme t Java"); cunt++; 8

9 Intrducing while Lps int cunt = 0; while (cunt < 100) { System.ut.println("Welcme t Java"); cunt++; Hw It Wrks The lp cntinuatin cnditin - blean expressin - is evaluated If the cnditin is true, the statements in the lp bdy are executed When executin f lp bdy statements is cmplete, cntrl returns t the lp cnditin The lp cntinuatin cnditin is evaluated again When the lp cnditin is false, cntrl ges t the 1st statement fllwing the lp Nte: if the lp cntinuatin cnditin evaluates t false the 1st time, the entire while lp is skipped 9 while (lp-cntinuatin-cnditin) { // lp-bdy; Statement(s);

10 while Lp Flw Chart while (lp-cntinuatin-cnditin) { // lp-bdy; Statement(s); int cunt = 0; while (cunt < 100) { System.ut.println("Welcme t Java!"); cunt++; 10

11 Rules fr While Lps The lp cnditin must be a blean expressin Blean expressin must be in parentheses Blean expressins are frmed using relatinal r lgical peratrs Lp cnditin Usually a statement befre the while lp "initializes" the lp cnditin t true Sme statement within the lp bdy eventually change the cnditin t false If the cnditin is never changed t false, the prgram is frever in the lp This is called an "infinite lp" Curly braces are nt necessary if nly ne statement in lp But best practice is t always include curly braces CS4500/5500 UC. Clrad Springs

12 Trace while Lp int cunt = 0; while (cunt < 2) { System.ut.println("Welcme t Java!"); cunt++; Initialize cunt (which we ften call cntrl variable) 12

13 Trace while Lp, cnt. int cunt = 0; while (cunt < 2) { System.ut.println("Welcme t Java!"); cunt++; (cunt < 2) is true 13

14 Trace while Lp, cnt. int cunt = 0; while (cunt < 2) { System.ut.println("Welcme t Java!"); cunt++; Print Welcme t Java 14

15 Trace while Lp, cnt. int cunt = 0; while (cunt < 2) { System.ut.println("Welcme t Java!"); cunt++; Increase cunt by 1 cunt is 1 nw 15

16 Trace while Lp, cnt. int cunt = 0; while (cunt < 2) { System.ut.println("Welcme t Java!"); cunt++; (cunt < 2) is still true since cunt is 1 16

17 Trace while Lp, cnt. int cunt = 0; while (cunt < 2) { System.ut.println("Welcme t Java!"); cunt++; Print Welcme t Java 17

18 Trace while Lp, cnt. int cunt = 0; while (cunt < 2) { System.ut.println("Welcme t Java!"); cunt++; Increase cunt by 1 cunt is 2 nw 18

19 Trace while Lp, cnt. int cunt = 0; while (cunt < 2) { System.ut.println("Welcme t Java!"); cunt++; (cunt < 2) is false since cunt is 2 nw 19

20 Trace while Lp, cnt. int cunt = 0; while (cunt < 2) { System.ut.println("Welcme t Java!"); cunt++; The lp exits. Execute the next statement after the lp. Let s lk at the first cde example. Questin: hw t get the rdinal right? 20

21 Infinite lp example In this example, nthing in the lp bdy changes the value f the cntrl variable cunt = 1; // Initializes the lp cntrl variable while (cunt <= 5) { System.ut.println("The value f cunt is " + cunt); This is an infinite lp because (cunt <= 5) is always true Nthing changes the value f cunt in the lp bdy If yu accidentally create an infinite lp, use terminate buttn (red square) in Eclipse t make it stp CS4500/5500 UC. Clrad Springs

22 Placing a semicln at the end f the while-clause creates an infinite lp - be careful! int iteratin = 1; while (iteratin <= 10); { System.ut.println("Iteratin = " + iteratin); iteratin++; CS4500/5500 UC. Clrad Springs

23 Off-by-ne Errr Cmmn issue with lps: Lp bdy executes ne mre/less than expected Example: System.ut.println("I'm ging t cunt t three, ready set..."); cunt = 1; while (cunt < 3) { System.ut.println(cunt); cunt++; CS4500/5500 UC. Clrad Springs

24 Off-by-ne Errr Cmmn issue with lps: Lp bdy executes ne mre/less than expected Example: System.ut.println("I'm ging t cunt t three, ready set..."); cunt = 1; while (cunt < 3) { Output: System.ut.println(cunt); cunt++; I'm ging t cunt t three, ready set CS4500/5500 UC. Clrad Springs

25 Prblem: Repeat Additin Until Crrect See sample cde. Practice! Limit the number f trials t 5, and exit regardless if the answer if crrect r nt. 25

26 Ending a Lp with a Sentinel Value Often the number f times a lp is executed is nt predetermined. Yu may use an input value t signify the end f the lp. Such a value is knwn as a sentinel value. Write a prgram that reads and calculates the sum f an unspecified number f integers (e.g., the sum f 2, 3, 5, 7, 11 ). The input 0 signifies the end f the input. See sample cde. 26

and inaccurate results. Cnsider the fllwing cde: duble item = 1; duble sum = 0; while (item!

27 Cautin Dn t use flating-pint values fr equality checking in a lp cntrl. Since flating-pint values are apprximatins fr sme values, using them culd result in imprecise cunter values and inaccurate results. Cnsider the fllwing cde: duble item = 1; duble sum = 0; while (item!= 0) { // N guarantee item will be 0 sum += item; if (item >=0) System.ut.println("when item is " + item + ", sum is " + sum); item -= 0.1; System.ut.println(sum); 27

28 Cautin duble item = 1; duble sum = 0; while (item!= 0) { // N guarantee item will be 0 sum += item; if (item >=0) System.ut.println("when item is " + item + ", sum is " + sum); item -= 0.1; System.ut.println(sum); Item = 0.9 Item = 0.8 Item = // Values are starting t get ff Item = Item = Item = Item = Item = Item = // Off enugh t miss 0 Item = E-16 28

29 d-while Lp d { // Lp bdy; Statement(s); while (lp-cntinuatin-cnditin) The lp bdy is executed The lp cnditin - blean expressin - is evaluated If the lp cnditin is true, then lp bdy is executed again If the lp cnditin is false, cntrl is transferred t the 1st statement fllwing the lp 29

30 D While Lp Rules (same as while lp) The lp cnditin must be a blean expressin Blean expressin must be in parentheses Blean expressin is frmed using relatinal and lgical peratrs Lp cnditin Generally, sme statement befre the while lp "initializes" the lp cnditin t true Sme statement within the lp bdy must eventually change the cnditin t false If the cnditin is never changed t false, the prgram will be frever stuck in the lp This is called an "infinite lp" Curly braces are nt necessary if nly ne statement in lp but best practice is t always include curly braces CS4500/5500 UC. Clrad Springs

31 Nte Recall hw placing a semicln at the end f the while-clause creates an infinite lp int iteratin = 1; while (iteratin <= 10); { // Unnecessary semicln System.ut.println("Iteratin = " + iteratin); iteratin++; CS4500/5500 UC. Clrad Springs

32 Nte In the case f d-while yu must include the semicln since it ends the lp! int iteratin = 1; d { iteratin++; System.ut.println("Iteratin = " + iteratin); while (iteratin <= 5); // Necessary semicln CS4500/5500 UC. Clrad Springs

33 Lp Design Strategies Fur steps when writing a lp. Step 1: Identify what statements need t be repeated Step 2: Wrap these statements in a lp using while r d while: while (true) { Statements; Step 3: Determine what cnditin the cde shuld check (replace true) Step 4: Add cde in the bdy that eventually causes the cnditin t becme false while (lp-cntinuatin-cnditin) { Statements; Additinal statements fr cntrlling the lp; CS4500/5500 UC. Clrad Springs

34 Annuncement HW3 psted Will pst practice questins fr midterm sn Als imprtant things fr the exam N hmewrk next week CS4500/5500 UC. Clrad Springs

35 Review If statements (nested) The else clause matches the mst recent if clause CS4500/5500 UC. Clrad Springs

36 Review If statements (nested) T frce the else clause t match the first if clause, must add a pair f braces int i = 1, j = 2, k = 3; if (i > j) { else if (i > k) System.ut.println("A"); System.ut.println("B"); CS4500/5500 UC. Clrad Springs

37 Review Switch statement True r false? Break is required in a switch statement While and d while lps Which lp is executed at least nce? Is this an infinite lp? Why (nt)? int cunt = 1; while (cunt <= 5) { System.ut.println("The value f cunt is " + cunt); CS4500/5500 UC. Clrad Springs

38 Review Is this an infinite lp? Why (nt)? int cunt = 1; while (cunt <= 5); { System.ut.println("The value f cunt is " + cunt); cunt++; CS4500/5500 UC. Clrad Springs

39 Review Is this an infinite lp? Why (nt)? int cunt = 1; while (cunt <= 5); { System.ut.println("The value f cunt is " + cunt); cunt++; int cunt = 1; d { cunt++; System.ut.println( Cunt = " + cunt); while (cunt<= 5); CS4500/5500 UC. Clrad Springs

40 Overview While lp D while lp (Ging ver hw3 hw t design a lp) Fr lp CS1150 UC. Clrad Springs

41 Fr lps Fr lps are used t execute a lp a preset number f times fr ( 1 initial-actin; 2 lp-cnditin; 4 actin-after-each-iteratin) { 3 Statement(s); 1. The cntrl variable is initialized t a start value 2. The cntrl variable is cmpared t the lp cnditin 3. If the cnditin is true, then lp bdy is executed 4. The cntrl variable is updated after lp bdy cmpletes 5. Steps 2-4 are repeated as lng as the lp cnditin is true CS4500/5500 UC. Clrad Springs

42 fr Lps fr (initial-actin; lp-cntinuatincnditin; actin-after-each-iteratin){ // lp bdy; Statement(s); int i; fr (i = 0; i < 100; i++) { System.ut.println( "Welcme t Java!"); 42

43 Rules f fr lps The cntrl structure f the fr-lp needs t be in parentheses fr (i=0; i<= 2; i++) { statements; The lp cnditin (i <= 2) must be a blean expressin The cntrl variable (i): nt recmmended t be changed within the fr-lp bdy Curly braces are nt necessary if nly ne statement in lp Best practice is t always include curly braces CS4500/5500 UC. Clrad Springs

44 Trace fr Lp int i; fr (i = 0; i < 2; i++) { System.ut.println("Welcme t Java!"); Declare i 44

45 Trace fr Lp, cnt. int i; fr (i = 0; i < 2; i++) { System.ut.println("Welcme t Java!"); Execute initializer i is nw 0 45

46 Trace fr Lp, cnt. int i; fr (i = 0; i < 2; i++) { System.ut.println( "Welcme t Java!"); (i < 2) is true since i is 0 46

47 Trace fr Lp, cnt. int i; fr (i = 0; i < 2; i++) { System.ut.println( "Welcme t Java!"); Print Welcme t Java 47

48 Trace fr Lp, cnt. Execute adjustment statement int i; i nw is 1 fr (i = 0; i < 2; i++) { System.ut.println("Welcme t Java!"); 48

49 Trace fr Lp, cnt. int i; fr (i = 0; i < 2; i++) { System.ut.println("Welcme t Java!"); (i < 2) is still true since i is 1 49

50 Trace fr Lp, cnt. int i; fr (i = 0; i < 2; i++) { System.ut.println("Welcme t Java!"); Print Welcme t Java 50

51 Trace fr Lp, cnt. Execute adjustment statement int i; i nw is 2 fr (i = 0; i < 2; i++) { System.ut.println("Welcme t Java!"); 51

52 Trace fr Lp, cnt. int i; fr (i = 0; i < 2; i++) { System.ut.println("Welcme t Java!"); (i < 2) is false since i is 2 52

53 Trace fr Lp, cnt. Exit the lp. Execute the next int i; statement after the lp fr (i = 0; i < 2; i++) { System.ut.println("Welcme t Java!"); Let s lk at the cde example. 53

54 Nte Yu may declare the cntrl variable utside/within the fr-lp fr (int j = 0; j <= 5; j++) { System.ut.println ("Fr lp iteratin = " + j); int j; fr (j = 0; j <= 5; j++) { System.ut.println ("Fr lp iteratin = " + j); Nte n variable scpe (the area a variable can be referenced) Declaring cntrl variable befre the fr lp cause its scpe t be inside and utside frlp Declaring the cntrl variable in the fr-lp causes its scpe t be nly inside the fr lp If I tried t use the variable j utside the fr-lp - errr CS4500/5500 UC. Clrad Springs

55 Nte The initial-actin in a fr lp can be a list f zer r mre cmma-separated expressins. The actin-after-eachiteratin in a fr lp can be a list f zer r mre cmmaseparated statements. Therefre, the fllwing tw fr lps are crrect. They are rarely used in practice, hwever. fr (int i = 1; i < 100; System.ut.println(i++)); fr (int i = 0, j = 0; (i + j < 10); i++, j++) { // D smething 55

56 Nte If the lp-cntinuatin-cnditin in a fr lp is mitted, it is implicitly true. Thus the statement given belw in (a), which is an infinite lp, is crrect. Nevertheless, it is better t use the equivalent lp in (b) t avid cnfusin: fr ( ; ; ) { // D smething (a) Equivalent while (true) { // D smething (b) Eclipse nly allwed this t happen if there are n statements fllwing the fr- lp If statements fllwed the lp, Eclipse places an errr "unreachable cde" n the statement fllwing the lp 56

57 Cautin Adding a semicln at the end f the fr clause befre the lp bdy is a cmmn mistake, as shwn belw: fr (int i=0; i<10; i++); { Lgic Errr System.ut.println("i is " + i); 57

58 Cautin, cnt. Similarly, the fllwing lp is als wrng: int i=0; Lgic Errr while (i < 10); { System.ut.println("i is " + i); i++; In the case f the d lp, the fllwing semicln is needed t end the lp. int i=0; d { System.ut.println("i is " + i); i++; while (i<10); Crrect 58

59 Which Lp t Use? The three frms f lp statements, while, d-while, and fr, are expressively equivalent; that is, yu can write a lp in any f these three frms. Fr example, a while lp in (a) in the fllwing figure can always be cnverted int the fllwing fr lp in (b): while (lp-cntinuatin-cnditin) { // Lp bdy (a) Equivalent fr ( ; lp-cntinuatin-cnditin; ) // Lp bdy (b) A fr lp in (a) in the fllwing figure can generally be cnverted int the fllwing while lp in (b): fr (initial-actin; lp-cntinuatin-cnditin; actin-after-each-iteratin) { // Lp bdy; (a) Equivalent 59 initial-actin; while (lp-cntinuatin-cnditin) { // Lp bdy; actin-after-each-iteratin; (b)

60 Recmmendatins Use the ne that is mst intuitive and cmfrtable. In general, a fr lp may be used if the number f repetitins is knwn, as, fr example, when yu need t print a message 100 times. A while lp may be used if the number f repetitins is nt knwn, as in the case f reading the numbers until the input is 0. A d-while lp can be used t replace a while lp if the lp bdy has t be executed befre testing the cntinuatin cnditin. 60

61 Practice! Predicting tuitin The tuitin fr a university is currently $10,000. It will increase by 7% each year In hw many years will the tuitin be dubled? Which lp t use? Statement(s) t be repeated? Cnditin t check? CS4500/5500 UC. Clrad Springs

62 Fahrenheit2CelsiusFrLp example Hw t design using a lp? Can we rewrite this using a while lp? Let s g ver hw3 Break dwn a big prblem int smaller pieces CS4500/5500 UC. Clrad Springs

63 RandmNumbers example Math.randm() Returns a duble value, greater than r equal t 0.0 and less than 1.0, i.e., [0.0, 1.0) duble randmduble; randmduble = Math.randm(); // [0.0, 1.0) Hw t generate a randm duble in [0.0, 10.0)? randmduble = Math.randm() * 10; Hw t generate a randm integer in [0, 10)? int randmint; randmint = (int) (Math.randm() * 10); CS4500/5500 UC. Clrad Springs

64 RandmNumbers example Math.randm() Hw t generate a randm integer between [lwer, upper)? Example: int lwer=100, upper=120; randmduble = Math.randm(); // [0.0, 1.0) randmduble = randmduble * (upper-lwer); // [0.0, 20.0) randmduble = lwer + randmduble; // [100.0, 120.0) randmint = (int) randmduble; // cast duble à int Or in ne step randmint = (int) (lwer + Math.randm() * (upper-lwer)); CS4500/5500 UC. Clrad Springs

65 Nested Lps If statements can be nested If (blean expressin) { if(blean expressin) { statement(s); // end f inner if // end f uter if Lps can be nested t CS4500/5500 UC. Clrad Springs

66 Nested Lps While lp within a while lp int uter = 1; while (uter < 3) { System.ut.println ("The uter lp iteratin is = " + uter); //inner lp: It will d all iteratins befre uter lp des anther iteratin int inner = 1; while (inner <= 5) { System.ut.println (" The inner lp iteratin is = " + inner); inner++; uter++; CS4500/5500 UC. Clrad Springs

67 Nested Lps Fr lp within a fr lp fr (int uter = 1; uter < 3; uter++) { System.ut.println ("The uter lp iteratin is = " + uter); // This is the inner fr lp // It will d all iteratins befre uter lp des anther iteratin fr (int inner = 1; inner <= 5; inner++) { System.ut.println (" The inner lp iteratin is = " + inner); CS4500/5500 UC. Clrad Springs

68 Nested Lps Prblem: Write a prgram that uses nested fr lps t print a multiplicatin table. What if I nly want lwer half f the table? 68

69 Using break and cntinue Break in lps Used "break" in switch statements t end a case Can be used in a lp t terminate a lp Breaks ut f lp Cntinue in lps Used t end current iteratin f lp Prgram cntrl ges t end f lp bdy CS4500/5500 UC. Clrad Springs

70 break public class TestBreak { public static vid main(string[] args) { int sum = 0; int number = 0; while (number < 20) { number++; sum += number; if (sum >= 100) break; System.ut.println("The number is " + number); System.ut.println("The sum is " + sum); 70

71 cntinue public class TestCntinue { public static vid main(string[] args) { int sum = 0; int number = 0; while (number < 20) { number++; if (number == 10 number == 11) cntinue; sum += number; System.ut.println("The sum is " + sum); 71

72 Guessing Number Prblem Revisited Let s lk at AdditinQuiz (randm number + break example) 72

73 Using break and cntinue There are cases when the use f a break r cntinue in a lp is necessary Overusing r imprper use can make prgram hard t read, maintain, and debug. Best practice is t never use these unless yu have a specific situatin CS4500/5500 UC. Clrad Springs

74 Summary While lp D while lp Fr lp Breack and cntinue CS1150 UC. Clrad Springs

CS1150 Principles of Computer Science Midterm Review

CS1150 Principles of Computer Science Midterm Review CS1150 Principles f Cmputer Science Midterm Review Yanyan Zhuang Department f Cmputer Science http://www.cs.uccs.edu/~yzhuang CS1150 UC. Clrad Springs Office hurs 10/15, Mnday, 12:05 12:50pm 10/17, Wednesday