CS1150 Principles of Computer Science Boolean, Selection Statements (Part II)

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1 CS1150 Principles f Cmputer Science Blean, Selectin Statements (Part II) Yanyan Zhuang Department f Cmputer Science CS1150

2 Review What s the scientific ntatin f 9,200,000? 9.2e6 What s the value f a after each statement? int a = 5; a /= 3; // a =1 a--;

3 Review Lgical peratrs!, &&,, ^ () Mixing peratrs: p1 && p2 p3, where p1 - p3 are blean variables x + y < 10 && x/y == 3 z!= 10 can be written as } ((x + y < 10) && (x/y == 3)) z!= 10!z x/y==0 && x > 12 Hw t test if x is between 10 and 20? } Can t write 10 <= x <= 20

4 Mre n ++ and -- Placement f prefix r pstfix cause different results when in expressins s be careful!!! int x = 1; int y = 3; y = ++x; // d increment then assignment => y is 2, x is 2 int x = 1; int y = 3; y = x++; // d assignment then increment => y is 1, x is 2

5 Mre n ++ and -- Simple (but cnfusing) example int numdays = 100; System.ut.println (numdays); System.ut.println (++numdays); System.ut.println (numdays++); System.ut.println (numdays); Recmmendatin: avid ++ and -- in expressins (t avid cnfusin, make cde mre readable); used in islatin is nt a prblem

6 One-way if Statements if (blean-expressin) { statement(s); }

7 Tw-way if Statement if (blean-expressin) { statement(s)-fr-the-true-case; } else { statement(s)-fr-the-false-case; }

8 What s wrng with this cde? Scanner input = new Scanner (System.in); int mathgrade = input.nextint(); String mathlettergrade = ""; // "" is an empty String if (mathgrade >= 90) mathlettergrade = "A"; System.ut.println ("Yu gt an A");

9 What s wrng with this cde? Scanner input = new Scanner (System.in); int mathgrade = input.nextint(); String mathlettergrade = ""; // "" is an empty String if (mathgrade >= 90); { } mathlettergrade = "A";

10 What s wrng with this cde? Scanner input = new Scanner (System.in); int mathgrade = input.nextint(); if (mathgrade = 90) { } System.ut.println ("Yu gt 90 in yur test!");

11 Prblem: A Math Learning Tl Create a prgram Subtractin.java t teach a first grade student subtractins. The prgram prmpts the student fr tw integers number1 and number2, and displays a questin such as What is 9 2? t the student. After the student types the answer, the prgram displays whether the answer is crrect r nt. 11

12 Multiple Alternative if Statements if (scre >= 90.0) System.ut.print("A"); else if (scre >= 80.0) System.ut.print("B"); else if (scre >= 70.0) System.ut.print("C"); else if (scre >= 60.0) System.ut.print("D"); else System.ut.print("F"); Equivalent This is better if (scre >= 90.0) System.ut.print("A"); else if (scre >= 80.0) System.ut.print("B"); else if (scre >= 70.0) System.ut.print("C"); else if (scre >= 60.0) System.ut.print("D"); else System.ut.print("F"); (a) (b)

13 Prblem: Bdy Mass Index Bdy Mass Index (BMI) is a measure f health n weight. It can be calculated by taking yur weight in kilgrams and dividing by the square f yur height in meters. The interpretatin f BMI fr peple 16 years r lder is as fllws: BMI Interpretatin BMI < 18.5 Underweight 18.5 <= BMI < 25.0 Nrmal 25.0 <= BMI < 30.0 Overweight 30.0 <= BMI Obese 13

14 If statement ntes The else clause matches the mst recent if clause What s the utput? CS1150

15 If statement ntes 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) { } if (i > k) else System.ut.println("A"); System.ut.println("B"); CS1150

16 If statement summary Blck statements {} can be mitted if there is nly ne statement Recmmendatin Use curly braces: it helps make the cde easier t mdify and less errr prne Avid deeply nested if statements An else" clause always belngs with the mst recent if clause CS1150

17 Practice! Calculating incme tax Penguin state s incme tax rule is as fllws If the incme is lwer than $2300, the tax rate is 0 If the incme is [$2300, $3400), the tax rate is 0.14 If the incme is [$3400, $4400), the tax rate is 0.16 If the incme is [$4400, $6500), the tax rate is 0.2 If the incme is abve $6500, tax rate is flat 0.3 The cde prmpts the user fr an incme, and prints ut the incme tax (incme*tax rate) Implement this using multiple alternative if statement

18 switch Statements Select a path f executin frm a grup f pssibilities Example: calculate tax returns fr different status switch (status) { case 0: cmpute taxes fr single filers; break; case 1: cmpute taxes fr married file jintly; break; case 2: cmpute taxes fr married file separately; break; case 3: cmpute taxes fr head f husehld; break; default: System.ut.println("Errrs: invalid status"); } 18

19 switch Statement Flw Chart 19

20 switch Statements Hw des it wrk? The switch "expressin (e.g., status) is evaluated If ne f the cases "values" matches the value f the switch "expressin" } The statements fr that case are executed } Executin stps when a break statement is reached, r the end f the switch statement is reached If n case "values" match the value f the switch "expressin" } A default case is executed if it exists switch (switch-expressin) { } case value1: statement(s)1; break; case value2: statement(s)2; break; case valuen: statement(s)n; break; default: statement(s)-fr-default; 20

21 switch Statement Rules The switch-expressin must yield a value f char, byte, shrt, r int type and must always be enclsed in parentheses. The value1,..., and valuen must have the same data type as the value f the switch-expressin. The resulting statements in the case statement are executed when the value in the case statement matches the value f the switchexpressin. switch (switch-expressin) { case value1: statement(s)1; break; case value2: statement(s)2; break; case valuen: statement(s)n; break; default: statement(s)-fr-default; } Nte: value1,..., and valuen are cnstant expressins, meaning that they cannt cntain variables in the expressin, such as 1 + x. 21

22 switch Statement Rules The keywrd break is ptinal, but it shuld be used at the end f each case in rder t terminate the remainder f the switch statement. If the break statement is nt present, the next case statement will be executed. The default case, which is ptinal, can be used t perfrm actins when nne f the specified cases matches the switch-expressin. switch (switch-expressin) { } case value1: statement(s)1; break; case value2: statement(s)2; break; case valuen: statement(s)n; break; default: statement(s)-fr-default; When the value in a case statement matches the value f the switch-expressin, the statements starting frm this case are executed until either a break statement r the end f the switch statement is reached. 22

23 switch Statement Ntes switch expressin Must evaluate t a value f type char, byte, shrt, int } switch (x > 1) // Nt allwed - evaluates t a blean value } switch (x == 2) // Nt allwed - anther blean expressin

24 switch Statement Ntes Case values Are cnstants expressins Cannt cntain variables } case 0: system.ut.println("..."); // valid } case (x+1): system.ut.println("..."); // nt valid Thugh this is valid way t write the cases int value = 3; switch (value) { case 1:case 2:case 3: System.ut.println("case 1, 2, and 3"); break; case 4: System.ut.println("case 4"); break; default: System.ut.println("default"); }

25 switch Statement Ntes break statement int day = 3; switch (day) { case 1: case 2: case 3: case 4: case 5: System.ut.println("Weekday"); break; case 0: case 6: System.ut.println("Weekend"); }

26 Practice! Day f the week Write a prgram that prmpts the user t enter a number between 0 and 6, and displays the name f the day (e.g., 0 fr Sunday, 1 fr Mnday, etc.) Please use a switch statement t implement this

27 Cnditinal Expressins Shrtcut way t write a tw-way if statement (if-else) Cnsists f the symbls? and : (aka the "ternary" peratr) result = expressin? value1 : value2 } expressin can be either a blean value r a statement that evaluates t a blean value } The cnditinal "expressin" is evaluated } If the expressin is true, value1 is returned } If the expressin is false, value2 is returned CS1150

28 Cnditinal Expressins if (a < 0) { absvalue = -a; } else { absvalue = a; } Can be re-written as: absvalue = (a < 0)? -a : a; CS1150

29 Summary Blean data type If statement Switch statement CS1150

CS1150 Principles of Computer Science Boolean, Selection Statements (Part II)

CS1150 Principles of Computer Science Boolean, Selection Statements (Part II) CS1150 Principles f Cmputer Science Blean, Selectin Statements (Part II) Yanyan Zhuang Department f Cmputer Science http://www.cs.uccs.edu/~yzhuang CS1150 Review What s the scientific ntatin f 9,200,000?