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

Transcription

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? What s the value f a after each statement? int a = 5; a /= 3; a--;

3 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--; // a = 0

4 Review Lgical peratrs!, &&,, ^ Mixing peratrs: p1 && p2 p3, where p1, p2, 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

5 Overview If statement Switch statement

6 If Statement Select a path f executin based n a cnditin A cnditin is evaluated (i.e. blean expressin) If the cnditin is true The statements in the true branch are executed If the cnditin is false The statements in false branch are executed (if there is an "else") CS1150

7 One-way if Statements if (blean-expressin) { statement(s); Example: Blean2.java, Even1.java

8 Tw-way if Statement if (blean-expressin) { statement(s)-fr-the-true-case; else { statement(s)-fr-the-false-case; Example: Even2.java

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"; System.ut.println ("Yu gt an A");

10 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";

11 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!");

12 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; case 1: cmpute taxes fr married file jintly; case 2: cmpute taxes fr married file separately; case 3: cmpute taxes fr head f husehld; default: System.ut.println("Errrs: invalid status"); 12

13 switch Statement Flw Chart 13

14 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; case value2: statement(s)2; case valuen: statement(s)n; default: statement(s)-fr-default; 14

15 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; case value2: statement(s)2; case valuen: statement(s)n; 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. 15

16 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 fllwing case statements 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; case value2: statement(s)2; case valuen: statement(s)n; 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. 16

17 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

18 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 This is valid way t write cases int value = 3; switch (value) { case 1:case 2:case 3: System.ut.println("case 1, 2, and 3"); case 4: System.ut.println("case 4"); default: System.ut.println("default");

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

20 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

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

22 Summary Blean data type If statement Switch statement CS1150