if statements Chapter 3

Selection Want to be able to do a statement sometimes, but not others if it is raining, wear a raincoat. Start first with how to make a condition

if statements if (booleanexpression) java statement; any Java statement you know about we don’t know about this

Boolean Expressions Evaluate to be true or false boolean variables –boolean isVisible = false; relational expressions (compares values) logical expressions (compares expressions with and, or, not)

Comparing Values, >=, <= != == 3 == 4 false 4 == 4 true 3 < 4 true 3 <= 4 true 3 >= 4 false 3 > 4 false These do not work for object types

Evaluating Boolean Expressions A true, B false, C error int x=3; int y = 4; int z=5; x < y x < y < z x = y y == 4 z >= x x != 3 (x + 4) < (y - 1) true error: < cannot be applied to boolean,int error: incompatible types - found int; expected boolean true true false 7 < 3; false

Logical Operators and (&&, single & very different) –both values must be true for the expression to be true –if it is cold and rainy, wear your winter raincoat (both must be true) or (|| - on keyboard, called pipe symbol) –either value can be true –if it is cold or rainy, wear a coat (if either or both is true, do) not (!) –changes the truth value of the expression –if it is not cold, do not wear a winter coat

Logical Operators A true, B false, C error int x=3; int y=10; (x < y) && (y < 20) (x == 3) || (y == 3) x < y; 3 < 10; true y < 20; 10 < 20; true true && true is true x == 3 true. short circuit evaluation (y==3 false true || false is true)

More logical operators A true, B false, C error int x=3; int y=10; !(y=10) (x != 3) || (y != 3) trick question error !(y==10) y == 10 true !true false x != 3 false Keep going. y != 3 true false || true is true

Yet more logical operators A true, B false, C error int x=3; int y=10; !((x+1 < 4) || (y <= 10)) !((x+1 < 4) && (y <= 10)) x+1 = 4 4 < 4 false.keep going y <= 10 true false || true true ! true is false 4 < 4 false. DONE with &&. Do not look at y <=10. !false true

if statements if statement form: –if (boolean expression) java statement; if (x < y) System.out.println(“x < y”); you know both parts now

if statements cautions MUST have ( )s around boolean expression no syntax error for non-boolean like expressions only ONE statement in an if statement no ';' after if condition Make sure you account for values that are equal use relational operators only with primitives use equals, compareTo with String

Write on board Write an if statement that prints "Before Middle" if int x is less than int middle Write an if statement that assigns 0 to average (double) if numberCounted (an int) is 0

if-else If you want to do one thing if a condition is true and something else if not, use if-else. –form: if (condition) Java statement else Java statement if (x < y) System.out.println(x + " is less than the other number"); else System.out.println(y + " is less than the other number"); What's wrong with this logic?

> one statement in an if If you want to have more than one statement inside an if or an else, use {}s: if (x < y) { System.out.println(x + " is less than the other number”); x = 0; } else { System.out.println(y + " is less than the other number”); y = 0; }

If-else cautions either if clause or else clause or both may have {}s. After statements inside if and else clause are executed, control passes back to next sequential statement no ';' after else Make sure you account for values that are equal

Computer Work public class MyTwoInts { public static void main(String[] args) { java.util.Scanner kbd = new java.util.Scanner (System.in); System.out.println(“Enter two numbers: “); int x = kbd.nextInt( ); int y = kbd.nextInt( ); // Write CODE IN HERE TO PRINT THE 2 // numbers IN ORDER // THE SMALLEST FIRST }

Watch Out if (3 < 4) x = 3; else System.out.println(“3 < 4 is false”); x = 7; System.out.println( "the value of x is " + x); Prints what? A. the value of x is 3 B. the value of x is 0 C. 3 < 4 is false D. the value of x is 7 E. none of the above

Embedded ifs If statements and if-else statements may be embedded (if within if). simply evaluate them as the Java code is executed: if-else example is most common. sets up a table of conditions

Embedded if-else for table if (ave >= 90) grade = 'A'; else if ((ave = 80)) // note: need ()s around entire condition grade = 'B'; else if ((ave =70)) grade = 'C'; else if ((ave =60)) grade = 'D'; else if ((ave < 70) && (ave < 60)) grade = 'F';

Tracing through the embedded if

Fixing the embedded if if (ave >= 90) grade = 'A'; else if (ave >= 80) // We know (ave < 90) or we wouldn't be here grade = 'B'; else if (ave >=70) // we know ave < 80 grade = 'C'; else if (ave >=60) grade = 'D'; else // if ((ave < 70) && (ave < 60)) grade = 'F';

Cautions for embedded ifs Don't use redundant comparisons Make sure you check for values that are equal Account for out of range values Embedded ifs can be implemented using switch statements. We will not go over these or test you on them. But switch statements are in the book.

1.Write a program to read a day (1 for Monday, 2 for Tuesday, etc. to 5 for Friday), then print “yipee! Today is CPSC150.” for days 1, 3 and 5; or “Only one more day until CPSC150!” for days 2 and 4. 2.Write a program to read an integer, and print “yes” if it is even and “no” otherwise. 3.Write a program that prints a message about academic status according to this table. GPA and attempted hours will be read from the keyboard. Credit Hours Attempted * Minimum Good Standing Academic Probation Academic Suspension Or Less Or Less Or Less Or Less 91 Or More Or Less