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 ExpressionsA true, B false, C error

int x=3; int y = 4; int z=5;int x=3; int y = 4; int z=5;

x < yx < y

x < y < zx < y < z

x = yx = y

y == 4y == 4

z >= xz >= x

x != 3x != 3

(x + 4) < (y - 1)(x + 4) < (y - 1)

truetrue

error: error: < cannot be applied to boolean,int< cannot be applied to boolean,int

error: incompatible types - found int; expected boolean error: incompatible types - found int; expected boolean

truetrue

truetrue

falsefalse

7 < 3; false7 < 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 = 44 < 4 false.keep goingy <= 10 truefalse || true true! true is false4 < 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 ifIf 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 Workpublic 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 Outif (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 3B. the value of x is 0C. 3 < 4 is falseD. the value of x is 7E. 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 tableif (ave >= 90) grade = 'A';else if ((ave < 90) && (ave >= 80)) // note: need ()s around entire condition grade = 'B'; else if ((ave < 80) && (ave >=70)) grade = 'C';else if ((ave < 70) && (ave >=60)) grade = 'D';else if ((ave < 70) && (ave < 60)) grade = 'F';

Tracing through the embedded if

Fixing the embedded ifif (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

1-30 2.00  1.99 - 1.61 1.60 Or Less

31-60 2.00 1.99 - 1.70 1.69 Or Less

61-75 2.00 1.99 - 1.80 1.79 Or Less

76-90  2.00  1.99 - 1.90 1.89 Or Less

91 Or More 2.00  1.99 - 1.98 1.97 Or Less