Decision Structures The IfElse If Statement Nested If

Decision Structures: The If-Else If Statement, Nested If Statements, Logical Operators, and String Comparison CS 0007: Introduction to Computer Programming

Review �In a decision structure’s simplest form certain statements are executed only when… � a specific condition exists. �It is said that the statements inside of the decision structure are… �conditionally executed. �Relational Operators determine whether… �a specific relationship exist between two values. �Some relational operators in Java are… �> , < , >= , <= , == , !=

Review � General for of an if statement: if(Boolean. Expression) statement; if(Boolean. Expression) { statement 1; statement 2; . . . } � A Flag is… � a boolean variable that signals when some condition exists in a program. � When Java compares characters, it compares the character’s… � Unicode values

Review �General form of an if-else statement: if(Boolean. Expression) statement or block 1 else statement or block 2

The if-else if Statement � Sometimes you need to be able to test a series of conditions � You can do this with the if-else if statement � General form: if (Boolean. Expression 1) statement or block 1 if else (Boolean. Expression 2) statement or block 2 else statement or block 3 � If Boolean. Expression 1 is true, then statement or block 1 is executed. � If Boolean. Expression 1 is false, then Boolean. Expression 2 is tested. � If Boolean. Expression 2 is true, then statement or block 2 is executed. � If Boolean. Expression 2 is false, then statement or block 3 is executed. � Note: You can have as many if else clauses as is needed.

if-else if Example �New Topics: �if-else if Statement �Using else as an error case

Nested if Statements � Nesting is enclosing one structure inside of another. � A block in Java can contain any valid Java code, this includes other if statements: if(Boolean. Expression 1) { if(Boolean. Expression 2) { statement 1; statement 2; } statement 3; statement 4; } � If Boolean. Expression 1 is true and Boolean. Expression 2 is true , what is executed? � statement 1 , statement 2 , statement 3 , statement 4 � If Boolean. Expression 1 is true and Boolean. Expression 2 is false , what is executed? � statement 3 , statement 4 � If Boolean. Expression 1 is false, what is executed? � Nothing

Nested if Statements Example �New Topics: �Nested if Statements

Logical Operators � Java provides logical operators. �The binary logical operators combine two boolean expressions into one. �The unary logical operator switches the value of a boolean expression. Operato r Meanin g Kind && AND Binar y || OR Binar y �Binary logical operators have lower precedence than ! NOTwill be. Unar relational operators (they evaluated after) y �NOT has the same precedence as negation.

Logical Operator Truth Tables �Truth Tables show the result of a logical expression based on the values of the operands. Op 1 Op 2 Op 1 && Op 2 Op 1 || Op 2 true true false true false true false false Op 1 !Op 1 true false true

Logical Operator Practice � 2 > 3 && 4 < 5 �false – first operand is false � 2 < 3 && 4 < 5 �true � 2 > 3 || 4 > 5 �false – both operands are false �!(2 > 3) �true – operand is false

Logical AND Example �New Topics: �Logical AND

Logical AND and Nesting � If we have the && operator, do we need nesting? if(Boolean. Expression 1) { if(Boolean. Expression 2) { Both conditions met } else { Both conditions NOT met } if(Boolean. Expression 1 && Boolean. Expression 2) { Both conditions met } else { Both conditions NOT met }

Logical AND and Nesting � Answer: Yes, to act if one of the conditions is not met: if(Boolean. Expression 1) { if(Boolean. Expression 2) { Both conditions met } else { Condition 2 not met } } else { Condition 1 not met } if(Boolean. Expression 1 && Boolean. Expression 2) { Both conditions met } else { Both conditions NOT met }

Exercise �Change Nested. If. Statement. java to tell the user if she does not meet one requirement, the other requirement, or BOTH. �Hint: we talked about something last lecture that will help us… �Flags

Logical OR Example �New Topics: �Logical OR

Exercise �Change Logical. Or. Operator. java to allow the user to enter capital or lower case Y or N as an answer. �Hint �Use another OR operator. �Also, check to make sure the user entered a valid response. �Y, N, y, or n.

Comparing String Objects � Imagine you have declared two String variables as such: String x = "buffalo"; String y = “bison"; � What does x == y resolve to? �false, but not for the reason you think! � The == operator compares what the reference values are (what objects they are pointing to), NOT what the value of the string is. � In some cases this causes problems � You should use methods in the String class in order to compare String variables �equals �compare. To

String equals method �To check if two strings are the same you can use the equals method. String. Reference. equals(Other. String); �If the string referred to by String. Reference is equal to the one referred to by Other. String, then true is returned. �Otherwise false is returned. �This comparison is case-sensitive ("buffalo" and "Buffalo" are not equal) �To do case-insensitive comparison, use equals. Ignore. Case

String compare. To method � If you want to determine which string is “greater” use the compare. To method. String. Reference. compare. To(Other. String); �If the string referred to by String. Reference is equal to the one referred to by Other. String, then 0 is returned. �If the string referred to by String. Reference is “less than” to the one referred to by Other. String, then a negative number is is returned. �If the string referred to by String. Reference is “greater than” to the one referred to by Other. String , then a positive number is returned. �This comparison is case-sensitive ("buffalo" and "Buffalo" are not equal) � To do case-insensitive comparison, use compare. To. Ignore. Case

String compare. To method �What does it mean for strings to be “greater than” or “less than” each other? �Java compares the two strings, character by character from left to right. �When there is a difference in a character it compares the Unicode values of the characters. �If a string is shorter than another, and the shorter one is the beginning of the longer one, the shorter one is considered less. �"hi" is less than "high"

String Comparison Example �New Topics: �String == operator �equals method �compare. To method

Block-Level Scope �If a variable is declared inside of a block, it is said to have block-level scope. �If a variable has Block-Level Scope it is in scope from its declaration to the ending of the block in which it was declared. if(Boolean. Expression) { int x; … } � The variable x has scope from the point it was declared to the } � This does not have much use now, but will be more useful when we learn loops.

The Conditional Operator � Java provides and operator to create short expressions that work like if-else statements. Boolean. Expression ? Value 1 : Value 2; �If Boolean. Expression is true, Value 1 is returned �If Boolean. Expression is false, Value 2 is returned � Example: if (score < 60) System. out. println("You Fail"); else System. out. println("You Pass"); System. out. println("You " + score < 60 ? "Fail" : "Pass");