Lec 6 Logical Operators String comparison

Review – if statement syntax OR

Logical expressions <boolean expression> is logical expression score == 100 x+y>10 ans!='y' (ans is a char) isValid (isValid is a boolean )

The 6 Comparison Operators

Sometimes we need more complicated logic example: age is between 18 and 55 months > 3 OR miles>5000 word is not "yes" (word is String)

Application: How to tell if n is in the correct range ...println("Enter a number from 1 to 10"); n = myInput.nextInt(); if (n>=1){ if (n<=10){ cout<<“OK, n is between 1 and 10!”; } else { cout<<“n is too big”; } } else{ cout<<“n is too small”;

Flowchart of previous slide

A better way using && ...println("Enter a number from 1 to 10"); n = myInput.nextInt(); if (n>=1 && n <=10){ cout<<“OK, n is between 1 and 10!”; } else{ cout<<“illegal value of n”;

The 3 Logical Operators logical operator meaning && and || or ! not example in English && and a < 3 && b > 10 a is less than 3 and b is greater than 10 || or a < 3 || b > 10 a is less than 3 or b is greater than 10 ! not !(a < 3) it is not the case that a is less than 3

Examples 1. T/F ( 3 < 2 ) && (5 > 7) 2. T/F ! ( 2 > 3 )   4. the expression: ( number > 10 && number < 40 )  is TRUE, when a)      number is larger than 40 b)      number is smaller than 10 c)      number is between 10 and 40 d) Never

Oil Change

In groups, flowchart previous slide

Primitives vs Objects Recall the primitives: int x = 3; double num = 3.42; char letter = 'Y'; boolean found = true; And objects (of Classes we've seen) Scanner myUserInput = new Scanner(System.in); String greeting = "hello";

Some differences between Primitives and Objects Objects have methods: greeting.trim(); // remove blanks before/after greeting.toUpperCase(); // convert to CAPS myUserInput.nextInt(); // get an integer Primitives just store values: int x = 3; double num = 3.42; char letter = 'Y'; boolean found = true;

Comparing Strings Normally, Strings (and objects in general) should not use the comparison operators to check values: WRONG: if ( greeting == "hello" ){ .... Use .equals( ) instead RIGHT: if (greeting.equals( "hello") { ... For strings, this is even better: if ( greeting.equalsIgnoreCase("hello"){ ...

True or false? String s1 = "hello", s2 = "HeLLo", s3 = " hello "; s1.equals("hello"); s2.equals("hello"); s3.equals("hello"); s2.equalsIgnoreCase("Hello"); s3.trim().equals("hello");

Lab 6 Department store Checkout.java Ask how many items to ring up must be 1-10 use do loop to verify correct numbers logical operators to check for too high or too low While loop repeats for as many items get item cost add to total Ask if a bag is wanted, $ 0.05 extra if only 1-3 items Add 8.25% sales tax ( total = total*1.0825)

How to avoid: total = $5.66666668 EITHER: truncate (chop) to two decimal places double d = 33.2482; int i = (int)(d*100); System.out.println(i / 100.0);   OR format (round) to two decimal places for output d = 33.2482; System.out.printf("%1.2f\n", d); // means print d with two digits after decimal // %1.2f is format, \n means go to new line after

Calculating the sum or average num k k < 10 Console int sum = 0; int k = 0, num; while ( k < 5) { num = scan.nextInt(); sum = sum + num; k = k + 1; } ...println("sum is " + sum); ...println("avg is " + sum/5.0);