Lecture 5 If Statements Last time 1 Variables and types 2 Expressions in Java 3 User input with Scanner objects Today 1 Finish Scanner 2 if statements 9 11 2006 CMSC 131 Fall 2006 Rance Cleaveland 2006 Univeristy of Maryland Objects From Example 5 Scanner sc new Scanner System in What s going on Scanner is a class defined in java util Scanner System in is a predefined object for keyboard input new Scanner System in creates a new object in the Scanner class and assigns it to sc Object sc is a variable Its type is Scanner A bundle of data instance variables and operations methods A class defines both instance variables and methods for objects A class is also a type for objects new creates new objects in the given class We will learn much more about objects later CMSC 131 Fall 2006 Rance Cleaveland 2006 University of Maryland 1 Control Flow and Conditionals Control flow the order in which statements are executed General rule top to bottom Conditional statements permit control flow to be dependent on true false conditions if if else CMSC 131 Fall 2006 Rance Cleaveland 2006 University of Maryland 2 The if Statement Form if boolean expression statement Example boolean expression condition if inchesOfSnow 7 System out println Go home statement The println statement is executed only if the variable inchesOfSnow is greater than 7 Otherwise it is skipped CMSC 131 Fall 2006 Rance Cleaveland 2006 University of Maryland 3 Java and White Space You can add carriage returns spaces tabs wherever you want in Java Properly used this makes your program easier to read and understand Proper indentation for if If statement part is short put on same line as condition Otherwise put statement part on line below indented by at least 2 spaces CMSC 131 Fall 2006 Rance Cleaveland 2006 University of Maryland 4 Example 6 public class Example6 public static void main String args Scanner sc new Scanner System in System out print Enter an integer int i sc nextInt if i 0 System out println That was a negative number System out println The number was i CMSC 131 Fall 2006 Rance Cleaveland 2006 University of Maryland 5 The if else Statement Form if condition statement 1 else statement 2 Example if inchesOfSnow 7 System out println Go home else System out println Go to school If inchesOfSnow 7 the first println statement is executed and the second is skipped Otherwise i e inchesOfSnow 7 the first println statement is skipped and the second is executed CMSC 131 Fall 2006 Rance Cleaveland 2006 University of Maryland 6 Indentation Convention for ifelse Line up if else in same column different lines If both if else statements are short put on same line as if else If at least one is long Put statements on different lines from if else Indent these statement the same amount at least 2 space from if else CMSC 131 Fall 2006 Rance Cleaveland 2006 University of Maryland 7 Example 7 public class Example7 public static void main String args Scanner sc new Scanner System in System out print Enter an integer int i sc nextInt if i 0 System out println That was a negative number else System out println That was a non negative number System out println The number was i CMSC 131 Fall 2006 Rance Cleaveland 2006 University of Maryland 8 Blocks What happens if i 10 i 10 saturate true else i i 1 Desired both i saturate are set if i 10 Actual syntax error Only one statement can be associated with if The saturate assignment statement is not part of the if The else has no if to belong to Blocks solve this problem CMSC 131 Fall 2006 Rance Cleaveland 2006 University of Maryland 9 What Blocks Are Blocks are sequences of statements glued together into one Form statement 1 statement 2 Example revisited if i 10 i 10 block saturate true else i i 1 CMSC 131 Fall 2006 Rance Cleaveland 2006 University of Maryland 10 Indentation Conventions for Blocks Either if statement statement Or if statement statement 1 2 1 2 CMSC 131 Fall 2006 Rance Cleaveland 2006 University of Maryland 11 Example 8 public class Example8 public static void main String args Scanner sc new Scanner System in System out print Enter an integer int i sc nextInt if i 0 System out println That was a negative number System out println I prefer positive ones so I ll fix it i i else System out println That was a positive number System out println That makes me happy System out println The number was i CMSC 131 Fall 2006 Rance Cleaveland 2006 University of Maryland 12 Logical Operators Used for forming more complex conditions and if temp 97 temp 99 System out println Patient is healthy or if months 3 miles 3000 System out println Change your oil not if phone equals 301 555 1212 System out println Sorry wrong number CMSC 131 Fall 2006 Rance Cleaveland 2006 University of Maryland 13 Example 9 public class Example9 public static void main String args Scanner sc new Scanner System in System out print Enter an integer from 1 to 10 int i sc nextInt if i 1 i 10 System out println Good job else System out println You didn t follow instructions CMSC 131 Fall 2006 Rance Cleaveland 2006 University of Maryland 14 Statement Constructors and Nesting if if else are statement constructors They take statement s and convert them into a new statement Example if i 1 i 10 System out println Good job else System out println Oops Two sub statements come in A single big statement if else comes out Implications if statements etc can also appear inside be nested within one another CMSC 131 Fall 2006 Rance Cleaveland 2006 University of Maryland 15 Example 10 public class Example10 public static void main String args Scanner sc new Scanner System in System out print Enter an integer int i sc nextInt if i 0 System out println I don t like negative numbers if i 100 System out println Also that one is REALLY negative else if i 100 System out println That s a small one else System out println That s a nice big number CMSC 131 Fall 2006 Rance Cleaveland 2006 University of Maryland 16 Java Eclipse and Uninitialized Variables Eclipse will complain if you try to use an uninitialized variable int i System out println i is i What is value of i This feature interacts strangely with if else statements sometimes Good programming practice always initialize new variables CMSC 131 Fall 2006 Rance Cleaveland 2006 University of Maryland 17 Example 10b public class Example10b public static void main String args int x Scanner scanner new Scanner System in String s scanner next if s equals dog x 10 System out println x is x CMSC 131 Fall
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