CSC 212 – Data Structures Lecture 5: Variables

Problem of the Day Why do underground subway stations always have more escalators going up than down?

Keys To Academic Success Cannot control class times, but can control when you wake up Complete in-class activities & homeworks  Review answers versus actual solutions  When you cannot explain differences, ask Read directions and follow them explicitly  Ask whenever something is unclear Practice, practice, practice  Do problems in book, from past classes, etc.

Variables All variables behave in similar manner  Assignment to primitive --- copy value  Assignment to reference --- alias instance But further divide variables by “lifetime”  Fields  Parameters  Locals How each of these used very different… ...but correct use makes coding & debugging simpler

Fields (redux) Fields declared in class, but not in method  By convention, declarations at start of class  Should be declared private (or protected) *  Document with javadoc or normal comments Each class instance have the fields  Field values in separate instances may * differ Objects’ fields initialized at instantiation  Initial value is 0, false, ‘’, or null

Tracing with Objects public class BankAccount { private Client owner; private float balance; private String identifier; BankAccount check = new BankAccount(haxx0r, 1.25, “0001”); BankAccount richOne = new BankAccount(billG, ,“2408”); check.balance -= 1.20; // Bought a coffee check.identifier = “2408”; // 1 st theft attempt richOne.owner = haxx0r; // 2 nd theft attempt owner balance  1.25 identifier  “0001” check haxx0r billG owner balance  identifier  “2408” richOne  0.05  “2408”

Parameters Variables named in method description  Created & initialized each time method called  “Live” until method completes Initialized with value of argument  Primitives  copy value into parameter  References  parameter aliases argument Documented as part of method comments  Done paramName paramDescript

Tracing Methods & Parameters Every method call, draw new frame  Write parameters in at top of frame  (Include this as a parameter) *  Initialize parameters using arguments Frame only used to trace that single call “Erase” frame when call completes/returns  Usually just cross it out to mark as done

Tracing Parameters public void badTransfer(BankAccount source, float amount) { if (amount >= source.balance) { amount = source.balance(); source = null; } else { source.balance -= amount; } balance += amount; } check.badTransfer(richOne, ); owner balance  0.05 identifier  “2408” check haxx0r owner balance  identifier  “2408” richOne this source amount  balance  

Tracing Parameters public void badTransfer(BankAccount source, float amount) { if (amount >= source.balance) { amount = source.balance(); source = null; } else { source.balance -= amount; } balance += amount; } check.badTransfer(richOne, Float.MAX_VALUE); owner balance  identifier  “2408” check haxx0r owner balance  identifier  “2408” richOne this source amount  ~3.4·10 38  

Locals Variables declared inside a method  Required to assigned value before using  “Live” only for code block in which declared Cannot be included in javadoc  Local variables help a method complete  Only comment as part of describing how method works

Tracing Locals Add local in frame for current call to method “Erase” local once the code block completes  Code block is one or more statements enclosed by braces

Code Blocks public void goodTransfer(BankAccount source, float amount) { if (amount >= source.getBalance) { amount = source.getBalance(); } else { source.balance -= amount; } balance += amount; for (int attempt = 0; attempt < 4; attempt++) { if (source.balance <= amount) { source.balance -= amount; break; } } }

Your Turn Get back into groups and try daily activity

Before Next Lecture… Finish week #2 homework Finish lab #1 Monday’s lecture continues with the difference between static and non-static data  Keep thinking & ask any questions you have  May want to review any old notes & handouts

Before Next Lecture… Finish week #2 homework Finish lab #1 Monday’s lecture continues with difference between static and non-static fields and methods  Keep thinking & ask any questions you have  May want to review any old notes & handouts