5. using variables, data, expressions and constants

Correct Use of Variables, Data, Expressions and Constants Correctly Organizing Data and Expressions Svetlin Nakov Technical Trainer www.nakov.com Telerik Software Academy academy.telerik.com

Table of Contents  Principles for Initialization  Scope, Lifetime, Span  Using Variables  Variables Naming  Naming convention  Standard Prefixes  Using Expressions  Using Constants 2

Principles for Initialization Best Practices

Initially Assigned Variables in C#  Static variables  Instance variables of class instances  Instance variables of initially assigned struct variables  Array elements  Value parameters  Reference parameters  Variablesdeclared in a catch clause or a foreach statement 4

Initially Unassigned Variables in C#  Instance variables of initially unassigned struct variables  Output parameters  Including the this variable of struct instance constructors  Local variables  Except those declared in a catch clause or a foreach statement 5

Guidelines for Initializing Variables  When the problems can happen?  The variable has never been assigned a value  The value in the variable is outdated  Part of the variable has been assigned a value and a part has not  E.g. Student class has initialized name, but faculty number is left unassigned  Developing effective techniques for avoiding initialization problems can save a lot of time 6

Variable Initialization  Initialize all variables before their first usage  Local variables should be manually initialized  Declare and define each variable close to where it is used  This C# code will result in compiler error: int value; Console.WriteLine(value);  We can initialize the variable at its declaration: int value = 0; Console.WriteLine(value); 7

Variable Initialization (2)  Pay special attention to counters and accumulators  A common error is forgetting to reset a counter or an accumulator int sum = 0; for (int i = 0; i < array.GetLength(0); i++) { for (int j = 0; j < array.GetLength(1); j++) { sum = sum + array[i, j]; The sum must be reset after } the end of the inner for loop Console.WriteLine( "The sum of the elements in row {0} is {1}", sum); } 8

Variable Initialization (3)  Check the need for reinitialization  Make sure that the initialization statement is inside the part of the code that’s repeated  Check input parameters for validity  Before you assign input values to anything, make sure the values are reasonable int input; bool validInput = int.TryParse(Console.ReadLine(), out input); if (validInput) { … 9

Partially Initialized Objects  Ensure objects cannot get into partially initialized state  Make all fields private and require valid values for all mandatory fields in all constructors  Example: Student object is invalid unless it has Name and FacultyNumber class Student { private string name, facultyNumber; public Student(string name, string facultyNumber) { … } } 10

Variables – Other Suggestions  Don't define unused variables  Compilers usually issues warnings  Don't use variables with hidden purpose  Incorrect example: int mode = 1; … if (mode == 1) …; // Read if (mode == 2) …; // Write if (mode == 3) …; // Read and write  Use enumeration instead: enum ResourceAccessMode { Read, Write, ReadWrite } 11

Retuning Result from a Method  Always assign the result of a method in some variable before returning it. Benefits:  Improved code readability  The returned value has self-documenting name  Simplified debugging The intent of the  Example: formula is obvious int salary = days * hoursPerDay * ratePerHour; return salary; We can put a breakpoint at this line  Incorrect example: and check if the result is correct return days * hoursPerDay * ratePerHour; 12

Scope of Variables  Scope – a way of thinking about a variable’s celebrity status  How famous is the variable?  Global (static), member variable, local  Most famous variables can be used anywhere, less famous variables are much more restricted  The scope is often combined with visibility  In C# and Java, a variable can also be visible to a package or a namespace 14

Visibility of Variables  Variables' visibility is explicitly set restriction regarding the access to the variable  public, protected, internal, private  Always tryto reduce maximally the variables scope and visibility  This reduces potential coupling  Avoid public fields (exception: readonly / const in C# / static final in Java)  Access all fields through properties / methods 15

Exceeded Scope – Example public class Globals { public static int state = 0; } public class ConsolePrinter { public static void PrintSomething() { if (Globals.state == 0) { Console.WriteLine("Hello."); } else { Console.WriteLine("Good bye."); } } } 16

Span of Variables  Variable span  The of lines of code (LOC) between variable usages  Average span can be calculated for all usages  Variable span should be kept as low as possible  Define variables at their first usage, not earlier  Initialize variables as late as possible  Try to keep together lines using the same variable Always define and initialize variables just before their first use! 17

Calculating Span of Variable 1 a = 1; 2 b = 1; 3 c = 1; span = 1 4 b = a + 1; 5 b = b / c; span = 0  One line between the first reference to b and the second  There are no lines between the second reference to b and the third  The average span for b is (1+0)/2 = 0.5 18

Variable Live Time  Variable live time  The number of lines of code (LOC) between the first and the last variable usage in a block  Live time should be kept as low as possible  The same rules apply as for minimizing span:  Define variables at their first usage  Initialize variables just before their first use  Try to keep together lines using the same variable 19

Measuring the Live Time of a recordIndex Variable ( line 28-line 25 + 1 ) = 4 25 recordIndex = 0; 26 while (recordIndex < recordCount) { 27 ... 28 recordIndex = recordIndex + 1; ... 62 total = 0; total 63 done = false; ( line 69-line 62 + 1 ) = 8 64 while ( !done ) { ... 69 if ( total > projectedTotal ) { done 70 done = true; ( line 70-line 63 + 1 ) = 8  Average live time for all variables:  (4+8+8)/3≈7 20

Unneeded Large Variable Span and Live Time 1 int count; 2 int[] numbers = new int[100]; 3 for (int i = 0; i < numbers.Length; i++) 4 { 5 numbers[i] = i; 6 } 7 count = 0; live time 8 for (int i = 0; i < numbers.Length / 2; i++) = 19 9 { 10 numbers[i] = numbers[i] * numbers[i]; 11 } 12 for (int i = 0; i < numbers.Length; i++) span = 13 { (5+8+2) 14 if (numbers[i] % 3 == 0) /3=5 15 { 16 count++; 17 } 18 } 19 Console.WriteLine(count); 21

Reduced Span and Live Time 1 int[] numbers = new int[100]; 2 for (int i = 0; i < numbers.Length; i++) 3 { 4 numbers[i] = i; 5 } 6 for (int i = 0; i < numbers.Length / 2; i++) 7 { 8 numbers[i] = numbers[i] * numbers[i]; 9 } 10 int count = 0; 11 for (int i = 0; i < numbers.Length; i++) 12 { 13 if (numbers[i] % 3 == 0) live time = 9 14 { 15 count++; span= 16 } (4+2) / 3 = 2 17 } 18 Console.WriteLine(count); 22

Keep Variables Live The best case As Short a Time  Advantages of short time and short span  Gives you an accurate picture of your code  Reduces the chance of initialization errors  Makes your code more readable 23

Best Practices  Initialize variables used in a loop immediately before the loop  Don’t assigna value to a variable until just before the value is used  Never follow the old C / Pascal style of declaring variables in the beginning of each method  Begin with the most restricted visibility  Expand the visibility only when necessary  Group related statements together 24

Group Related Statements – Example You have to keep track of void SummarizeData(…) oldData, newData, { numOldData, numNewData, … totalOldData and GetOldData(oldData, numOldData); totalNewData GetNewData(newData, numNewData); totalOldData = Sum(oldData, numOldData); totalNewData = Sum(newData, numNewData); PrintOldDataSummary(oldData, totalOldData); PrintNewDataSummary(newData, totalNewData); SaveOldDataSummary(totalOldData, numOldData); SaveNewDataSummary(totalNewData, numNewData); … }  Six variables for just this short fragment 25

Better Grouping– Example The two blocks are void SummarizeDaily( … ) each shorter and { individually contain GetOldData(oldData, numOldData); fewer variables totalOldData = Sum(oldData, numOldData); PrintOldDataSummary(oldData, totalOldData); SaveOldDataSummary(totalOldData, numOldData); … GetNewData(newData, numNewData); totalNewData = Sum(newData, numNewData); PrintNewDataSummary(newData, totalNewData); SaveNewDataSummary(totalNewData, numNewData); … }  Easier to understand, right? 26

Using Variables Best Practices

Single Purpose  Variables should have single purpose  Never use a single variable for multiple purposes!  Economizing memory is not an excuse  Can you choose a good name for variable that is used for several purposes?  Example: variable used to count students or to keep the average of their grades  Proposed name: studentsCountOrAvgGrade 28

Variables Naming  The name should describe the object clearly and accurately, which the variable represents  Bad names: r18pq, __hip, rcfd, val1, val2  Good names: account, blockSize, customerDiscount  Address the problem, which the variable solves – "what" instead of "how"  Good names: employeeSalary, employees  Bad names: myArray, customerFile, customerHashTable 29

Poor and Good Variable Names x = x - xx; xxx = aretha + SalesTax(aretha); x = x + LateFee(x1, x) + xxx; x = x + Interest(x1, x);  What do x1, xx, and xxx mean?  What does aretha mean ? balance = balance - lastPayment; monthlyTotal = NewPurchases + SalesTax(newPurchases); balance = balance + LateFee(customerID, balance) + monthlyTotal; balance = balance + Interest(customerID, balance); 30

Naming Considerations  Naming depends on the scope and visibility  Bigger scope, visibility, longer lifetime  longer and more descriptive name: protected Account[] mCustomerAccounts;  Variables with smaller scope and shorter lifetime can be shorter: for (int i=0; i<customers.Length; i++) { … }  The enclosing type gives a context for naming: class Account { Name: string { get; set; } } // not AccountName 31

Optimum Name Length  Somewhere between the lengths of x and maximumNumberOfPointsInModernOlympics  Optimal length – 10 to 16 symbols  Too long numberOfPeopleOfTheBulgarianOlympicTeamFor2012  Too short а, n, z, щ  Just right numTeamMembers, teamMembersCount 32

Naming Specific Data Types  Naming counters UsersCount, RolesCount, FilesCount  Naming variables for state ThreadState, TransactionState  Naming temporary variables  Bad examples: a, aa, tmpvar1, tmpvar2  Good examples: index, value, count 33

Naming Specific Data Types (2)  Name Boolean variables with names implying "Yes / No" answers canRead, available, isOpen, valid  Booleans variables should bring "truth" in their name  Bad examples: notReady, cannotRead, noMoreData  Good examples: isReady, canRead, hasMoreData 34

Naming Specific Data Types (3)  Naming enumeration types  Use build in enumeration types (Enum) Color.Red, Color.Yellow, Color.Blue  Or use appropriate prefixes (e.g. in JS / PHP) colorRed, colorBlue, colorYellow  Naming constants – use capital letters MAX_FORM_WIDTH, BUFFER_SIZE  C# constants should be in PascalCase: Int32.MaxValue, String.Empty, InvariantCulture 35

Naming Convention  Some programmers resistto follow standards and conventions  But why?  Conventions benefits  Transfer knowledge across projects  Helps to learn code more quickly on a new project  Avoid calling the same thing by two different names 36

Naming Convention (2)  When should we use a naming convention?  Multiple developers are working on the same project  The source code is reviewed by other programmers  The project is large  The project will be long-lived  You always benefit from having some kind of naming convention! 37

Language-Specific Conventions  C# and Java / JavaScript conventions  i and j are integer indexes  Constants are in ALL_CAPS separated by underscores (sometimes PascalCase in C#)  Variable and method names use uppercase in C# and lowercase in JS for the first word  The underscore _ is not used within names  Except for names in all caps 38

Standard Prefixes  Hungarian notation – not used  Semantic prefixes (ex. btnSave)  Better use buttonSave  Do not miss letters to make name shorter  Abbreviate names in consistent way throughout the code  Create names, which can be pronounced (not like btnDfltSvRzlts)  Avoid combinations, which form another word or different meaning (ex. preFixStore) 39

Kinds of Names to Avoid  Document short names in the code  Remember, names are designed for the people, who will read the code  Not for those who write it  Avoid variables with similar names, but different purpose it UserStatus / UserCurrentStatus  Avoid names, that sounds similar decree / degree / digRee  Avoid digits in names 40

Kinds of Names to Avoid (2)  Avoid words, which can be easily mistaken  E.g. adsl, adcl, adctl, atcl  Avoid using non-English words  Avoid using standard types and keywords in the variable names  E.g. int, class, void, return  Do not use names, which has nothing common with variables content  Avoid names, that contains hard-readable symbols / syllables, e.g. Csikszentmihalyi 41

Using Expressions Best Practices

Avoid Complex Expressions  Never use complex expressions in the code!  Incorrect example: What shall we do if we get at this line IndexOutOfRangeException? for (int i=0; i<xCoords.length; i++) { for (int j=0; j<yCoords.length; j++) { matrix[i][j] = matrix[xCoords[findMax(i)+1]][yCoords[findMin(j)-1]] * matrix[yCoords[findMax(j)+1]][xCoords[findMin(i)-1]]; } } There are 10 potential sources of IndexOutOfRangeException in this expression!  Complex expressions are evil because:  Make code hard to read and understand, hard to debug, hard to modify and hard to maintain 43

Simplifying Complex Expressions for (int i = 0; i < xCoords.length; i++) { for (int j = 0; j < yCoords.length; j++) { int maxStartIndex = findMax(i) + 1; int minStartIndex = findMin(i) - 1; int minXcoord = xCoords[minStartIndex]; int maxXcoord = xCoords[maxStartIndex]; int minYcoord = yCoords[minStartIndex]; int maxYcoord = yCoords[maxStartIndex]; int newValue = matrix[maxXcoord][minYcoord] * matrix[maxYcoord][minXcoord]; matrix[i][j] = newValue; } } 44

Using Constants When and How to Use Constants?

Avoid Magic Numbers and Strings  What is magic number or value?  Magic numbers / values are all literals different than 0, 1, -1, null and "" (empty string)  Avoid using magic numbers / values  They are hard to maintain  In case of change, you need to modify all occurrences of the magic number / constant  Their meaning is not obvious  Example: what the number 1024 means? 46

The Evil Magic Numbers public class GeometryUtils { public static double CalcCircleArea(double radius) { double area = 3.14159206 * radius * radius; return area; } public static double CalcCirclePerimeter(double radius) { double perimeter = 6.28318412 * radius; return perimeter; } public static double CalcElipseArea(double axis1, double axis2) { double area = 3.14159206 * axis1 * axis2; return area; } } 47

Turning Magic Numbers into Constants public class GeometryUtils { public const double PI = 3.14159206; public static double CalcCircleArea(double radius) { double area = PI * radius * radius; return area; } public static double CalcCirclePerimeter(double radius) { double perimeter = 2 * PI * radius; return perimeter; } public static double CalcElipseArea( double axis1, double axis2) { double area = PI * axis1 * axis2; return area; } } 48

Constants in C#  There are two types of constants in C#  Compile-time constants: public const double PI = 3.14159206;  Replaced with their value during compilation  No field stands behind them  Run-time constants: public static readonly string ConfigFile = "app.xml";  Special fields initialized in the static constructor  Compiled into the assembly like any other class member 49

Constants in JavaScript  JS does not support constants  Simulated by variables / fields in ALL_CAPS: var PI = 3.14159206; var CONFIG = { COLOR : "#AF77EE", DEFAULT_WIDTH : 200, DEFAULT_HEIGHT : 300 }; document.getElementById("gameField").style.width = CONFIG.DEFAULT_WIDTH; document.getElementById("gameField").style. backgroundColor = CONFIG.COLOR; 50

When to Use Constants?  Constants should be used in the following cases:  When we need to use numbers or other values and their logical meaning and value are not obvious  File names public static readonly string SettingsFileName = "ApplicationSettings.xml";  Mathematical constants public const double E = 2.7182818284;  Bounds and ranges public const int READ_BUFFER_SIZE = 5 * 1024 *1024; 51

When to Avoid Constants?  Sometime it is better to keep the magic values instead of using a constant  Error messages and exception descriptions  SQL commands for database operations  Titles of GUI elements (labels, buttons, menus, dialogs, etc.)  For internationalization purposes use resources, not constants  Resources are special files embedded in the assembly / JAR file, accessible at runtime 52

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Homework 1. Refactor the following code to use proper variable naming and simplified expressions: public class Size { public double wIdTh, Viso4ina; public Size(double w, double h) { this.wIdTh = w; this.Viso4ina = h; } } public static Size GetRotatedSize( Size s, double angleOfTheFigureThatWillBeRotaed) { return new Size( Math.Abs(Math.Cos(angleOfTheFigureThatWillBeRotaed)) * s.wIdTh + Math.Abs(Math.Sin(angleOfTheFigureThatWillBeRotaed)) * s.Viso4ina, Math.Abs(Math.Sin(angleOfTheFigureThatWillBeRotaed)) * s.wIdTh + Math.Abs(Math.Cos(angleOfTheFigureThatWillBeRotaed)) * s.Viso4ina); } 54

Homework (2) 2. Refactor the following code to apply variable usage and naming best practices: public void PrintStatistics(double[] arr, int count) { double max, tmp; for (int i = 0; i < count; i++) { if (arr[i] > max) { max = arr[i]; } } PrintMax(max); // continue on the next slide 55

Homework (3) tmp = 0; max = 0; for (int i = 0; i < count; i++) { if (arr[i] < max) { max = arr[i]; } } PrintMin(max); tmp = 0; for (int i = 0; i < count; i++) { tmp += arr[i]; } PrintAvg(tmp/count); } 56

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