Video: High-Quality Code - Use of Variables, Data, Expressions and Constants (Bulgarian)

Topics covered:

• Principles for Initialization
• Scope, Lifetime, Span
• Using Variables
  • Variables Naming
  • Naming convention
  • Standard Prefixes
• Using Expressions
• Using Constants

Video (in Bulgarian)

Presentation Content

Initially Assigned Variables in C#

• Static variables
• Instance variables of class instances
• Array elements
• Value parameters
• Reference parameters
• Variables declared in a catch clause or a foreach statement

Initially Unassigned Variables in C#

• Output parameters
  • Including the this variable of struct instance constructors
• Local variables
  • Except those declared in a catch clause or a foreach statement

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  • The value in the variable is outdated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

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);

• 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];
  }
  Console.WriteLine(
    "The sum of the elements in row {0} is {1}", sum);
}

The sum must be reset after the end of the inner for loop

• 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)
{
  …
}

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)
  { … }
}

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 }

Returning 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
  • Example:

int salary = days * hoursPerDay * ratePerHour;
return salary;

• Incorrect example:

return days * hoursPerDay * ratePerHour;

The intent of the formula is obvious
We can put a breakpoint at this line and check if the result is correct

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

Visibility of Variables

• Variables’ visibility is explicitly set restriction regarding the access to the variable
  • public, protected, internal, private
• Always try to 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

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.");
      }
   }
}

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!

Calculating Span of Variable

• 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

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

Measuring the Live Time of a Variable

25 recordIndex = 0;
26 while (recordIndex < recordCount) {
27 ...
}
28 recordIndex = recordIndex + 1;
...
62 total = 0;
63 done = false;
64 while ( !done ) {
...
}
69 if ( total > projectedTotal ) {
70 done = true;
}

• Average live time for all variables:
  • ( 4 + 8 + 8 ) / 3 ≈ 7

recordIndex ( line 28-line 25 + 1 ) = 4
total ( line 69-line 62 + 1 ) = 8
done ( line 70-line 63 + 1 ) = 8

Keep Variables LiveAs 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

Best Practices

• Initialize variables used in a loop immediately before the loop
• Don’t assign a 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

Group RelatedStatements – Example

void SummarizeData(…)
{
  …
  GetOldData(oldData, numOldData);
  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
  You have to keep track of oldData, newData, numOldData, numNewData, totalOldData and totalNewData

Better Grouping– Example

• Easier to understand, right?

void SummarizeDaily( … )
{
  GetOldData(oldData, numOldData);
  totalOldData = Sum(oldData, numOldData);
  PrintOldDataSummary(oldData, totalOldData);
  SaveOldDataSummary(totalOldData, numOldData);
  …
  GetNewData(newData, numNewData);
  totalNewData = Sum(newData, numNewData);
  PrintNewDataSummary(newData, totalNewData);
  SaveNewDataSummary(totalNewData, numNewData);
  …
}

The two blocks are each shorter and individually contain fewer variables

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

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

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);

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

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

• Just right

numTeamMembers, teamMembersCount

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

• 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

• 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

Naming Convention

• Some programmers resist to followstandards 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

• 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!

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#)
  • 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

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)

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

UserStatus / UserCurrentStatus

• Avoid names, that sounds similar

decree / degree / digRee

• Avoid digits in names

• 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

Avoid Complex Expressions

• Never use complex expressions in the code!
  • Incorrect example:

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]];
  }
}

• Complex expressions are evil because:
  • Make code hard to read and understand, hard to debug, hard to modify and hard to maintain
    What shall we do if we get at this line IndexOutOfRangeException?
    There are 10 potential sources of IndexOutOfRangeException in this expression!

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;
  }
}

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?

The Evil Magic Numbers

public class GeometryUtils
{
  public static double CalcCircleArea(double radius)
  {
    double area = 3.14159265 * radius * radius;
    return area;
  }
  public static double CalcCirclePerimeter(double radius)
  {
    double perimeter = 6.28318531 * radius;
    return perimeter;
  }
  public static double CalcElipseArea(double axis1, double axis2)
  {
    double area = 3.14159265 * axis1 * axis2;
    return area;
  }
}

Turning MagicNumbers into Constants

public class GeometryUtils
{
   public const double PI = 3.14159265;
   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;
   }
}

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

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;

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;

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