VB.NET Loop Statements

Loop Statements

• Loop statements repeat a process until a condition is met.
• The loop executes blocks of statements as long as the condition remains true.
• Several statements repeatedly execute a code sequence until a condition is satisfied.
• Do While Loop, For Next Loop, and While End Loop are types of loop statements.

For Loop Components

• Step determines the counter variable's increment or decrement after each iteration.
• Next is a keyword that signifies the end of For loops.
• start To end parameters represent the initial/final value of the variable in the loop.

For Each Loop

• For Each iterates through a block of statements in an array or collection.
• The while loop initially checks the defined condition.
• The Do loop executes statements and then checks a condition.
• In VB.NET, a Do Until loop executes as long as the Until condition is True.

Nested Loops

• Not a nested loop - for i=0 to 9 Console.Write("i={0}",i) Next for j=1 to i+3 Console.Write("j={0}",j)Next

While Loop Characteristics

• One untrue statement about the while loop is: "The while loop is a posttest loop."

VB.NET Loop Types

• do while loop is not a pretest loop.

VB.NET Code Segment Output

• This VB.NET code segment: x=10 While(x>5) Console.WriteLine("{0}",x) x-=1 End While would output 10 9 8 7 6.

Data Types

• Data Type represents the type of a variable.

For Each Loop Variables

• var_name is the variable name to access elements from the array or collection object.
• A variable checks whether the element is available or not in For Each loops.
• If an element exists in a collection object, the For Each block continues while the condition is true.
• After all elements are read in a For Each loop, control passes to Next to terminate the program.

String Definition

• A String is a sequential collection of characters called text.
• String creates a string variable to store text.

System.String Class

• The String class is named System.String.

String Object

• In VB.NET, the string object is immutable.

String Functions

• Chars gets a character from the current string object.
• Length returns the number of characters in the current string object.
• Length counts the number of characters in a string.
• Compare() compares two strings; if one is less than the other, it returns an integer value.
• Contains() validates if specified text exists in a string.
• Concat() joins two or more strings.
• Copy() creates a new string object with the same content as the original string.
• EndsWith() checks if the string ends with specified text.
• Equals() determines if two string objects have the same value.
• Format() replaces objects with a specified string.
• Replace() replaces one or more objects within the specified string object.
• IndexOf() returns the index position of the first character or string occurrence, starting from zero.
• Insert() inserts a specified string at a given position.
• Remove() deletes characters from a specified position to the end of a string.
• Replace() replaces all occurrences of a character or string and returns a new string.
• Split() divides a string into substrings using delimiters
• Collections are special purpose classes, useful for storing data, dynamically allocating memory, retrieving a list of items or elements on the basic index number, etc.

List Definition

• List stores generic types of collection objects.
• Count counts the number of elements in the List.
• Item gets or sets an item at a specified position in the List.
• AddRange adds the elements of a collection to the end of the List.
• ToArray copies all list elements to a new array object.

Functions

• A function consists of separate code groups performing specific tasks.
• Access_Specifier defines the access level (public, private, friend, protected) of a function.
• ParameterList lists parameters for sending/retrieving data from a method.
• Recursive function is when a function calls itself until a condition is satisfied.
• Sub is a separate code set for executing specific tasks that does not return values.
• Add() method adds an object at the end of the List.
• Clear() clears all records from the List.
• Find() method searches for a specified element from a list that matches the defined state and returns the first occurrence.
• CopyTo() method copies all records of a list into a one-dimensional array.
• OrderBy() method used to sort the elements of the list.
• Opposite() method used to reverse the order list of elements
• Remove() method in a list, we can delete an element from the specified location in a list.
• Insert is used to insert all elements of a specified collection in a list start from a specified location.
• Contains is used to validate whether the specified object exists in the List or not.

Class Definition

• Class is a group of data members/objects with the same properties, processes, events, and relationships.
• Access_Specifier defines a class's access levels (Public, Private, Friend, etc.).
• Implements specifies interfaces from which the class inherits.
• NotInheritable means the class cannot be a base class.
• Shadows re-declares variables and hides identical elements in a base class.
• Constructor is a special method implemented when a class object is created.
• A constructor without arguments creates a default constructor.
• Inheritance creates a new class by inheriting characteristics from an existing class.
• Derived Classes - The old class is known as the base class, and the inherited class is known as the derived class.
• Classes - An interface in VB.NET allows multiple inheritances of classes.
• Multithreading is when two or more processes execute simultaneously.
• A thread executes each process; the execution of a process is known as the thread.
• Single Thread is used to execute a single logic or task in an application.
• Main thread - By default, each application has a main thread to execute each process.
• To create and access a new thread in the Thread class, import the System.Threading namespace.
• Child Thread - A new thread becomes the child thread for what it inherited from the main thread.
• ThreadStart() is the method executed by the new thread.
• Join() is a synchronization method that stops the calling thread until the execution thread completes.
• Interrupt() is used to stop the currently executing thread.
• Dead state is when a thread is complete or aborted.

User Interface Elements

• Object is a user interface element created on a form using a toolbox control.
• Properties describe the object.
• Methods cause an object to do something.
• Value is a new property setting.
• Event alerts an application that something has occurred.
• Forms contains all the controls for the user interface
• ListBox displays a list of items.
• RadioButton enables single selection from a group of choices.
• PictureBox displays an image.
• TreeView displays hierarchical, labeled items.
• ListView displays items in different views.
• DateTimePicker allows date and time selection and display.
• ColorDialog displays available colors for custom color definition
• FontDialog allows font selection, size, and color choice.
• OpenFileDialog allows the user to select a file to open.
• SaveFileDialog prompts the user to select a location for saving a file and specifies the file's name.
• Abort returns DialogResult.Abort value.
• Cancel returns DialogResult.Cancel.
• Default returns nothing and the dialog box continues running.
• Ignore returns DialogResult.Ignore.

VB.NET Form Applications

• Form creates a form-based or window-based application.
• Locked determines if the form is disabled.
• FormBorderStyle sets/gets border style in a form.
• MDI stands for Multiple Document Interface.
• TopMost determines whether to put the window form above all others (Boolean value).
• GotFocus is activated when the form control receives a focus.
• KeyDown is activated when a key is pressed while focussing on the form.
• KeyUp is activated when a key is released while focusing on the form.
• LostFocus is activated when the form loses focus.
• MouseEnter is activated when the mouse pointer put on the form.
• MouseLeave event is activated when the mouse pointer leaves the form surface.
• MouseEnter event is activated when the mouse pointer enters the form.
• Load event is used to load a form before it is first displayed.
• Resize event is activated when a form is resized.
• DragDrop event is activated when a drag and drop operation is performed.
• Exception is a wanted error that occurs during the execution of a program and can be a system exception or application exception.
• Catch block is used to monitor a particular exception that may throw an exception within the application.
• Try is a block of code that catches an exception with an exception handler at the place in a program where the problem arises.
• Throw handler is used to throw an exception after the occurrence of a problem.
• "Finally" is used to execute a set of statements in a program, whether an exception has occurred.

JavaScript Variables

• Variables declared in Javascript: let variableName, var variableName, const variableName
• An uninitialized variable has a default value of undefined in JavaScript.
• const declares a constant variable in JavaScript.
• Reassigning a value to a constant variable throws an error.
• An invalid variable name is 2ndVar.
• let a, b, c; declares multiple variables in a single statement
• Variables can have block-level scope using let.
• The output of the following code is undefined -

let x;
console.log(x);

• An object can store multiple values in Javascript.
• Variable that can be declared but not initialized: let.
• Variables declared with var are function-scoped in JavaScript.
• Variables declared with let and const are block-scoped.
• JavaScript variable names can start with a number: False.
• The keyword const allows variable reassignment: False.
• Variables declared without a keyword become global variables: True.
• Javascript is case-sensitive: True.
• Using var for variable declaration is preferred in modern JavaScript: False.
• Uninitialized variables have a default value of null in JavaScript: False.
• Constants must be initialized at the time of declaration: True.
• Re-declaring a variable using let in the same scope throws an error: True.

Javascript Operators

• The "+" operator concatenates strings when used with strings.
• ++ is NOT a comparison operator in JavaScript.
• The result of the expression 5 + "5" in JavaScript is 55.
• The && operator returns true only if both operands are true.
• The typeof operator returns "string" when used on a string.
• The === operator checks equality in value and type in JavaScript.
• The result of the expression 5 % 2 javascript is 1.
• The = operator is used to assign values in JavaScript.
• ! is a logical NOT operator in JavaScript.
• The result of ""10" == 10" in JavaScript? - Returns/evaluates to: true.
• Javascript uses the + operator for both addition and string concatenation - True.
• The == operator checks for both value and type equality - False.
• The expression 5 === "5" evaluates to true - False.
• The && operator returns true if both operands are true - True.
• The result of "5" + 5 is 10 - False.
• The != operator checks for strict inequality - False.
• JavaScript's typeof operator returns a string representing the type of a value - True.
• The ++ operator can be used to increment a variable by 1 - True.
• The expression 10 % 3 evaluates to 3 - False.
• The || operator returns true if at least one operand is true - True.

JavaScript Popup Boxes

• alert() displays a simple alert box with a message.
• prompt() asks the user for input and returns it as a string.
• confirm() confirms an action from the user in JavaScript.
• The return value of confirm() if the user clicks "Cancel" is false.
• prompt() returns a string data type in JavaScript.
• The following code will output if "John" is entered: Hello John

let name = prompt("Enter your name:");
alert("Hello " + name);

• console.log() is often used for simple debugging to check values.
• Closing a prompt box without entering any input returns null.
• A JavaScript alert box's appearance cannot be customized.
• The purpose of the alert box in JavaScript is to display warning messages.
• The alert() function can accept multiple arguments: False.
• The confirm() function always returns a string value: False.
• JavaScript pop-up boxes are blocking, meaning script execution pauses until the box is closed: True.
• The prompt() function can accept two parameters: a message and a default value: True.
• The alert() function can return a value: False.
• The confirm() function returns true if the user clicks "OK": True.
• Using pop-up boxes is generally recommended for complex user interactions: False.
• If a user clicks "Cancel" on a prompt(), the return value is null: True.
• Pop-up boxes can be styled using CSS in JavaScript: False.
• The prompt() function can be used to get input from the user: True.

Javascript Functions

• A function in JavaScript is a block of code designed to perform a specific task.
• You define a function in JavaScript using: function functionName() {}.
• The keyword return is used to return a value from a function.
• The output of the following code is 7:

function add(a, b) {
  return a + b;
}
console.log(add(2, 5));

• A function can be assigned to a variable.
• An anonymous function is a function without a name.
• The following code outputs Hello undefined:

function greet(name) {
  console.log("Hello " + name);
}
greet();

• Immediately Invoked Function Expression (IIFE) type does NOT have a name and is often used immediately after creation.
• The output of the following code is 6:

const multiply = (a, b) => a * b;
console.log(multiply(2, 3));

• If a function does not explicitly return a value, it returns undefined.
• A function in JavaScript can accept zero or more arguments: True.
• Functions in JavaScript cannot return values: False.
• Anonymous functions are often used as arguments in higher-order functions: True.
• The return statement immediately stops function execution: True.
• JavaScript functions can be assigned to variables and passed as arguments: True.
• Functions in JavaScript must always have a name: False.
• An arrow function automatically binds this based on the surrounding code: True.
• JavaScript does not allow nested functions: False.
• A function defined with the function keyword cannot be called before its declaration: False.
• JavaScript functions can only be declared using the function keyword: False.

JavaScript Loops

• Loops execute a block of code repeatedly in JavaScript.
• While loop runs a block of code while a specified condition is true.
• do-while guarantees at least one execution, even if the condition is false initially
• The following loop will run 5 times:

for (let i = 0; i < 5; i++) {
  console.log(i);
}

• The break statement exits the loop completely.
• continue is used to skip the current iteration and continue with the next iteration.
• The output of the following code is 0 2:

for (let i = 0; i < 3; i++) {
  if (i === 1) {
    continue;
  }
  console.log(i);
}

• The following is NOT a valid loop in JavaScript: loop-until
• If the condition in a while loop is always true, it creates an infinite loop.
• The for loop is best suited for iterating over an array.
• A for loop can be used to iterate over an array: True.
• The while loop checks the condition before executing the code block: True.
• The do-while loop executes the code block at least once, even if the condition is false: True.
• Using break inside a loop skips the current iteration: False.
• The continue statement terminates the loop completely: False.
• A loop with an always-true condition may create an infinite loop: True.
• JavaScript has a built-in foreach loop for arrays: False.
• The for loop can have multiple initializations and increments: True.
• Loops cannot be nested in JavaScript: False.
• It’s good practice to create infinite loops for long-running tasks: False.

JavaScript Error Trapping

• Error trapping handles runtime errors gracefully in JavaScript.
• try, catch, throw are keywords used to handle errors in JavaScript.
• The catch block catches and handles errors.
• The output of the following code is: Start Error occurred

try {
  console.log("Start");
  throw new Error("Something went wrong");
  console.log("End");
} catch (error) {
  console.log("Error occurred");
}

• CatchError is NOT a valid error type in JavaScript.
• The throw keyword is used to manually trigger an error in JavaScript.
• The finally block executes code regardless of errors in JavaScript.
• A true statement about error handling is: The throw statement is used to create user-defined errors
• If an error is not caught, the script stops executing immediately.
• A ReferenceError is thrown when an undefined variable is accessed.
• The catch block in JavaScript can access the error message from the throw statement: True.
• The finally block always executes, regardless of whether an error occurs: True.
• JavaScript can handle both syntax errors and runtime errors: True.
• The throw statement can only throw built-in errors: False.
• The finally block is optional in a try-catch-finally statement: True.
• A catch block must always follow a try block: True.
• JavaScript does not support user-defined errors: False.
• The catch block can catch multiple types of errors: False.
• Errors inside a finally block are ignored: False.
• Error handling improves the reliability and robustness of JavaScript applications: True.