Fundamentals of Variables

Introduction to Values

A value is a piece of information (called data) you need to use in your program. One way to use a value is to display it to the user. To do this, you can include the value in the parentheses of Write(). Here are two examples:

using static System.Console;

Write(248);

Practical LearningPractical Learning: Introducing Variables

  1. Start Microsoft Visual Studio
  2. To create a new application (if Microsoft Visual Studio was already opened, on the main menu, click File -> New -> Project...), on the Visual Studio 2022 dialog box, click Create a New Project)
  3. In the right list of the Create a New Project dialog box, click Console App and click Next
  4. Change the Name to GeorgetownDryCleaningServices1
  5. Click Next
  6. In the Framework combo box, make sure .NET 9.0 (Standard Term Support) is selected (if it is not, select it in the Framework combo box).
    Click Create
  7. Based on our introductions in the previous lesson, change the document as follows (you will delete both lines in the document and replace them with the following line of code): 
    using static System.Console;

Introduction to Variables

A variable is a value that is stored in the computer memory (the random access memory, also called RAM). Such a value can be retrieved from the computer memory and displayed to the user.

Declaring a Variable

Before using a variable in your application, you must let the compiler know. Letting the compiler know is referred to as declaring a variable.

To declare a variable, you must provide at least two pieces of information. Actually, you have various options. We will start with one of them. One option to declare a variable is to provide the type of value, also called a data type, followed by a name for the variable. The formula to follow is:

data-type variable-name;

Initializing a Variable

When declaring a variable, you can give it a primary value. This is referred to as initializing the variable. You can use the following formula:

data-type variable-name = value;

This would be done as follows:

using static System.Console;

data-type variable-name = desired-value;

The Name of a Variable

A variable must have a name. There are rules you must follow to specify the name of a variable. To start, you must avoid reserved words, also called keywords. These keywords are:

abstract and as async await base bool
break (loops) break (switch) byte case char checked class
const continue decimal default (type) default (switch) delegate do
double dynamic enum else explicit extern event
false finally fixed float for foreach get
global goto if implicit in (foreach) in (generic) in (parameter)
init int interface internal is (Conditions) is (Nullity) is (Pattern)
lock long nameof namespace new (generic) new (inheritance) new (classes)
not null object operator or out (generic) out (parameters)
override params private protected public readonly record
ref (variable) return sbyte sealed set short sizeof
stackalloc string struct switch static this (class) this (indexer)
throw true try typeof unsafe using (namespace) using (namespace)
uint ulong ushort virtual value var void
when (Conditions) when (patterns) where while with yield  

There are other names that are not considered C# keywords but should be avoided because they may cause a conflict in your code. They are referred to as contextual keywords. They are:

add ascending descending
from group into join let
orderby partial (method) partial (type) remove  
select where (generic) where (query)    
       

There are some rules we will follow to name variables:

Besides these rules, you can also create your own rules but that follow the above restrictions. For example:

C# is case-sensitive. This means that the names Case, case, and CASE are completely different.

Practical LearningPractical Learning: Introducing Variable Names

A Primary Introduction to Strings

Introduction

A string one or a group of symbols, readable or not. The symbols can be letters (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z, A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, and Z), digits (0, 1, 2, 3, 4, 5, 6, 7, 8, and 9) are non-readable characters (` ~ ! @ # $ % ^ & * ( ) - _ = + [ { ] } \ | ; : ' < ? . / , > "). To create a string, include its symbol, symbols, or combination inside double-quotes.

A String Variable

One way to use a string in a program is to store that string in the computer memory. To do that, you can declare a variable that holds a string as value. To let you do this, the C# language provides a data type named string. Use it to declare a string-based variable. Here is an example:

using static System.Console;

string firstName;

To provide the value of the variable, specify its value in double-quotes and assign it to the variable. Here is an example:

using static System.Console;

string firstName = "James";

Displaying a String

Displaying a value consists of printing it to the computer screen. If you simply have a string to display, include that string in the parentheses of Write(). Here is an example:

using static System.Console;

Write("Welcome to the World of C# Programming!");

This would produce:

Welcome to the World of C# Programming!Press any key to close this window . . .

Primary Topics on Strings

Creating a New Line

If you use Write(), everything displays on the same line. This means that, after a string has displayed, the caret stays on the same line. As an alternative, you can display a string on one line and then move the caret on the next line. To do this, use WriteLine(). In this case, after the string is displayed, whatever item comes next would display in the next line. Just as you can include a value in the parentheses of Write(), you can put a value in the parentheses of WriteLine(). Consider the following example:

using static System.Console;

WriteLine("Welcome to the World of C# Programming!");

This would produce:

Welcome to the World of C# Programming!
Press any key to close this window . . .

In the same way, you can use any combination of Write() or of WriteLine() in your code. Here are examples:

using static System.Console;

Write("Country Name:");
Write(" ");
WriteLine("Australia");

This would produce:

Country Name: Australia
Press any key to close this window . . .

You can leave the parentheses of WriteLine() empty or include something in it but you can never leave the parentheses of Write() empty.

Practical LearningPractical Learning: Creating a New Line

  1. Change the document as follows: 
    using static System.Console;

// data-type variable - name = value;

WriteLine("-/-Georgetown Cleaning Services -/-");
WriteLine("===================================");
  2. To execute the application, on the main menu, click Debug -> Start Without Debugging. This would produce: 
    -/-Georgetown Cleaning Services -/-
===================================
Press any key to close this window . . .
  3. To display many strings, change the document as follows: 
    using static System.Console;

string customerName = "James Burreck";
string homePhone = "(202) 301-7030";

WriteLine("-/-Georgetown Cleaning Services -/-");
WriteLine("===================================");

Write("Customer:   ");
WriteLine(customerName);
Write("Home Phone: ");
WriteLine(homePhone);
WriteLine("===================================");
  4. To execute the application, on the main menu, click Debug -> Start Without Debugging. This would produce: 
    -/-Georgetown Cleaning Services -/-
===================================
Customer:   James Burreck
Home Phone: (202) 301-7030
===================================
Press any key to close this window . . .
  5. Press Enter and return to your programming environment

Ordering Value Display

Imagine you have a certain value stored in a variable and you want to display the value of that variable on the screen. We already know that you can type Write() or WriteLine() and you can include a string in those parentheses. You can create an expression that includes the string in those parentheses and the value(s) of your choice. To proceed, the string in Write() or WriteLine() must be divided in two parts separated by a comma. The left side of the comma is the rough string you want to display. The right side of the comma is the name of a variable or a list of variables. If you use a list of variables (on the right side of the commas), the variables must be separated by commas. In the left string, type some curly bracjets, {}, for each variable that is on the right side of the comma. The {} combination is called a placeholder. If you are displaying only one variable, include 0 in the {} placeholder. If you are displaying more than one variable, the numbers in the {} placeholders muts be cumulative, as {0}, {1}, {2}, etc. The formulas to follow are:

Write("Something {0}", value);
Write("Something {0} {1}", value1, value2);
Write("Something {0} {1} {2}", value1, value2, value3);
Write("Something {0} {1} {2} {3}", value1, value2, value3, value4);

WriteLine("Something {0}", value);
WriteLine("Something {0} {1}", value1, value2);
WriteLine("Something {0} {1} {2}", value1, value2, value3);
WriteLine("Something {0} {1} {2} {3}", value1, value2, value3, value4);

String Interpolation

String interpolation consists of inserting one string in another existing string to get a new string. To perform this operation, start the existing string with the $ sign. In the existing string, include the curly bracjets placeholder {}. In the {} placeholder, include the desired string. Normally, you usually perform this operation for a string variable. Here is an example:

using static System.Console;

string firstName = "Robert";

WriteLine($"First Name: {firstName}");
WriteLine("===================================");

This would produce:

First Name: Robert
===================================
Press any key to close this window . . .

If you want to interpolate more strings, create as many {} sections as you want. In each {} placeholder, include the variable of your choice. Here is an example:

using static System.Console;

string firstName = "Robert";
string lastName = "Ellis";

WriteLine($"Customer: {firstName} {lastName}");
WriteLine("===================================");

This would produce:

Customer: Robert Ellis
===================================
Press any key to close this window . . .

The Width to Display a Value

The {} placeholder is primarily used to hold a value. One way to control the display of the value is to specify how wide an area must be reserved for the value. To specify this information, on the right side of the value in the placeholder, type a comma and a number for the desired width. Such a value is aligned to the right side of the reserved area. If you want the value to be aligned to the left, provide the value as a negative. Here are examples:

using static System.Console;

string firstName = "Robert";
string mi = "T";
string lastName = "Ellis";

WriteLine("First Name: '{0,10}'", firstName);
WriteLine("Middle:     '{0,10}'", mi);
WriteLine("Last Name:  '{0,10}'", lastName);
WriteLine("-----------------------------------");
WriteLine("First Name: '{0,-10}'", firstName);
WriteLine("Middle:     '{0,-10}'", mi);
WriteLine("Last Name:  '{0,-10}'", lastName);
WriteLine("===================================");
WriteLine($"First Name: {firstName}");
WriteLine($"First Name: '{firstName}'");
WriteLine($"First Name: '{firstName,20}'");
WriteLine($"First Name: '{firstName,-20}'");

This would produce:

First Name: '    Robert'
Middle:     '         T'
Last Name:  '     Ellis'
-----------------------------------
First Name: 'Robert    '
Middle:     'T         '
Last Name:  'Ellis     '
===================================
First Name: Robert
First Name: 'Robert'
First Name: '              Robert'
First Name: 'Robert              '

Press any key to close this window . . .

Requesting a String

Program interactivity allows a user to submit value to an application so that you would process those value. To do this, write ReadLine(). When the program executes, the carret would blink to the user to type something. To indicate what the user is supposed to type, you can precede ReadLine() with with a string in Write() or WriteLine().

When the user has typed something in response to ReadLine(), you can get that value and store it in a variable. To do that, assign ReadLine() to a string. You can then use that variable normally. In tje same way, you can use ReadLine() as many times as you want.

Introduction to Natural Numbers

Overview

A natural number is a value that contains only digits. To recognize such values, the C# language provides a data type named int. Use it to declare a variable that would hold a natural number. Here is an example:

using static System.Console;

int age;

The Value of an Integral Variable

To specify the variable of an int type, provide a value that contains only digits. Here is an example:

using static System.Console;

int age = 15;

Displaying an Integral Value

To display a number to the user, you can include that number in the parentheses of Write() or WriteLine(). Here is an example:

using static System.Console;

WriteLine(9285);

This would produce:

9285
Press any key to close this window . . .

If the value is stored in a variable, you can either display that value to the user, you can type the name of the variable in the parentheses of Write() or WriteLine(). Here are two examples:

using static System.Console;

int monthlySalary = 3288;

Write("Monthly Salary: ");
WriteLine(monthlySalary);

This would produce:

Monthly Salary: 3288
Press any key to close this window . . .

You can also use string interpolatione to display the value of an integral variable. To do this, as seen with strings, start the interior of the parentheses of Write() or WriteLine() with $ followed by double-quotes. In the double-quote, write {}. In those square brackets, include the name of the integral variable.

Creating a Large Integer

The value of an integer can be between -2147483648 and 2147484647 (or -2,147,483,648 and 2,147,484,647). If you need to use a large value, to make it easier to humanly read, you can separate the thousands by underscores. Here are examples:

using static System.Console;

int areaOfChina = 9_596_961;
int areaOfCanada = 9_984_670;
int areaOfBurkinaFaso = 275_200;
int areaOfDjibouti = 23_200;

WriteLine("Countries Areas");
WriteLine("---------------------");
Write("Djibouti: ");
WriteLine(areaOfDjibouti);
Write("Burkina Faso: ");
WriteLine(areaOfBurkinaFaso);
Write("China: ");
WriteLine(areaOfChina);
Write("Canada: ");
WriteLine(areaOfCanada);

This would produce:

Countries Areas
---------------------
Djibouti: 23200
Burkina Faso: 275200
China: 9596961
Canada: 9984670
Press any key to close this window . . .

Practical LearningPractical Learning: Introducing Integers

  1. Change the document as follows: 
    using static System.Console;

string customerName = "James Burreck";
string homePhone = "(202) 301-7030";
int numberOfShirts = 1;
int numberOfPants = 1;
int numberOfDresses = 1;
int orderMonth = 3;
int orderDay = 15;
int orderYear = 2020;

WriteLine("-/-Georgetown Cleaning Services -/-");
WriteLine("===================================");

Write("Customer:   ");
WriteLine(customerName);
Write("Home Phone: ");
WriteLine(homePhone);
Write("Order Date: ");
Write(orderMonth);
Write("/");
Write(orderDay);
Write("/");
WriteLine(orderYear);
WriteLine("-----------------------------------");
WriteLine("Item Type  Qty");
WriteLine("-----------------------------------");
Write("Shirts      ");
WriteLine(numberOfShirts);
Write("Pants       ");
WriteLine(numberOfPants);
Write("Dresses     ");
WriteLine(numberOfDresses);
WriteLine("===================================");
  2. To execute the application, on the main menu, click Debug -> Start Without Debugging. This would produce: 
    -/-Georgetown Cleaning Services -/-
===================================
Customer:   James Burreck
Home Phone: (202) 301-7030
Order Date: 3/15/2020
-----------------------------------
Item Type  Qty
-----------------------------------
Shirts      1
Pants       1
Dresses     1
===================================
Press any key to close this window . . .
  3. Press Enter and return to your programming environment

Converting a Value to an Integer

Your programs will deal with various types of values. Some of those values must be involved in arithmetic operations, but sometimes, when you have a value, you don't know what type that value is. Normally, beformed involving a value in a number-based operation, you may have to first convert that value to a number. To do this, type int.Parse(). In the parentheses of int.Parse(), type the value to be converted. Here is an example:

using static System.Console;

string value = "3528";

int number = int.Parse(value);
WriteLine(number);

Requesting an Integral Value

We already saw that, to request a value, you can use ReadLine(), but ReadLine() gets a string. As a result, you must convert the value of ReadLine() to an integer. To do that, ou can first get the value, then convert it. This can be done in two steps as follows:

using static System.Console;

Write("Enter a number: ");
string request = ReadLine();

int number = int.Parse(request);

WriteLine(number);
WriteLine("===================================");

To reduce the number of lines of your code, you can include ReadLine() in the parentheses of int.Parse().

Introduction to Floating-Point Numbers

Overview

A floating-point number is a number made of either only digits or two digit parts separated by a symbol referred to as a decimal separator. As one way to support floating-point numbers, the C# language provides a data type named double. Use it to declare a variable for a number. To provide a value for the variable, you can use the same types of numbers we saw for integers. Here are examples:

using static System.Console;

double areaMaine = 35385;
double areaAlaska = 1_723_337;

WriteLine("States Areas");
Write("Maine: ");
WriteLine(areaMaine);
Write("Alaska: ");
WriteLine(areaAlaska);

This would produce:

Monthly Salary: 3288
Press any key to close this window . . .

A Floating-Point Number with Precision

The primary difference between an integer and a floating-point number is that a decimal number can include a second part referred to as a precision. To specify the precision of a number, after the natural part, add the symbol used as the decimal separator. In US English, this is the period. Here is an example:

using static System.Console;

double hourlySalary = 25.85;

Write("Hourly Salary: ");
WriteLine(hourlySalary);

This would produce:

Hourly Salary: 25.85
Press any key to close this window . . .

If the integral part is large, you can use it "as is" or you can separate its thousansds with underscores. Here are examples:

using static System.Console;

double areaGuam = 570.62;
double areaAlaska = 665_384.04;
double areaSouthDakota = 77115.68;
double areaTennessee = 42_144.25;

WriteLine("States Areas");
WriteLine("=======================");
Write("Guam: ");
WriteLine(areaGuam);
Write("Alaska: ");
WriteLine(areaAlaska);
Write("Tennessee: ");
WriteLine(areaTennessee);
Write("South Dakota: ");
WriteLine(areaSouthDakota);

This would produce:

States Areas
=======================
Guam: 570.62
Alaska: 665384.04
Tennessee: 42144.25
South Dakota: 77115.68
Press any key to close this window . . .

In the above examples, we used the underscore separator only on the integral part of the number. In reality, you can also use that separator in the precision side. Here are examples:

using static System.Console;

double number = 085.24_497;
double value  = 947_596.75_038;

Write("Number: ");
WriteLine(number);
Write("Value: ");
WriteLine(value);
WriteLine("===================================");

This would produce:

Number: 85.24497
Value: 947596.75038
===================================

Press any key to close this window . . .

Converting a Value to Double-Precision

To convert a value to a floating-point number with double-precision, type double.Parse(). As seen with integers, in the parentheses of double.Parse(), type the value to be converted. Here is an example:

using static System.Console;

string value = "495.86";

double number = double.Parse(value);
WriteLine(number);

Requesting a Decimal Number

We saw that, to request a value, you can use ReadLine(). Once you have the value, you must convert it to a double value. We saw that you can use double.Parse() to do that. Here is an example:

using static System.Console;

Write("Enter a number: ");
string request = ReadLine();

double number = double.Parse(request);

WriteLine(number);
WriteLine("===================================");

You can also include ReadLine() in the parentheses of double.Parse().

Displaying a Decimal Number

Introduction

So far, to display the value of a decimal variable, we simply typed it in the parentheses of Write() or WriteLine() as we saw with strings (and integers). You can also use string interpolation to display the value. Here are examples:

using static System.Console;

double number = 285.24_497;
double value  = 947_596.75_038;

WriteLine($"Number: {number}");
WriteLine($"Value: {value}");
WriteLine("===================================");

This would produce:

Number: 285.24497
Value: 947596.75038
===================================

Press any key to close this window . . .

Displaying a Number with Fixed Precision

Because floating-point numbers use a precision part, sometimes you want to control how the number displays. The string interpolation mechanism provides many options. To start, in the {} placeholder, after the name of the variable, type a colon (:). After the colon, if you want to display the number with two decimal places, type f or F, n. Here are examples:

using static System.Console;

double nbr = 4848075.5279;
double val = 63828493.806;
double number = 28835.24_497;
double value  = 947_596.75_038;

WriteLine($"Number: {nbr:f}");
WriteLine($"Value: {val:f}");
WriteLine($"Number: {number:F}");
WriteLine($"Value: {value:F}");
WriteLine("===================================");

This would produce:

Number: 4848075.53
Value: 63828493.81
Number: 28835.24
Value: 947596.75
===================================

Press any key to close this window . . .

Formatting the Number with Thousands Separators

If the integral part of the number is large and you want to display it with decimal separators in the integral part but two decimal places, in the {} placeholder of the string interpolation, after the colon (:), type n or N. Here are examples:

using static System.Console;

double nbr = 484.5279;
double val = 63.806828493;
double number = 288358075.24_497;
double value  = 947_596.75_038;

WriteLine($"Number: {nbr:n}");
WriteLine($"Value: {val:n}");
WriteLine($"Number: {number:N}");
WriteLine($"Value: {value:N}");
WriteLine("===================================");

This would produce:

Number: 484.53
Value: 63.81
Number: 288,358,075.24
Value: 947,596.75
===================================

Press any key to close this window . . .

Practical LearningPractical Learning: Introducing Double-Precision Numbers

  1. Change the document as follows: 
    using static System.Console;

string customerName = "James Burreck";
string homePhone = "(202) 301-7030";
int numberOfShirts = 1;
int numberOfPants = 1;
int numberOfDresses = 1;
double priceOneShirt = 0.95;
double priceAPairOfPants = 2.95;
double priceOneDress = 4.55;
int orderMonth = 3;
int orderDay = 15;
int orderYear = 2020;

WriteLine("-/-Georgetown Cleaning Services -/-");
WriteLine("===================================");

Write("Customer:   ");
WriteLine(customerName);
Write("Home Phone: ");
WriteLine(homePhone);
Write("Order Date: ");
Write(orderMonth);
Write("/");
Write(orderDay);
Write("/");
WriteLine(orderYear);
WriteLine("-----------------------------------");
WriteLine("Item Type  Qty Sub-Total");
WriteLine("------------------------");
Write("Shirts      ");
Write(numberOfShirts);
Write("     ");
WriteLine(priceOneShirt);
Write("Pants       ");
Write(numberOfPants);
Write("     ");
WriteLine(priceAPairOfPants);
Write("Dresses     ");
Write(numberOfDresses);
Write("     ");
WriteLine(priceOneDress);
WriteLine("===================================");
  2. To execute the application, on the main menu, click Debug -> Start Without Debugging. This would produce: 
    -/-Georgetown Cleaning Services -/-
===================================
Customer:   James Burreck
Home Phone: (202) 301-7030
Order Date: 3/15/2020
-----------------------------------
Item Type  Qty
-----------------------------------
Shirts      1
Pants       1
Dresses     1
===================================
Press any key to close this window . . .
  3. Press Enter and return to your programming environment
  4. Close your programming environment
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