Datatypes Nomenclature

Value types
- Numeric
  - Signed integer (sbyte, short, int, long)
  - Unsigned integer (byte, ushort, uint, ulong)
  - Real number (float, double, decimal)
- Logical (bool)
- Character (char)
Reference types
- String (string)
- Object (object)

(In italics, the one we will mainly be using.)

Integers are “whole” numbers (ℤ = {…, − 1, 0, 1, 2, 3, …}), floating point numbers are real numbers (ℝ), strings are “text messages”, …

Please refer to the “Datatypes in C#” cheatsheet for more information about datatypes.

String and Int Variables

Literals are fixed values (“Hi Mom”, 40, 1.2404, …) in the source code.

using System;
class MyFirstVariables{
    static void Main(){
    // Declaration
       int myAge;
       string myName;
    // Assignemnt
       myAge = 40;
       myName = "Clément";
    // Displaying
       Console.WriteLine($"I am {myAge} old and my name is {myName}.");
    }
}

A variable has a name (which must be an identifier), a type, a size, and a value.

Variable Name	Variable Type	Variable Size	Variable Value
`myAge`	`int`	32 bit	`40`
`myName`	`string`	Variable¹	`"Clément"`

Variable Initialization

You can declare and assign a variable in one statement using what is called an “initialization statement”.

string myMessage = "Hey Mom";
int myValue = 12;

There is now one additional rule when it comes to chosing a valid identifier for your variable name: you can not take an identifier that was already used. That is, you can have only one variable named myMessage: if you want to re-assign a variable, you can not use an initialization again (that would re-declare the variable), you need to use an assignment statement again.

Remarks

The value can change (hence the name!) if you re-assign it. The previously stored value is simply wiped out, and lost.
You can store one variable’s value into another, but that value in the other variable won’t change when the original variable’s value changes:
```
int a = 12;
int b = a; // b's value is 12
a = 0; // a's value changed to 0, but b's value is still 12.
```
We can perform basic math operations with numeric datatypes: + (sum), * (multiplication), - (substraction) but also the modulo operation (%), which corresponds to the remainder. More details will be given in lab #3, in homework #2, and during lecture #4.

There is a difference between

int sum = num * 2; // The value of sum is num's value times two
Console.WriteLine($"{sum}"); // The value of sum is displayed.

and

Console.WriteLine($"{num * 2}"); // The value of num times two is displayed, but the value of num is still the same.

You can combine multiple declarations, initializations, and even mix both in one statement:

int a=0, b, c;  // a, b, c are declared as three int variables, and a's value is set to 0.

It’s actually 20 + (n/2) * 4 bytes, for n the number of characters in the string, so 7 in that case.↩︎

PCP − Lecture 03

Last Time - First Program

Datatypes Nomenclature

String and Int Variables

Variable Initialization

Remarks