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# Programming language

A programming language is a type of written language that tells computers what to do. Programming languages are used to make all computer programs and computer software. A programming language is like a set of instructions that the computer follows to do something.

A programmer writes source code text in the programming language to create programs. Usually the programming language uses real words for some of the commands, so that the language is easier for a human to read. Many programming languages use punctuation just like a normal language. Many programs now are compiled. This means that the computer translates the source code into new languages such as assembly language or machine language, which are much faster and easier for the computer to read, but much harder for a person to read.

Computer programs must be written very carefully. If the programmer makes mistakes, or the program tries to do something the programmer did not design it to do, the program might then "crash" or stop working. When a program has a problem because of how the code was written, this is called a bug. A very small mistake can cause a very big problem. For example, forgetting a period or typing a plus sign instead of a minus sign can cause a bad bug.

## Types of programming languages

There are many types of programming languages. Most programming languages do not follow one type alone, so it is difficult to assign a type for each language. The examples of each type are given in each section below because they are the best well-known examples of that type.

### Declarative vs. Imperative programming

Declarative programming languages describe a "problem" but they usually do not say how the problem should be solved. The problem description uses logic, "solving" the problem often looks like automatically proving a system of logical axioms. Examples for such programming languages are Prolog, XSLT, LISP and SQL

Imperative programming languages describe a system of state changes. At the start, the program is in a certain state, and the computer is given steps to follow, in order to perform an action. Following the steps causes the program to "change state".

In general, declarative programming languages are safer and shorter. Imperative programming languages are more common, because they are easier to use.

### Functional vs. Procedural

Functional programming looks at programming like a function in mathematics. The program receives input, some information, and uses this information to create output. It will not have a state in between, and it will also not change things that are not related to the computation.

Procedural programs are a set of steps or state changes.

### Stack based

Stack based languages look at the some of the program's memory like a stack of cards. There are very few things that can be done with a stack. A data item can be put on the top of the stack. This operation is generally called "push". A data item can be removed from the top of the stack. This is called a "pop". You can look at the item at the top of the stack without removing it. This is called a "peek".

If a program is written as "push 5; push 3; add; pop;" it will put 5 on the top of the stack, put 3 on top of the 5, add the top two values (3 + 5 = 8), replace the 3 and 5 with the 8, and print the top (8). Examples for programming languages that are stack-based are the languages Postscript and Forth.

### Object-oriented

Object-oriented programming languages place data and functions that change data into a single unit. This unit is called an "object". Objects can interact with each other, but one object can not change another object's data. This is usually called encapsulation or information hiding. Most modern programming languages are object-oriented. An example of this is Java or C++.

### Flow-oriented

Flow oriented programming sees programming as connecting different components. These components send messages back and forth. A single component can be part of different "programs", without the need to be changed internally.

## Rules

Every programming language has rules about what it can and can not do. These include:

• Correct numbers (types of numbers, and how large or small the numbers can go)
• Words (reserved words, case-sensitivity)
• Limits on what the programming language can do

Most languages have official standards that define the rules of how to write the source code. Some programming languages have two or more standards. This can happen when a new standard replaces an old one. For example, the Perl 5 standard replaced Perl 4 in 1993. It can happen because two people made two standards at the same time. For example, there are several standards for APL.

## Object-Oriented Programming

Object-Oriented Programming (sometimes shortened to OOP) is a form of programming where all parts of the program are objects. Objects are pieces of memory with the same structure that can be used again and again. A bank account, bitmap, or hero from a video game could all be objects within a program. Objects are made up of properties (pieces of information the object stores) and methods which are things the object can do. A Dog object might have properties like height and hairColor. Its methods might include bark() and wagTail().

All objects are created from templates called classes. You can think of a class as a mold from which objects are made. The class defines all the properties and methods that its objects will have. Objects created from a class are called instances of the class. A class can extend another class, which means that it takes all the properties and methods of the class but can add its own.

Here is an example of what a class might look like in a programming language:

class Dog extends Mammal{

//These are properties:
String breed = "Collie"
String type = "Herding Dog"

//These are methods
void wagTail(){
//Do some wagging
}

void bark(){
//Do the barking here
}

}


Notice that the Dog class extends the Mammal class, so all dogs will have the properties of a mammal, like hairLength, and methods, like eat() or sleep().

Object-oriented programming is used in many of today's most popular programming languages, such as Java, C#, Objective-C, C++, Python, Ruby, Javascript, and ActionScript.

## Examples

### Example of Visual Basic

Here is a simple program written in Visual Basic:

 Dim Input
Input = InputBox("How old are you?")
If Not IsNumeric(Input) Then
MsgBox "That's not a number!"
ElseIf Input < 0 Then
MsgBox "You cannot be less than zero!"
ElseIf Input > 100 Then
MsgBox "That's old!"
Else
MsgBox "You're " & Input & " years old."
End If


This program asks the user his or her age and responds based on what the user typed. If the user typed something that is not a number, the program says so. If the user typed a number less than zero, the program says so. If the user says he or she is older than 100 years old, the program says "That's old!" If the user typed a correct age the program says back to the user how old he or she is.

### Example of Python

Here is a program that does the same thing as the program above, but in Python:

 try:
age = int(raw_input("How old are you? "))
except ValueError:
print "That's not a number!"
else:
if age < 0:
print "You cannot be less than zero!"
elif age > 100:
print "That's old!"
else:
print "You're %s years old." % age


### Example of C#

The same thing as the program above, but in C#:

using System;

public class Hello
{
public static void Main()
{
int age;