04. Variables

Variables :

Variables are place holders in RAM, which provides us ability to store data.

Rules for Variables :

Variable Names could contain letters, digits and underscore character.
Variable name should start with a letter or underscore
variable name should not contain spaces.
Variable name length should be atleast one character.
Variable name should not contain any special character except underscore.

Note: In case of GCC compiler $ symbol is allowed.

Types of Variables :

Global Variable / Public Variables
Local Variable / Private Variables

Syntax of declaring variable :

    datatype variable_name;
    datatype variable_name = value;

Examples of variables :

    int marks = 10;

    int age;    // age declared without any value.
    age =10;    // age assigned a value.

Local Variables :

The variables which are present within a function are called as local variables.

Examples of Local Variables :

    int main() {
            //local scope variables
        int age = 10;
        char grade = 'A';
        float pi = 3.14f;

        return 0;
    }

Examples of Global Variables :

    float PI = 3.14;
    int main(){
        printf("Value of PI is %f",PI);
        return 0;
    }

Scopes of Variables :

Public Scope - means any part of program is able to use them.
Private Scope - means only specific part of Program is able to use them.

You Tell me...

Which type of variable according to you has public scope?
Which type of variable according to you has private scope?

Basic Conventions For Global Variables :

Conventions are optional,
They are rules followed by most of developers around the world.
You may choose to implement them in your programs to keep seperate of concerns.
Name your variable in UPPER CASE
Declare your variables with pre allocated value.(to prevent garbage value)
In case you do not know what value the variable will hold then, provide a suitable value to prevent garbage value.
Keep length of characters in variable name to be minimum.
Variable name should not exceed 50 characters.
Variable name should be descriptive.

Basic Conventions for Local Variables :

Name you variable in CAMEL CASE.
Declare your variable with default value to prevent allocation of garbage value.
Variable name should be descriptive.

On program execution :

OS will create account of Program in STACK.
OS will allocate some space in RAM.
OS will generate address of allocation unit.
OS will store address of variable in STACK.
STACK Exists in RAM itself somewhere.

Stack :

Stack is a space in RAM created by OS to keep account of program and its variables.
Stack helps OS keep record of RAM bits used by saveral programs during concurrent operations.
Makes Multitasking possible.

How Program Execution Works :

$$$

Program asks ---> OS allocate me some memory

OS Creates Account of Program ---> OS Allocates Space in RAM

OS Stores Address of Memory in STACK

Program Asks OS to store value in variable

OS fetches Address of variable from stack.

OS fetches data of variable stored at that address

OS provides program data in binary form

Program converts data as required

$$$

Why were local variables developed ?

Let's see source code of some program to understand this,

    int Multiply(int first, int second){
        return first * second;
    }
    int Sum(int a, int b){
        return a + b;
    }

    int main(){
        printf("a = %d and b = %d",a,b);
        return 0;
    }

In above situation,

Our program has 3 functions.
But None of Sum and Multiply functions are used in program.
Our Sum() function has two local variables a and b.
Our main function prints values of a and b on execution.

We notice that, in our situation all three variables are local.

Let's Execute the program and see what happens :

Result : We got an error that a and b are not defined.
It means a and b variables are not allocated any space in RAM by OS.
So, Local variables prevent unnecessary space allocation in RAM. and this is the reason, why local variables were created.

Why were global variables developed ?

Let's again see source code of some program to understand this,

    float PI =3.14;

    float AreaOfCircle(float radius){
        return PI * (radius * radius);
    }

    int main(){
        printf("Value of PI is %f",PI);

        float area = AreaOfCircle(23.4f);

        printf("Area of Circle is %f",area);
    }

In above situation : * We have a global variable PI. * We have two functions that uses value of PI.

In above situation we notice that PI is a global variable.

Conclusion :

So global variables to created to avoid duplicate variable declaration. This makes OS Fast and Efficient.

Conclusions of why variables were created :

Prevents duplicate code.
Makes system faster.
Prevents unnecessary usage of RAM.
Reduces overall software size.