How to Define Constants in C Using #define and const Keyword

now we continue our discussion on constants in see how to define constants we already know this thing that how to

define constants using hash define we would be able to define constants like for example 3.14159 which is a famous

irrational number called pi we would be able to use this irrational number in our code by simply using a macro name

associated with it by using hash define maybe we want to calculate the area of circle maybe we want to calculate the

circumference of circle in that case writing this constant value multiple number of times would be little bit

cumbersome we can simply use this macro instead of using this constant value and that is the use of hash define or you

can say greatest advantage of hash define that we already learned in our previous lesson now in this lesson we

consider another way of defining constants by using Const keyword now where do we use this Const keyword let's

try to understand this thing we would be able to use this Const keyword in front of a variable for example we can define

a variable like int var by simply putting a Const keyword in front of that variable we won't be able to change its

value anywhere in the code now this is quite weird suppose for example we want to assign a different value to a

variable then we won't be able to do that because when we define it as a constant it won't be able to change its

value and that is the meaning of a constant right constraint simply means that this value whatever is being

assigned to a variable is permanent and we won't be able to change it it's just a read-only value and that is the

meaning of a constant sometimes this functionality we required from a variable and we simply put Const keyword

in front of some datatype and some variable name let's understand this thing with the help of code I define a

variable and assigned it a value is equals to 67 and I'm simply going to print this value with the help of printf

function let's see what would be the output you got 67 as the output right now I'm going to assign a different

value to this variable and I'm simply going to print this value let's see what would be the

output it is equals to 57 we would be able to assign any number of time values to it as whenever we want in our code

that is the meaning of a variable it is not necessary that once we initialize with this value we won't be able to

assign any different value later in our code now if we put a Const keyword in front of this variable then let's see

what will happen we got an error assignment of read only variable you can see over here in the build messages tab

that error is produced which is assignment of read only variable there because we are trying to assign a

different value to it which is after the initialization step we already initialized it with value is equals to

67 and now this becomes a constant we won't be able to change its value further in the code and that is why this

kind of error is produced which is read only variable assignment right and we won't be able to do such things in our

code and that is the meaning of a constant if you put this variable outside of this main function and of

course there is obviously the rule of scope in this case this variable is visible within this function only right

if we try to change the value of this variable in this function then definitely it will produce the error we

can also declare this variable as global and no function would be able to assign value to it that is the functionality of

the constant variable by simply putting the Const keyword in front of a variable name we would not be able to assign any

different value to it forever in the core [Applause]

[Music]