in the series of learning programming in c today in this video i am going to talk about identifiers and keywords we have already discussed what are constants types of constants what are variables in c language now we will see what are keywords and see what are identifiers in c because sometimes um they can ask you an exam like a difference between identifier and keyword so we will see the difference also and some other important points also about keywords and identifiers right here i am talking about a nsic american national standard institute because a nsi has standardized c in 1989. i have already discussed this in the first video when we were discussing introduction to programming and introduction to c language right so now first of all we'll discuss about keywords in c we have 32 keywords now what are these keywords see keywords are some reserved words you can say keywords or reserved words or predefined words see in see we have predefined functions like printf printf means that is already been defined in the c library and to include that thing to tell the meaning of that printf to compiler we include what header file hash include stdio.h right because the meaning of that printf is already defined in that library right we cannot change that meaning same way there are some words also meanings are already defined in c right so those words are called keywords reserved words predefined words and how many keywords are there in c 32 keywords in a and s i c there are 32 keywords right now see what is use of keywords in c c when you write a program we will we write what instructions program is what sequence of instructions we write some instructions some declaration instruction arithmetic instructions or many instructions so to form those instructions we use keywords we use identifiers we will discuss later when we'll see programs like these are keywords these are identifier yeah i'll give some example also in this video but you can say keywords are basic building blocks for writing a c program for building basic building blocks for writing instructions in a c program like 32 keywords so some examples are like int int is what a keyword this meaning is already defined like it is an integer data type data it is for data type float it is also a keyword we cannot change their meanings and here we can say break go to these are keywords if for continue while do so these are some keywords i have written i haven't written all the 32 keywords but you don't have to remember like at this time at this point of time like these are 32 keywords yeah you can remember some keywords but when we will be familiar with the programming in c when we'll see later videos when we used to write down programs then automatically you will come to know these are keywords you don't have to memorize you just just have to understand when we will write a program but yeah these are some you can say few keywords and every keyword is having their own meaning the meaning i'll tell you uh when we'll discuss the program when we'll write programs at that time we'll discuss what is meaning of this double what is meaning of continue why i am writing for why we are writing go to statement why you are writing switch statement that will discuss so every keyword is having its own meaning and keywords cannot be used as identifiers yeah and if i will discuss then you will come to know this point in previous video i have told you when we were discussing variables in c the rules you cannot use keyword as a variable name right so that's exactly i want to say here keywords cannot be used as as identifiers right so they are having fixed meanings and their meaning cannot be changed during the program you cannot change their meaning fine and keywords are see you can see here all the keywords are in small letters c c is what case sensitive here you write in and you write like int intent these are having different meaning you cannot write like this keyword you can all you can always write keyword in small words or i would rather say it is must to write keywords in your program when you write when you use keywords in program it is must to write keywords in lowercase letters see all the keywords are in lowercase letters right now let us see what are identifiers identifiers are these are predefined reserved words identifier refers to a name of a variable function arrays structure union right see identifier as the name suggest i had to identify something if someone has to identify me means you can say my identity is my name you can call my name that is you can say identifier for me so when we use variables in various variables in a program functions so those are having their names also that is why we call as the name of a variable is identifier name of a function we refer as identifier name of any array structure union because those are user defined we define their names that is why these are known as identifiers and if user defined their names if these are user defined words then obviously there should be some rule for these to define identifier to write identifier to construct the name of identifier and what are those rules first rule is identifier consists of you can say a sequence of letters and numbers and underscore is also allowed no other special symbols are allowed in identifier right suppose i am writing here like this sum yeah this is an identifier this is name of a variable so this is valid it is having only letters sum one two this is also valid it is having letters and digits only that is also allowed underscore is also allowed if i write some underscore 12 this is also allowed and the first letter of an identifier is always beer you can say this alphabet or underscore is also allowed in some compilers so this is valid this is valid but you write 1 2 sum this is invalid because the first letter is what number this is not allowed right next you can say any special symbol is also not allowed like you cannot say dollar sum you cannot write some dollar dwell you cannot write some space 12 this is invalid this is invalid this is invalid right yeah but this is valid you have to tell me this is valid or not underscore sum underscore 12 and one is sum two underscore are there continuous and two l these two you have to tell me in comment box this is allowed or not right next is what keywords cannot be used as identifiers we can we cannot use like int and here i'm using uh suppose float you can use int a a is name of variable that is fine this is identifier this is what keyword this is keyword but here in declaration we cannot use another keyword this is invalid right next is the length of identifier is what ah according to this ansi 31 characters first 31 characters are significant right and here we can use uppercase letter as well as lowercase letters in keywords we can only use lowercase letters no uppercase letters are allowed but here uppercase and lowercase letters are treated differently like if you are writing sum and you are writing s-u-m this is different this is different if you are writing s-u-m this is also different these are three names right and if you write down a name like this are referred as name of function also so you can also write if you are creating a function sum how to declare a function how to initial how to define it that will discuss in later videos so you can also write like this sum and angular bracket this is what function or if you i want to write down the data type that also you can write this is what you can say declaration of a function yeah this is also identifier and this is what keyword word is also a keyword right and suppose if you write sum dot 12 this is also invalid identifier if you write add the rate sum this is also invalid if you write like this one if i write like this in brackets i am writing sum this is also invalid not allowed and if you write like average this is valid you can say department or you can say department name this is also valid but if you write like this department hyphen name this is not underscore this is invalid this is you can say hyphen so this is not allowed if you uh write like here percentage max this is also invalid i hope you got now the basic idea about identifiers and keywords so i hope uh you can also write down the differences between identifier and keywords because i have already discussed what are the differences here fine and if you want me to provide notes then you can also tell me in the comment box i'll put the notes like the pdf file of the notes in the description box of every video or maybe in a separate video so now in the next video we are going to discuss what are data types in c so i'll see in the next video till then bye take care