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Friends here we are going to

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discuss about how to define a function, and

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how to use or call or execute defined function.

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Right. Let me explain this with examples.

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So let me create a simple script to define and to use your

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function. So 'define_function.py'.

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[no audio]

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See that. If you want to define a function, you have to use

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keyword called 'def'.

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With that keyword you need to define your function, and anyway guys before

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going to work with functions, why we need a function? Whenever

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if you have repeated logic in your code, instead of

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repeating your logic in different places

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we are going to write that logic in one place, and we are

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giving some name.

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[no audio]

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So giving name is nothing but defining a function.

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Right. So I am defining simply 'display' function. That's it.

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Assume that

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I'm repeating two lines of code in different places. Now instead

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of writing two lines of code in different places,

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what I am doing is, those two lines I am writing here.

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So, "This is first line", just assumption.

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"This is second line".

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So these two lines I am repeating suppose in ten places, instead

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of repeating in ten places

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I am writing two lines here, and I am giving the name for

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that. So this is the way how to define a function. You have

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to define your function with 'def' keyword. 'def', your function

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name, then parentheses, after that colon.

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Guys, actually you are defining for these two lines

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you are giving one name called 'display'.

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That means these two lines are there under 'display'.

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So whenever if you want to write some lines under your function,

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you should provide some indentation, or some space for all

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your lines which are there under your function.

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[no audio]

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If you define any function without any line there is no use.

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[no audio]

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Right. See that.

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I am defining in this way.

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Your Python will give an error. What is that error?

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"SyntaxError: unexpected EOF while parsing".

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You don't have anything,

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that's why it is giving an error.

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[no audio]

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You should have at least one line inside of your function.

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Now, I have two lines.

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No problem. Yes, it's fine.

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[no audio]

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So I defined a function, and then I am running my Python script.

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You're not getting any output.

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[no audio]

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So whenever if we define a function, it won't execute by

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itself. If you want to run your Python function you should

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call somewhere.

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So let's say I am calling that function from sixth line, 'display'.

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Now let me run it. Now,

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you're going to get your two lines of output.

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So guys whenever your Python is running your code, how it

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is going to run? From top to down.

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So whenever it is coming from top to down, if Python identifies

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a keyword called 'def', then at that time your Python won't

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execute this logic, simply it will remember the name of your

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function name.

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Then it will come out from your function.

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Then do you have any line here?

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No problem, you don't have any line here.

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Now, we have a line here,

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but this line is 'display'. Already Python knows that 'display' is

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somewhere a function.

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That's why whenever 'display' is there, because already Python

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knows that where 'display' is there, now your Python flow of

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control, execution part will come to here, and now will execute

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these two lines. After completion of these two lines, again

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it will come to the same line, it will come automatically to sixth line itself.

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Then it will try to go to seventh line if you have any line here.

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As of now we don't have any seventh line.

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Right. So now what I am doing is, I am going to call it once again.

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[no audio]

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See that. So what you are doing?

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[no audio]

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You're calling first your function,

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then you are coming back to here only, then you're going to

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seventh line. Seventh line, again you're calling same function.

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Then you come back to here. Then you'll go to eighth line. As

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of now you don't have any eighth line. If you have it

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will, it may be any line, eighth line.

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Let's say I am going to print, "Now, we called

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display function two times".

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[no audio]

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Now once again, I'm going to call

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it. Yes, you can call it. No problem.

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[no audio]

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Right. So if I run my code, first thing, very first line is a 'def'.

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That's why Python won't execute this 'display' function.

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Simply it will remember the name where your 'display' is there, then

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it'll come out from your function, then is there anything

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here? No. Here 'display'

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is there. Now your Python will go to here, and it'll execute

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that, and will come back to same line. Then

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anyway, you don't have anything to execute in the same line.

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Then it'll, Python will go to seventh line.

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Now again 'display'. 'display' is a function. Now your python will

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come here, and will execute these two lines.

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Then it'll come back to same line, seventh line. As of now

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once again you don't have anything to do on seventh line.

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Then it will go to eighth line. Eighth is a 'print' statement.

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It'll print that line.

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Then it'll will go to ninth line. Ninth is again a function.

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Now, you will come to here, you will execute these two lines, and

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it'll come back to here, after that

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we don't have any lines, then your Python script will stop.

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Now, let me run it and see the output. That's it.

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[no audio]

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So guys, this is a simple way to define your function, and

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is the way how to call or execute your function.

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Make sure that your Python function won't execute by itself.

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If you want to execute or use your function, you should somewhere

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call that function in this way.

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So whenever if you are calling your function, your function

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name with parenthesis you have to mention. While defining

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your function with the 'def', function name, parentheses, ':'.

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Under function name with some indentation, with some space

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you have to mention some number of lines.

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That's it.

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Right. Now what I am doing here is, forget about this logic.

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See, I am going to define somewhere your function, simply

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'display()'.

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'print("

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Welcome to functions concept")'.

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'print("Simple way to define your function")'.

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Right. Let me run it.

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No output. No error. No output. So you just defined a function.

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So if you want to execute that function, you should call somewhere.

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Yes, I'm calling. Now,

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it is executing. But instead of calling your function from ninth line

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I am going to call it from very first line.

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Then see the result what you are getting. You are getting

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an error called, name 'display' is not defined.

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Why you are getting this error means, see your Python will execute

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your code line by line.

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It will try to execute first line, as of now 'display'.

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[no audio]

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As of now your Python don't know what is the 'display'. In case

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if it encounters 'def' keyword first, then your Python understands

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that 'display' is a function because of this helping keyword.

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[no audio]

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But you defined somewhere, but before definition defining your

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function you are calling, this is not the correct way to define your function.

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So always you should define your function first, and after

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defining your function you have to call your function.

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So before calling your function, somewhere you should define

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your function in your sequence.

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[no audio]

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Fine, and it is a better practice to write at the end

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'return None' as of now. As of now, 'return None'.

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See, let me run it.

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Is there any problem? No problem.

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It is working fine. Even though if you include 'return None',

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it's working fine.

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So actually it is a good practice to write at the end of

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your function 'return None'.

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So what is the use of that, we'll discuss while going forward.

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Right. Fine. And guys, one more thing, while defining your

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function name you have to follow some rules.

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[no audio]

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Rules to define function name. So already we know that guys.

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If you remember, what are the rules to define variables.

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[no audio]

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Variable name should have only 'a' to 'z', or capital 'A' to 'Z', then

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number 0 to 9, and then underscore. Only these three group

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of characters

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you should have in a variable name. Now even function also same thing.

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There is no difference. The same rules which we have to define

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a variable, all rules are applicable to define your function.

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[no audio]

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That's it. So capital letters function name and small letters function

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names are different, case sensitive, we know that. So just try

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to recollect the rules to define variables,

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same variable name rules are applicable for your function as well.

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Right. You should not start with any number fir your variable. Same here,

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also functions should not start with

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[no audio]

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number, but we can start with

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[no audio]

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underscore, no problem. Underscore or letters, characters,

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and don't include any space.

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[no audio]

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That's it. So these are the rules. Exactly the variable rules are applicable

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even while defining your function name as well.

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That's it.

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and finally,

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[no audio]

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so your function must be defined.

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[no audio]

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before calling it.

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See, I am defining at very first your function.

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I am calling at sixth line,

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then there is no problem.

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But if I call our function at very first line, and if I define

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this function somewhere in 10th or 11th somewhere below

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that then it will create a problem.

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It will give an error. That is not the correct

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way. First define your function, then somewhere you can

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call that. Right.

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So guys, once again this is a calling function, and you are calling

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this. This is a called function. Called function you're calling

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from here. So of course both are same but because of this you

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are able to execute these lines.

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So this is calling, this is called function.

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You need to remember these names as well. And you can call

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your function any number of times in your code. Right. Fine.

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So finally I want to give this definition

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once again. A function is a block of code for some specific

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operation, and functions are reusable.

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You can reuse your function code any number of times in your

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logic. There is no restriction for that.

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We can use that any number of times. And one more thing,

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it won't execute itself.

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It will execute only when it is called in this way. You're

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calling from here. This function we called.

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So if you don't have this thing, it won't execute, right?

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See, let me save it and run it. You don't have any output.

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But if I do, if I call that in this way then only it is going

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to execute these two lines.

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[no audio]

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That's it. So guys now tell me why we need functions? Why functions are

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required? Why functions are used in our code? First thing

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is code reusability purpose.

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So we started our introduction of functions with

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this concept only. Whenever if you are repeating your logic

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in different places, so instead of repeating your logic,

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we are writing your logic in one place and we are calling

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that in different places.

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Then second one, is improve modularity. Means,

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[no audio]

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see even though if you don't have any requirement with your

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functions in your code, it's better to write your code in

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terms of functions.

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Why? Why means, just assume that you are writing some thousands of lines.

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[no audio]

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Thousand lines of code you're writing.

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So if you don't have functions concept, what you will

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do? Line-by-line you will go and write.

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Assume that these first four lines are for some specific

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purpose, and next hundred lines are for some specific purpose.

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[no audio]

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Now instead of writing straightforward, if I use some name for this,

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[no audio]

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right, if I give some name for your different lines, what

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is the purpose of that logic? In future easily

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I can understand

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what is the logic you've written, or the big code

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I can split into some parts and each part

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I can write as a separate function, and finally I will call

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all the functions in one place.

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So that the readability will increase by seeing your code,

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for your code.

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Right. So from now onwards even though if it is having only one

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line of code try to write in terms of functions so that you

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will be good with functions after sometime.

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So from next class onwards, we will try to write each and

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every code in terms of functions only, so that you will be

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good with functions. Functions are very easy.

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You don't need to worry much about your functions.

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[no audio]

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Right. Then, types of functions. So guys, basically there are two

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types of functions. One is built-in functions. Built-in functions

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means by default

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they are with your Python.

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[no audio]

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Then user-defined functions, means we are defining those functions.

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Just now we defined, right?

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We defined one function called 'display'.

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[no audio]

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So if there is a function, then only you can use it, right?

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Let me remove this function.

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Right. What I am doing is I am running this.

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[no audio]

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You're going to get an error called, 'display' is not defined.

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Because nowhere we define our function, right. Now,

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it is giving perfect output. Same way suppose assume that

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I'm writing some 'print(len("hi"))'.

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[no audio]

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You're getting 2. Actually, 'len()' is a function.

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Do you have that in our code? No, but still it is working.

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See if I remove this part, 'display' is not working.

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Right. If I remove this part 'display' is not working.

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[no audio]

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But 'len()', nowhere we define 'len()' function, but still

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we are able to use that.

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So that means this is the default function with your Python,

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built-in functions with your Python.

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So for built-in functions, you don't need to worry, directly you

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can use it. Not only length, if you remember we have suppose,

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if I write 'x = 40', then 'print(id(x))', 'id' is also

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one built-in function with your Python. It is going to give

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you the address of your variable. Not only that, let me take

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[no audio]

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'x = '

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[no audio]

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(5,6),

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[no audio]

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right. Then 'print(len(x))'.

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Now tuple length, you're going to get. 'len()' is already

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defined. Not only this we have so many functions

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we've already seen. Let's say. 'print(min(x))'.

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What is the result? 5. 'min()' is a function here. 'print(max(x))'.

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Maximum value among your values in 'x', 6. So these all

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are some of the default functions with your Python.

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[no audio]

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Not only that, 'int(x)', right.

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I mean if you want to do some type conversion what you are

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doing? 'int', 'list', conversion, right. Suppose if I have a

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'5', that is a string actually. Now,

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what I am doing is, I am trying to convert that as a integer.

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So 'int(x)', 'int()' is a function here, but that is

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a built-in function or default function with your Python.

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[no audio]

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Right. So you have two types of functions - one is user defined

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functions, and one more is built-in functions.

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So very rarely we used built-in functions.

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But sometimes you have very importance with your built-in

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functions, with some functions, some built-in functions,

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but most of the cases you are going to define your user defined

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functions. Right.

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So now your concentration is how to define and use and work

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with your user-defined functions.

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[no audio]

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Right. So as of now we have seen simple way to define your function,

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right. In next video

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we will see in detail with your functions, how to define, and

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how to call with some arguments. See 'display' in parentheses

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I'm not writing anything, but in 'len()', I am writing something.

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So why should we write this?

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So to understand this function you need to work with your

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user defined functions by passing some value here so that

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you can understand effectively why 'len()' requires some input.

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Right.

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Okay guys, so finally you need to remember this, how to define

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a function and how to call a function. Right.

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Okay guys, thank you for watching this video.

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[no audio]