R/C2/Functions-in-R/English-timed

Time	Narration
00:00	Welcome to this tutorial on Functions in R
00:05	In this tutorial, we will learn: Need for functions
00:10	How to create a user-defined function
00:15	Scope of variables
00:17	To understand this tutorial, you should know,
00:21	Basics of permutation and combination
00:25	Basic data structures
00:28	Conditional statements
00:31	If not, please locate the relevant tutorials on R on this website.
00:38	This tutorial is recorded on Ubuntu Linux OS version 16.04
00:47	R version 3.4.4
00:51	RStudio version 1.1.463
00:56	Install R version 3.2.0 or higher.
01:02	For this tutorial, we will use
01:05	A script file myFunctions.R.
01:10	Please download this file from the Code files link of this tutorial.
01:18	I have downloaded and moved this file to functions folder.
01:24	This folder is located in myProject folder on my Desktop.
01:31	I have also set functions folder as my Working Directory.
01:38	A function is a set of statements organized together to perform a specific task.
01:45	R has a large number of built-in functions.
01:49	In spite of that, sometimes we need to define our own functions.
01:56	User-defined functions are specific to user’s requirements.
02:02	Once created these functions can be used as the built-in functions.
02:08	An R function is created by using the keyword function.
02:14	The syntax of an R function is as follows:
02:20	The different parts of a function are: Name, Arguments, Body, Return value.
02:32	Let us switch to RStudio.
02:35	Open the script myFunctions.R in RStudio.
02:42	Here, we have declared a sample vector named sampleVec.
02:48	Let us find the summary of this vector by using the built-in function.
02:55	In the Source window, type the following command.
02:59	Save the script and run this script by clicking on the Source button.
03:07	The summary of sampleVec is displayed.
03:12	It includes median, mean, etc.
03:18	Similarly, R has many built-in functions to make our life easy.
03:24	But we can always create a function of our own, depending on the need.
03:30	We will create a function that computes the permutation of two numbers.
03:36	In the Source window, type the following command.
03:42	Here, we have named our function as permutation.
03:48	Next, we have passed two values as arguments.
03:53	Inside the function, the definition of permutation has been written.
03:58	Run the current line by pressing Ctrl + Enter keys simultaneously.
04:05	Now, let us test this function to check whether it gives correct results.
04:11	First, we will calculate the permutation of 5 and 2.
04:17	In the Source window, type permutation and in parentheses 5 comma 2.
04:27	Run the current line.
04:29	The required value of 20 is displayed.
04:34	Thus, our function has given the correct result.
04:39	While creating functions, the arguments are optional.
04:44	Let us create a function that does not take any arguments.
04:49	We will create a function named as votingEligibility.
04:54	In the Source window, type the following commands.
04:59	This function should print a message whether an Indian citizen is eligible to vote or not.
05:08	Remember the minimum age for Indian citizens to vote is 18 years.
05:15	Here, we have not passed any arguments.
05:20	readline function reads a line from the terminal in interactive use.
05:27	Run this block of code to execute the function votingEligibility.
05:33	Now, we will run this function to see whether it works.
05:38	In the Source window, type the following command.
05:42	Please note that we are not passing any arguments inside this function.
05:49	Run the current line.
05:52	We are asked to enter our age.
05:56	Let us type 21 and press Enter.
06:02	We get a message, You can cast your vote.
06:08	Now let us say we want to create a function sum_between_two such that
06:17	it takes two natural numbers num1 and num2 as its arguments.
06:26	Then, it returns the sum of all numbers from num1 to num2.
06:32	For example, if we pass 2 and 6 to the function sum_between_two,
06:42	it should return the value of 2 + 3 + 4 + 5 + 6.
06:50	Let us switch back to RStudio.
06:54	In the Source window, type the following command.
06:59	We have named our function as sum_between_two.
07:08	Also, we have specified the two arguments num1 and num2.
07:14	Inside the function, we have initialized an object named result to store the sum.
07:22	We have created a for loop which will increment the result at each iteration.
07:31	Run this block of code to execute the function.
07:36	Now we test our function sum_between_two by running it with two arguments.
07:45	In the Source window, type the following command.
07:51	Run the current line.
07:53	The function with arguments 2 and 6 was executed, but we got nothing as output.
08:03	Let us investigate what went wrong.
08:07	We know that our function got executed as we did not get any error after executing this line.
08:16	It means that the object named result got incremented.
08:22	Now, we will check the value of the result.
08:26	In the Source window, type the following command.
08:31	Run the current line.
08:34	It shows, Error in print(result) : object 'result' not found.
08:40	To understand what went wrong, we need to learn a concept named as Scope.
08:48	Scope is the term used to describe how objects and variables get defined within R.
08:56	If a variable is defined inside a function, then it can be accessed inside the function only.
09:04	So, if we try to access the same variable outside the function, it will throw an error.
09:12	Let us switch to RStudio.
09:17	Here, the variable result is defined inside the function.
09:22	So its scope is limited to this function only.
09:27	Hence, when we tried to access this variable outside the function, it threw an error.
09:34	For this problem, we can use return function.
09:39	Let us get back to our function sum_between_two.
09:47	Press Enter at the end of for loop.
09:53	Now type the following command.
09:56	I will resize the Console window.
10:01	Select all lines of the function sum_between_two..
10:08	Now press Ctrl + Enter keys together to run the updated function.
10:17	Let us run the function again with arguments 2 and 6.
10:23	For this, we will run this line.
10:27	We see the output 20 as expected.
10:32	Let us summarize what we have learnt.
10:36	In this tutorial, we have learnt: Need for functions
10:41	How to create a user-defined function
10:45	Scope of variables
10:48	We now suggest an assignment.
10:51	Create a function which computes combination of the two numbers.
10:57	Create a function which takes a natural number as an argument, and prints Fibonacci series
11:05	For example, consider fibonacci(5). It should print the first 5 elements of Fibonacci series, i.e. 1, 1, 2, 3, 5.
11:19	The video at the following link summarises the Spoken Tutorial project.
11:24	Please download and watch it.
11:27	We conduct workshops using Spoken Tutorials and give certificates.
11:33	Please contact us.
11:36	Please post your timed queries in this forum.
11:41	Please post your general queries in this forum.
11:44	The FOSSEE team coordinates the TBC project.
11:48	For more details, please visit these sites.
11:52	The Spoken Tutorial project is funded by NMEICT, MHRD, Govt. of India
11:59	The script for this tutorial was contributed by Varshit Dubey (College of Engineering Pune).
12:07	This is Sudhakar Kumar from IIT Bombay signing off.
12:12	Thanks for watching.

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