Lecture 19

[Music] [Music] hello friends welcome to the course foundations of r software and in this lecture we are going to begin with the new topic this is about the conditional executions the first question comes here what are these conditional executions so you know that suppose we take a call that in case a student gets more than 50 percent marks then the student is going to be declared as pass and and in case if the student gets marks less than 50 percent then the student is going to be classified as fail so now in this case how are you going to execute this command first you want that some condition has to be checked what is that condition the condition is to check whether the student has got marks more than 50 percent or is smaller than 50 percent based on that you are going to take a call what is going to be the outcome whether pass or fail so similarly we have many more such conditions where the complication increases or the need of the situation changes so in order to fulfill those things we have different kinds of conditional executions and essentially we are now beginning a topic on the control structure control structure means you are trying to control the control different parts of your programming so the first part which we are going to talk about under the heading of under the broad heading of control structure is the conditional execution so to curry in this lecture we are going to talk about the if and if else conditional execution commands and we will try to see that how these conditional executions can be done in the r software and and in particular if and if else conditions what are these conditions and how they have to be executed so in order to explain you i will try to take here couple of examples and through those good examples i will try to show you that how these conditional executions are implemented in the r software so let us begin our lecture so as i said now we are going to begin with the topic of control structures in the r software so first we are going to discuss about the control statements then functions and after that loop so this will continue in the next couple of lectures and we are going to cover all these topics one by one right so as i already have explained you that why do we need the conditional executions the conditional executions are needed when we want to execute a command when certain conditions are true or they are satisfied now these conditions can be one or more than once so based on that we have different types of contacts and commands in the r software and among them we are going to discuss first the if condition right that is the popular name that is how we try to address it so the syntax for the conditional execution using if is like this first we try to write down here if i f then inside the parenthesis you can see here we try to write the condition which we want to test so this condition is to be tested now this condition that is going to be a sort of logical statement is outcome is going to be either true or false so in case if the condition is true then whatever commands we want to execute that we have to write inside this curly bracket right so i will write down here these brackets and here i will try to write down all the commands which are going to be executed when the condition is true and in case if the condition is false then whatever is written inside this curly bracket all these conditions are ignored so this is how the if condition works in the case of our software so now you can see here couple of things number one when you learned about the logical statements now you can see one application of those logical statements that you want to first test and this outcome is going to be a logical variable in terms of true and false so for that you would also like to check whether the outcome of the condition is a is logical or not so all the commands which we have learnt in the earlier lectures they are now going to be useful as we proceed further and after that i had discussed in the beginning that in mathematics we have three types of brackets this simple bracket then curly bracket and then a square bracket and i had explained you that in r software these brackets have got different roles and in mathematics i had requested you to use only this parenthesis this small brackets this simple brackets and i had earlier explain you one utility of this square brackets and now i am i have explained you here one utility of this curly bracket if you recall at that time i had told you that in r software this parenthesis curly and this square bracket they have got different types of utilities and this is one of the utility where it is going to be used right so what you have to keep in mind that when you are trying to use this if condition try to use it only when there is a scalar and this if condition should not be applied when the condition whatever this condition here that we want to evaluate is a vector so this is a best use only when we are trying to meet a single element conditions so what is this single element condition i will try to show you that there is only one condition which has to be met and so on so the basic syntax for this if condition is very simple if then inside the parenthesis write down the condition and inside the curly bracket you try to write down the statements or the commands which need to be executed in case if you try to look at the flowchart of this if conditional execution it goes like this that you start you try to test here the condition now there are two option the condition is true or the condition is false in case if the condition is false here like this then we come and we stop here and in case if the condition is true then we try to execute the commands and then the program is stop so that is how the if condition works so now let me try to take here some example and try to show you that how it works suppose we want to multiply all the values in some program which are greater than four and we want to multiply such values by three right so somebody will enter some value the condition whether the value is greater than four or smaller than 4 that is going to be checked and in case if the this condition that x is or the number what you have entered is greater than 4 if this is found to be true then whatever is the number this is going to be multiplied by three and if this is false the input number is not greater than four then nothing will happen right so that is the thing which we want to do it and you can see here now that using the conditional execution concept and using the if commands we can do it very easily so suppose i give here an input x is equal to 5 and then i try to write down if condition here like this if and inside the parenthesis you try to write down here x greater than four and after that you write here x star three that means the number is going to be multiplied by three right here in this case either you write here curly bracket or not this will not make much difference because there is only one condition one an element but later on you will see that when we are trying to increase such conditions then this curly bracket will be necessary so here on the r console i will try to show you the application of actually both so in this case since you have taken the value to be 5 and this 5 is greater than 4 so the condition is here true and so this number is multiplied by 5 and you get here 3 into 5 as 15. now in case if you try to take here another number here x equal to 3 then what will happen 3 is less than 4 this is false so now in this case no outcome will be obtained if you try to execute this condition if x greater than 4 and then x into 3 node no response will be obtained here so you can see here when i am trying to take it x equal to 5 then i get this outcome but when i try to take here x equal to 3 then you can see here as soon as i enter on the r console there is no outcome right so let me try to show you and now i would like to show you the same example in the bracket for this execution so that you do not get confused right so you can see here that this is the same program i take x equal to five this is the same condition if x is greater than four but now i use here these curly brackets and you can see here this gives you the same outcome and similarly if you try to take here this x equal to 3 and if you use the same condition here but now i am using here this curly brackets then it is giving you no response you can see here this is the same condition but here i have used the curly bracket and in the earlier example i had not used here the curly brackets right so as i said it will not make much difference as long as you are working with the scalars single conditions now let me try to show you this execution on the r console so that you get more confident so you see when you are trying to execute such statements on the r console directly then first you have to give the value of the variable so for example if i say here x equal to 5 and then i have to give here this condition and as soon as i give this condition you can see here that this condition is going to give you the value 15 right and here in case if you try to take here the same condition but if you try to now put here the curly brackets you will see here you get the same outcome but in case if you try to take here x equal to suppose here 2 then you can see here what happens to this condition this will give you no output because there is nothing because the condition is false so there is no outcome and even if you try to write down here this curly brackets also this will not give you anything so this is what i meant when i was trying to explain you on this example so now let us try to take one more example and try to see what happens so you know there is a command here print so print is used to print something on the screen right so well we are going to discuss about different types of such options in the forth coming lectures but here i would like to show you something which is based on the use of print so now i want to print a value in case if the value is more than 3 so suppose i give here the value x equal to 6 and then i try to write down this requirement in the form of r contacts so i try to give here the condition the condition here is x greater than three so it is if inside the parentheses x greater than three and now in this curly bracket i am trying to write down the condition here so the condition here is print and whenever we are trying to print something which is scalar we try to give it inside the double quotes so i try to write down here within double quotes the value is more than three and i try to enclose it inside the parenthesis so now if you try to ah execute it because the value of x is six so 6 is greater than 3 this condition is true so whatever is there inside the curly bracket that will be executed and you get here this outcome so you can see here that if you try to take here x equal to 6 then it gives you here this outcome but if you try to take here at x equal to 2 then it will give you here no outcome y because 2 is greater than 3 no this condition is false right so yeah that is the same condition i want to show you here so let me try to show you here what happened with x x equal to two this two is greater than three this condition here is false and so this condition is false so whatever is written inside this curly brackets this will not be executed so if you try to execute it here no outcome is going to be obtained so let us try to do this example on the r console and then we try to see what happens ok so let me clear the screen and you know the command here is control l so now if i try to take here some value here x equal to suppose six and then i try to give this statement like is here you can see here the outcome is the value is more than three but on the other hand in case if i try to take here some value here two and if i try to repeat the same thing here you can see here there is no outcome here no outcome but in case if you just change it x to be here 10 and then if you try to repeat the command you can see here the outcome comes here once again the value is more than three right yeah it is also possible that you can also write what is the value of three so these things we are going to learn in the forthcoming lectures that you can write that you here that the value is more than three that is ok but what is the value which you are trying to compare so so you can also print the value of x but that we leave it at the moment for the future lectures right so now you have understood that how this if condition is going to work and but now you have screen here that in this case you have a constraint that if the condition is true only then whatever the condition you want that is executed but in case if the condition is false then nothing is happening but now in case if your wish or your requirement is that if the condition is true then something should be printed or something should be executed and if the condition is false then something else should be computed for example the example which i took in the beginning that if a student has got more than 50 percent marks then it should be for example printed that the student has passed and if the student has received the marks in less than 50 percent then it should be printed that the student is failed like this so these type of conditions can be executed using the conditional execution with the help of the concept of if else conditions so as the name suggests if else if it is true then something should be executed and else that means if it is not true then something else should be computed so if you try to understand this situation by the flow chart of this if else execution then it goes like this you start with something then there is a condition which is here checked now there are two options that the condition is true or the condition is false in case if the condition is true then it will work just like your if condition and whatever is the condition or the code you have written that is going to be executed and after executing it the program will stop but in case if the condition is false then in the earlier case there was no output but now in this case you are going to write down here some more here core that is called as else code means when the condition is false then under else these contacts are going to be executed and then after its execution the program will stop so this is how this if else condition works so in r the syntax for this if else condition is like as follows you try to write down here if or in lowercase alphabets and then inside the parenthesis you try to write down the condition now this condition is going to be either true or this condition is going to be here false in case if the condition is true then whatever you want to execute that has to be written inside the curly bracket just after this so this will go very true condition so there can be commands here they will be executed and in case if the condition is false then what will happen so after that you write here else e l s e all in lowercase alphabets and then within the curly brackets you try to write down the commands which you want to execute when this condition is false so that is a very simple thing that is a sort of extension of the if condition that you are adding here one more here syntax for the else so in this if else condition ah you have to be ah little bit watchful that the condition in the control statement may not be vector valued and in this case only the first element of the vector is used right and means ideally if else condition should not be applied when the condition which we want to judge or which we want to evaluate in terms of true and false is a vector and this if else condition is best use when you are trying to have a single element right and when you are trying to use this condition means you want to judge whether this is true or this is false this condition may be a simple expression or any complex expression where you can use all your logical operator like s and ah like i say or say or right whatever logical operators we have studied they can be used here in defining the conditions in the proper way ok so now let me try to take here one example and try to show you how the things are going to work and the more important part which i always say is is that you have to understand what and how are is going to execute it so now ah suppose i try to write down here a function like this means i can write down here this function takes value x minus 1 if x is equal to 3 and it takes value to x otherwise so that is the most simple language of mathematics in which we study so now in case if you want to understand what are you going to do you are trying to say that in case if the input here x is equal to 3 then this has to be executed that is x minus 1 and in case if number is something else that x is not equal to 3 like this then whatever the number is there that has to be multiplied by two so now in case if you want to write this statement then what you have to do the first thing is this you when you try to see here what about this x equal to 3 which i am writing here you are trying to say that if x is equal to 3 then execute x as x minus 1 and in case if x is not equal to 3 then execute x as twice of x what is this x this is an outcome that will be obtained after giving the input value and after this if you try to see this equality operator this is mathematical no this is logical so that is why when you try to write down the condition you have to use the logical operator which consists of two equality sign and that is indicating the exactly equal to sign under the logical operations so now if you try to see here how are we going to execute it so the inputs here suppose is x equal to 5 and now i write down here if and inside this parenthesis i try to write here x equal to 3 that is exactly equal to using the double equal to operator and now i am saying that in this case x should be replaced by x minus one so i try to write down here this curly brackets and within this curly bracket i write here x equal to x minus 1 and in case it is not true that means x is not equal to true then i try to write down here l and then within the curly bracket i write to i write down here x is equal to twice of x one thing i would like to have your attention here i mean and my advice is do not misunderstand this statement for example here i am trying to write down here x is equal to x minus 1 please understand this is not a mathematical operation what we are trying to say is that replace x by x minus 1 i mean some people may write it think it mathematically that x is equal to x minus one and they try to cancel x x it will be simply rubbish it has no meaning and similarly if you try to look at here this is here x is equal to twice of x this means replace x by twice x this is the meaning it is not like that you write x is equal to two x and then try to cancel x and x and then try to prove that one is equal to two which is wrong right so this is also wrong this is also wrong so please ah do not misunderstand this expression ok so now if you try to see here what will be the output so if i try to take here x equal to 5 then in this case x is not equal to 3 and so x will be operated as 2 2 into x so the value will come out to be here 10 and you can see here this is here the 10 and this value here is coming out to be here 10 right so let me try to execute this expression first on the r console and then i will try to move further so let me try to copy this condition so that i can save some time and now if i try to take here x equal to here 5 remember first you have to give the value of x as input and then i try to write down this condition and then i try to write down the value of here x so now you can see here you had given the value of x to be as five but now the value of x is transformed to ten similarly if you try to take it here x equal to here nine so what will happen here this condition is going to be operated once again and then if you try to print out the value of here x this is here eighteen that is two into nine is eighteen but now i try to take it here x equal to three and then if you try to repeat this condition and if you try to see what happened to the value of x this becomes here x minus 1 that is 3 minus 1 which is equal to 2 right so this is how you have to operate with this if else right so exactly this is the same thing which i just explained you on the r console that you try to take here x equal to 3 and then if you try to operate because x equal to 3 is now true so this is going to be operated and x will be replaced by three minus one is equal to two and then you get here an outcome two right ok so now i try to take here one more example quick what is the same example or a sort of extension of the earlier example which we considered that we want to print if a value is more than 3 or less than 3. so earlier we had printed the value is more than three but uh what if the value is smaller than three then there was no outcome right so now i try to extend it and i try to show you here how you can do so you can see here you have here two condition that you try to give here an input x and this condition is that that x is going to be checked whether this value is more than 3 or a smaller than 3 and based on that there will be an outcome true or false and based on that there will be two outcome if the condition is true then the then you have to print the value is more than three right like more than three and if the value is here false the outcome here is false then you have to print here less than three ok so now how to get it done so i try to write down here this statement if inside the parenthesis x greater than three and then within this curly bracket first set of curly brackets i write here print and then within the parenthesis and within the double quotes i write down here whatever i want to type the value is more than three and if this condition is false because this x greater than three will have here two outcomes one is here true so when this two is there this is going to be operated and if this outcome here is false then i have to write down here else and after that i have to write another pair of curly brackets and i have to write here what we want to do so here i want to print the value is less than 3 so i try to use here this parenthesis and i try to write down the statement within the double quotes so now in case if you consider here this x equal to here 6 because 6 is greater than 3 this condition is true so whatever is the condition under the true part that the value is more than 3 that is going to be printed here right and similarly if you try to take here say here one more value here that x equal to here 2 then what will happen 2 is greater than 3 this condition here is false when this condition is false then you are going to print whatever is written after the else command and it is here print the value is less than 3 right so you can see here the outcome will come here like this and you can see here this is here the screenshot of both these operations but let me try to ah show you these operations on the r console to make you more confident right so let me try to copy this command so that i can save some time and i try to take it suppose here x equal to five and if i paste this command you can see here it is printed here the value is more than three and in case if i try to take here x equal to two then you can see here it is printed here the value is less than three so you can see here it is not a very ah difficult command here and yeah so this is the screenshot so now ah we come to an end to this lecture and you can see here i have explained you ah two types of conditional execution one is using the condition if and another is the if else so these if and if else now you can very clearly understand they have got different types of role the if condition is going to be used only when you have one condition that if the condition is true then this has to be executed but in case if you have suppose two outcome and you want that in case of the if condition is true then this should be done and if the if condition is false then something else should be done then you have to use the if else condition and believe me these are very useful conditions because whenever you are trying to write down the programs basically in statistics at least i know that you have these types of condition that this has to be evaluated only when the condition is true for example many times you want to evaluate a mathematical expression only in case of the value of that expression is greater than 0 right suppose you want to find out the square root of some complicated expression and when you are trying to evaluate it you would like to find out the square root of that the outcome only when the outcome is positive and suppose the outcome can be positive or outcome can be negative so in this case this type of conditional execution help us and we always say please try to check this condition if this condition is greater than 0 only then you try to find out the square root so these are the ingredients of a program so now my request to all of you is that you try to take some example try to create the example yourself because when you are trying to create an example yourself you also know the answer and then you try to see that when you are trying to execute the same concept in the r software are you getting the same outcome and that will give you more confidence and after that you will become a good programmer so you try to practice it and i will see you in the next lecture with more details on the conditional executions till then good bye [Music] do [Music] [Music] you