Control Flow in Python - If Elif Else Statements

In almost every program, there are times you need to make decisions and that's when you use an if statement here is an example let's say we have a variable called temperature we set it to 35 now if temperature is greater than 30 perhaps we want to display a message to the user so we use an if statement if after if we add a condition which is basically a boolean expression an expression that produces a boolean value so if temperature is greater than 30 here we have a boolean expression if this expression evaluates to true the following statements will be executed let me show you now here is the important part that a lot of beginners miss when you use an if statement you should always terminate your statement with a colon now let's see what happens when I press ENTER our cursor is indented so here we have two white spaces this is very important because using these indentations Python interpreter will know what statements should be executed if this condition is true here we want to print a message like it's warm we can print another message as well drink water so we can have as many statements as we want here as long as they are indented they belong to this if block now when we finish here we should remove indentation to indicate the end of this if block so here we can add a print statement with a message like done this statement will always be executed whether this condition is true or not now note that when I save the changes this indentation you see here is going to be doubled up take a look save there you go so when we save the changes auto kept eight reformat our code and uses four white spaces for indentation so one two three four it uses four white spaces because that's what pepp eight recommends alright now let's run this program so because temperature is greater than 30 we see the first two messages and we see the dawn message regardless so if I change the temperature to let's say 15 and run the program one more time look this dawn message is executed whether our condition is true or not so pay great attention to these indentations that's one of the issues I see in beginners code let's say they want both these print statements to be executed if the condition is true accidentally they remove the indentation on the fourth line and that's why their program doesn't work as they expect so be careful about this now what if you want to have multiple conditions we use an elephant so L if that is short for else/if here we can add another condition another expression so temperature is greater than 20 once again : enter a by default here vs code is using two white spaces so don't worry about this as soon as you save the changes those two white spaces will be converted to four white spaces so let's print a different message it's nice save the changes now look all these lines are indented consistently you can have as many Elif statements as you want and optionally you can also have an else statement so if none of the previous conditions are true then what you have in the else block will be executed once again we add the colon annotation print here we can add a message like it's called saying the changes in this case temperature is 15 so none of these two conditions will be true and we will see it's called let's run the program there you go in this lecture I'm going to show you a technique for writing cleaner code so let's say we're building an application for university and we want to check to see if the person who's applying for this university program is eligible or not so we start by defining a variable called age set it to 22 now if H is greater than or equal to 18 colon print eligible remove the indentation else colon print not eligible let's run a program make sure it works beautiful now there is nothing wrong in this piece of code but I want to show you a cleaner way to achieve the same result instead of having a print statement here we can define a variable like message and set it to this string that is the first step so message equals this string and then we will print this message now when you have an if-else statement with this structure where you're basically assigning a value to a variable you can rewrite this in a simpler way so this is how it works all we want to do over these few lines is to assign a value to this message variable right so we start with message we set it to eligible if H is greater than or equal to 18 else we set it to not eligible this statement is almost like plain English so what we have on line 7 is exactly equivalent to these 4 lines of code delete save the changes run the program you can see this person is eligible if I change the age to 12 and run the program we get not eligible so what we have here is called ternary operator in Python we have three logical operators and we use these operators to model more complex conditions so these operators are and or and not let's see a real-world example of using these operators so imagine we're building an application for processing loans so we need two variables high income we can set this to true and good underline credit we set it to true now here's the condition we want to implement if the applicant has high income and good credit score then they are eligible for the loan so if high income and good credit we add the colon and print eligible now note that here I have not compared the value of this variable with true that is one of the issues I see in a lot of beginners code this is redundant and unprofessional because high income is a boolean so it's either true or false we don't need to compare true with true so if this condition is true and this second condition is true then we will print eligible in the terminal so save the changes and run the program obviously this person is eligible however if one of these conditions is false we will not see eligible in the terminal so let's add an else statement here and print not eligible run the program we see not eligible so this is how the and operator works with an operator if both conditions are true the result will be true in contrast with the or operator as long as at least one of the conditions is true the result will be true so if I replace and with or here we should see eligible and the terminal let's run it one more time there you go so these are the and an or operator now let's take a look at an example of the nut operator so I'm gonna define another variable steel and set it to true temporarily I'm gonna remove this expression and simplify it we'll come back to this later so let's say if the person is eligible if they are not a student the nut operator basically inverses the value of a boolean so in this case student is true when we apply the not operator the result will be false so in this case our condition will be false and that's why this print statement will not be executed let me show you so save run the program they are not eligible if student was false when we apply the nut operator will get true so our condition will be true and we'll see it eligible let's run it one more time there you go with this operators we can model even more complex conditions here is an example a person can be eligible if they have either high income or good credit and they should not be used to do that let me show you how to implement this condition so if high income or good credit we want at least one of these conditions to be true so we put these in parentheses we want to separate these from the other condition which is not a student now the result of this should be true which means at least one of these conditions should be true after that we'll add and not student and finally caught so we this operators you can model all kinds of real-world scenarios so here's the example from the last lecture a person is eligible for a loan if they have high income and good credit and they are not a student now one thing you need to know about these boolean operators is that their short circuit what do we mean by that well when python interpreter wants to evaluate this expression it starts from the first argument if this is true it continues the evaluation to see if the second argument is also true so it continues the evaluation all the way to the end of this expression however as soon as one of these arguments is false the evaluation stops let me show you what I mean so if I change high-income to false when Python interpreter sees this expression it starts here it knows that high-income is false so it doesn't matter what comes after the result of this entire expression will always be false because at least one of the arguments or one of the operands is false this is what we call short-circuiting just like the short-circuit concept we have in electronics so the evaluation stops as soon as one of these arguments evaluates to false we have the same concept with the or operator so if I change these and operators to or let's see what happens with the or operator we know that at least one of the arguments should be true so the evaluation stops as soon as we find an argument that evaluates to true in this case when Python interpreter evaluates this expression it sees that high-income is false so it continues the evaluation hoping that the next argument will be true here good credit is true so evaluation stops and the result of this entire expression will be true so in Python logical operators are short-circuit in this lecture I'm gonna show you how to chain comparison operators this is a very powerful technique for writing clean code here is an example let's say we want to implement a rule that says age should be between 18 and 65 here's how we can implement it so we define a variable like age set it to 22 now if age is greater than or equal to 18 and age is less than 65 then we print eligible now here's a question for you how do we write this rule in mass we can write it like this well more accurately we should have an equal sign here so age should be between 18 and 65 this is how we write this rule in math now I've got some good news for you we can write the exact same expression in Python so I'm going to move this up put an if statement here line 4 and line 3 are exactly equivalent but as you can see line 4 is cleaner and easier to read so let's get rid of line 3 this is what we call chaining comparison operators all right here is a little quiz for you I want you to pause the video and think about this quiz for 10 to 20 seconds what do you think we'll see on the terminal when we run this program so pause the video figure out the answer when you're ready come back continue watching all right let's see what happens when we run this program first we get this if statement in this case we're comparing two different objects for equality and these objects have different types we have a number compared for this string so number 10 and string 10 are not equal that is why a will not be printed on the terminal so the control moves to the Elif part here we have two boolean expressions here's the first one here's the second one and they are combined using the logical end so if both these expressions are evaluated to true then this entire expression will be true and we will see beyond the terminal let's see if both these expressions are evaluated to true here's the first part bag is greater than Apple that is true because when we sort this words bag comes after Apple but look at the second part this expression is evaluated to false because bag is not greater than cat so when we applied the logical end between true and false the result will be false that is why this statement will not be executed so to control moves to the else part and when we run this program the letter C will be printed on the terminal hi guys thank you for watching this tutorial my name is Mohammad Ani and I have tons of tutorials like this for you on my channel so be sure to subscribe and also please like and share this video if you want to learn Python properly from scratch with depth I have a comprehensive python tutorial for you the link is below this video so click the link to get started thank you and have a fantastic day

Channel: Programming with Mosh

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Keywords: control flow, conditional statements, if elif else, conditional statements in python, elif, elif capítulo, python if, python if else statement, if statements python, python if else, python if statement, if else statements, python elif, if else statement in python, if statement in python, if statement, if else, python tutorial, python tutorial for beginners, code with mosh, learn python, how to use if else statements, conditionals, conditional statements python

Id: Zp5MuPOtsSY

Length: 16min 8sec (968 seconds)

Published: Tue Nov 06 2018