Learning to Code with Python: Lesson 1.7 - Input and Conditional Statements

Key Takeaways

welcome back to learning to code with python lesson number seven input and conditional statements so up to now all of our programs have been non-interactive we just run them and sit back to watch what happens but what if you want to be able to ask a question in your program and have the person running it be able to answer well let's find out so I'm starting a new program here and we're going to leave Fred the turtle behind for now because we won't need him for this so first let's ask the person the program the user to tell us their name to do that I'm going to use the input function so the way the input function works is in the parentheses you put the question that you want to ask so this is what the user will see when they run the program and then it'll wait for them to type in a response and whatever response they type in will get saved into the variable name so now that we have the user's name saved in the variable name we can say hello a print function is pretty neat it lets you put more than one item inside the parentheses if you want and it'll print all of them out you just have to put a comma between them so I'm going to print out this part in quotes saying hello and then whatever the VAR variable name is equal to so let's run it and see what happens what's your name there we go right at this point we need to talk about something that the input function does that you need to watch out for here's an example let's say in our program we're going to ask the user what their Agee is so we ask them how old are you okay and when it asks you the question let's say we type in a number okay I'll say a number okay so now that number 30 has been stored in the variable age and we could print out age but here's something that you may not expect what if we said next year you will be and we'll put age + one now you would expect it would print out 30 + 1 but instead you get an error message the error message says it can't convert int to string what does that mean well the variable age if we look at what type it is and type is a way to tell you what kind of data is stored in that variable it's a string anytime you type in something with input it saves it as a string meaning it doesn't think of it as a number it just thinks of it as some string like this but that means that we can't add to it right because that's an error message so we need a way to tell the computer that we want this data that we input to be treated as a number so that we can add one to it we can do that with a function called int int basically takes a string and we'll convert it into a number so if we say int age plus one that's what we wanted so if we were to try our print statement again we need to make sure we just put int before age and that's what we wanted let's ask for a different number now in our program we're going to ask how many ninjas are attacking and remember we want this to be a number so I want to make sure and put the int function around that now I want my program to do something different depending on how many ninjas are attacking if there's more than 50 I want to print out that's too many otherwise we can print out something like we can take them but here's the thing that requires us to do something called a branch or a conditional statement that means we want different commands to run depending on a different circumstance so for example so here's what that looks like we use the command if to indicate we're going to do a branch so this conditional statement will say if ninjas is greater than 50 so the computer is going to make that comparison it's going to compare whatever Ninjas the value of ninjas is and if it's greater than 50 then it's going to execute this block of code that comes after remember just like we did in our for Loops we have a block of code that's indented underneath the for Loop for what commands we wanted it to repeat same thing here so this command print that's too many will only execute if ninjas is greater than 50 if if this comparison right here is false then this command does not get executed so let's try that out so if I say let's put a bigger number first if I say 51 then it prints out that's too many what about if I run it again and this time I'm going to say that there's fewer so there's only 24 attacking I see nothing happened so how do we add a second option well we can use the keyword else so this gives us another block of code that's the alternative case so if ninjas is greater than 50 you should do this but if it's not you're going to do this which is print we can take them so now our program has two paths it can follow depending on the value of ninjas it will either run this print statement or this print statement but only one let's add one more option let's say that we have three branches that we want this program to take if it's greater than 50 that's fine but what about maybe somewhere in between well we can add a third choice by adding L if now L if you can think of as short for else if so it's basically going to do another test we're going to say l if ninjas is greater than 20 and if that's the case then we're going to print maybe we can do it okay so now we have three different paths if ninjas is greater than 50 this happens none of the rest does if ninjas is not greater than 50 then it goes to the next option is it greater than 20 if it is do this and if it wasn't either of these then the last case is this one so let's try that out let's try 18 18 is not greater than 20 so we can take them and we'll try one more and let's say we're somewhere in the middle branching is a key Concept in programming and we'll make use of it quite a lot as we go through this series in our next lesson we'll use some if statements to make a simple game