Lecture Notes: User Input in C

Hey, welcome to Draft Academy. My name is Mike. In this tutorial, I'm gonna show you guys how to get input from a user in C. So a lot of times in our C programs, we're going to be working with all different types of information. And a lot of times we're going to want to get some of that information from the user. So I'm gonna show you guys how we can prompt the user to enter in some information, we can take that information, store it inside of variables, and then we can use it inside of our program. So this is going to be pretty cool. So down here, I will basically write out the code for this. And the first thing we want to do whenever we're getting input from the user is we want to prompt them. So I want to prompt the user, you know, as far as what they should enter in. So I'm just gonna say print F. And over here, I'm just going to print out a prompt. So why don't we ask the user to enter in their age. So we'll I'll show you guys how we can get a number from the user. So I'm just gonna say enter your age. And now that we've prompted them to enter their age, I need to do two things. So the first thing I want to do is create a variable where we can store the input that the user puts into the program. So I want to create a variable where we can store the age that the user inputs. So I'm going to create an in over here, it's called age. And I'm not going to give this a value. So all I'm going to do up here is just declare the variable, I'm just going to tell C that I want to use this variable, but I'm not going to give it a value. In other words, I'm going to allow the user who's inputting the age to give this a value. So down here, I want to be able to get input for from the user. So I can use another function, which is called scan F. And scan F is basically going to allow the user to enter in some information into our program. And this works similar to print F, it's kind of doing the opposite of print F. printf is printing something out onto the screen scan f is allowing the user to input something into the program. And we're gonna make an open and close quotation marks. And in here, I basically want to tell see what type of information I'm asking the user to enter in. So in our case, we're asking for an integer, right age is going to be an integer, it's a whole number. So I'm going to accept as an input an integer. And over here, we want to tell see where we want to put that integer. So I'm basically going to tell see what variable I want to store this in. So I'm going to store this inside of my age variable. And I can essentially just type out the name of the variable here. But in order to get input from the user, I'm actually going to have to type an ampersand in front of this. So instead of just typing out age, I'm going to have to type ampersand age. And when I say ampersand age, this is what's called a pointer. And we're going to talk about pointers in a later tutorial. I'm going to cover everything you need to know about pointers. But for now, that's a little bit beyond what we need to learn. So all you need to know is that when you're using scan F, and you want to store information inside of like, an integer or a float, or like a character, you need to use this ampersand over here. So down here, now that we've scanned for the user's age, I'm just going to go ahead and print it out. So we'll just print out like you are, and I'll say percent D years old. And over here, I'm just going to print out that age variable. So essentially, what I'm doing is I'm prompting the user to enter their age, I'm storing whatever age they enter inside of this age variable, and then I'm printing it out to them. So let's go ahead and run this program. And we'll see how we did. So I'm gonna run the program. And it says enter your age. So let's say someone is 50. When I click Enter, it's going to take that value, it's gonna take that integer that we entered 50, store it inside of that age variable and print it out. you'll see over here, it says you are 50 years old. So that's how we can get an integer from the user, we can also get like a double from the user. So for example, I could say enter your GPA. And so now instead of asking for an integer, we're going to be asking for a double. So I can come up here and say like double, and we can call this GPA. And now I can do the same thing. But instead of saying D, I want to say l f, and l f is basically going to tell this scanner. function that we're looking for a double. And then obviously, instead of age, we're just going to put GPA inside of here. And so now we can just say like, your GPA is, and then we can put percent f, because we're going to be printing out a double, and I'll just say GPA. So notice here in printf, when we want to use a double, we're using percent f to print it out. But when we're using scan f, we're going to use percent lf. So that's just like a little difference. And now let's go ahead and run this program. And we should be able to get a GPA. So let's say someone's GPA is like 3.1. Now it says your GPA is 3.1. So that's how we can get a double. I also want to show you guys how we can get a character. So why don't we create a character up here, and we'll just call it grade. And we'll say enter your grade. And now when we want to get a character, we can just say percent C. And again, I can just come down here and say ampersand grade. And then down here, we can just say your grade is percent C. And we'll go ahead and print out the grade. So now we should be able to get a character from the user. So enter the grade, let's say I got an A, and it says your grade is A. So we can use this scan F to get specific types. of input from the user, we can store those specific types of input inside of variables. Now I also want to show you guys one more thing we can do, which is we can actually get a string from the user. So in addition to getting numbers and a character, we could also get like a string of characters from the user. And this is going to be a little bit different from doing like numbers and characters. So I want to kind of show you guys how we can do this really quick. So over here, let's create a variable, we'll just call it Let's just call it name. So we'll be storing someone's name. And whenever we're creating a string of characters, we always need these open and close square brackets. And inside of here, I'm actually going to specify how many characters I want to be able to store inside this string of characters. So I'm just going to put 20. And in the past in this course, when we've been creating strings, we haven't put a number in there, we've just kind of said like, whatever, right, we've basically just given this a value straight away. But in this particular situation, we're not going to be giving this variable of value right away. In other words, like I don't know what the user's name is going to be like, I don't know that right up front, they're going to tell us what that is. And so if I'm not going to give this a value right away, I do actually need to tell see how big I want this variable to be. In other words, I need to tell see how many characters I want this variable to be able to store. And that way, see can go ahead and allocate enough memory for this variable. So I'm just going to put 20 and 20 basically means this will be able to store 20 characters. And I think that's enough for a name. So down here, we'll just say enter your name. And I can use scan F in order to get input in the form of a string. But instead of saying percent C, I'm just going to say percent s. And over here, instead of saying ampersand grade, I'm just going to type out the name of the string of characters. So I don't actually need this ampersand here, I can just specify the name of the string. So down here, it says print f your grade is, and actually, let's just say your name is, I will just say percent s. And then over here, we can print out the name. So this should work just like it did in the other cases. So let's go ahead and run this. And it says enter your name. So my name is Mike. And you'll see it says your name is Mike. So that works out really well. But there is one problem when we're using scan F in order to get a string from the user. So for example, if I came in here, and I said, enter your name, and I said, My name was John Smith, when I click Enter, you'll notice that it's only saying your name is John, it's not including Smith over here. And here's the problem. This is because whenever I use this scan F function, and I use it with a string, it's only going to grab the characters up to the first space. So once it sees this space, it's going to be like, okay, we're done getting characters, right? So that's kind of a problem. And that's just kind of how scan F works. It's not really, you know, scan F's fault. There is a way that we can modify scan F in order to be able to get input with spaces. But there's another function, which I want to show you guys, which we can use to get a line of text from the user, and it's called F gets. And f gets is another function, it's similar to scan F. But f gets is basically going to be more general. So f gets is essentially just going to grab like a whole line of text, it's not going to be able to grab it and store it inside of like an integer or a variable, or a character, it's just going to be able to store it inside of like a string of characters. So when we're using f gets, the first argument we want to give it is the name of the variable where we want to store the line of text. So in our case, it's just going to be this name variable. The next thing we want to do is specify how many characters we want to be able to input from the user. So this will essentially limit the amount of characters that the user can input. And this is always a good idea when we're trying to get a string from the user. Because one common problem is that the user could try to enter in like a million characters, and they would overflow the like the buffer. In other words, like C wouldn't be able to handle accepting that many characters and the program might break. So over here, with F guts, we can specify how many characters we want to be able to accept. So I'm just going to say 20. Because that's how many characters we can store inside of this variable. And over here, I'm just going to say st di n. And this stands for standard in essentially, what we're doing over here is we're telling f gets where we want to get the information from and standard input is set as essentially just like that little console that we've been using. So now I'm using f gets and this is going to do exactly what scan F did. So we'll be able to store the input inside of this name variable. But now we'll be able to store multiple words instead of just one single word. So let's run this program. And it says enter your name. So now if we entered in John Smith, you'll see it's able to enter in john Smith. Now the one downside with just using this F gets is you'll notice that it printed out this new line over here. So let me actually demonstrate this a little bit. So if I was to print out like, you know, just some random text over here, whenever I get input using scan F, so if I entered in john Smith, when I click the Enter key, so when I'm entering this by clicking the Enter key, that's actually going to represent a new line character. And that's going to get stored inside of the string. So when I click Enter, you'll notice it says your name is John Smith, then it prints out a new line character and it prints out this text. And so that's just something that you need to be aware of when you're using this f gets function. But for the most part, I would say if you're getting input from a user in the form of a string, you can use f gets. You could also use scan F and you could use multiple percent s's. And I'm actually going to show you guys how we could do something like that in a future tutorial. But for the most part, I would say whenever you're getting a string from the user, you just want to use F gets. So that's the basics of getting input from the user. Like I said, there's a couple other things that we can do. And I'm going to talk about at least one other way that we can get strings from the user in a future tutorial. But this is kind of the basics. And you can kind of play around with doing this stuff.

And a lot of times we're going to want to get some of that information from the user. So I'm gonna show you guys how we can prompt the user to enter in some information, we can take that information, store it inside of variables, and then we can use it inside of our program. So this is going to be pretty cool. So down here, I will basically write out the code for this.

And the first thing we want to do whenever we're getting input from the user is we want to prompt them. So I want to prompt the user, you know, as far as what they should enter in. So I'm just gonna say print F.

And over here, I'm just going to print out a prompt. So why don't we ask the user to enter in their age. So we'll I'll show you guys how we can get a number from the user. So I'm just gonna say enter your age.

And now that we've prompted them to enter their age, I need to do two things. So the first thing I want to do is create a variable where we can store the input that the user puts into the program. So I want to create a variable where we can store the age that the user inputs.

So I'm going to create an in over here, it's called age. And I'm not going to give this a value. So all I'm going to do up here is just declare the variable, I'm just going to tell C that I want to use this variable, but I'm not going to give it a value.

In other words, I'm going to allow the user who's inputting the age to give this a value. So down here, I want to be able to get input for from the user. So I can use another function, which is called scan F.

And scan F is basically going to allow the user to enter in some information into our program. And this works similar to print F, it's kind of doing the opposite of print F. printf is printing something out onto the screen scan f is allowing the user to input something into the program. And we're gonna make an open and close quotation marks. And in here, I basically want to tell see what type of information I'm asking the user to enter in.

So in our case, we're asking for an integer, right age is going to be an integer, it's a whole number. So I'm going to accept as an input an integer. And over here, we want to tell see where we want to put that integer. So I'm basically going to tell see what variable I want to store this in. So I'm going to store this inside of my age variable.

And I can essentially just type out the name of the variable here. But in order to get input from the user, I'm actually going to have to type an ampersand in front of this. So instead of just typing out age, I'm going to have to type ampersand age.

And when I say ampersand age, this is what's called a pointer. And we're going to talk about pointers in a later tutorial. I'm going to cover everything you need to know about pointers. But for now, that's a little bit beyond what we need to learn. So all you need to know is that when you're using scan F, and you want to store information inside of like, an integer or a float, or like a character, you need to use this ampersand over here.

So down here, now that we've scanned for the user's age, I'm just going to go ahead and print it out. So we'll just print out like you are, and I'll say percent D years old. And over here, I'm just going to print out that age variable.

So essentially, what I'm doing is I'm prompting the user to enter their age, I'm storing whatever age they enter inside of this age variable, and then I'm printing it out to them. So let's go ahead and run this program. And we'll see how we did.

So I'm gonna run the program. And it says enter your age. So let's say someone is 50. When I click Enter, it's going to take that value, it's gonna take that integer that we entered 50, store it inside of that age variable and print it out. you'll see over here, it says you are 50 years old.

So that's how we can get an integer from the user, we can also get like a double from the user. So for example, I could say enter your GPA. And so now instead of asking for an integer, we're going to be asking for a double.

So I can come up here and say like double, and we can call this GPA. And now I can do the same thing. But instead of saying D, I want to say l f, and l f is basically going to tell this scanner. function that we're looking for a double. And then obviously, instead of age, we're just going to put GPA inside of here.

And so now we can just say like, your GPA is, and then we can put percent f, because we're going to be printing out a double, and I'll just say GPA. So notice here in printf, when we want to use a double, we're using percent f to print it out. But when we're using scan f, we're going to use percent lf.

So that's just like a little difference. And now let's go ahead and run this program. And we should be able to get a GPA.

So let's say someone's GPA is like 3.1. Now it says your GPA is 3.1. So that's how we can get a double.

I also want to show you guys how we can get a character. So why don't we create a character up here, and we'll just call it grade. And we'll say enter your grade. And now when we want to get a character, we can just say percent C.

And again, I can just come down here and say ampersand grade. And then down here, we can just say your grade is percent C. And we'll go ahead and print out the grade. So now we should be able to get a character from the user.

So enter the grade, let's say I got an A, and it says your grade is A. So we can use this scan F to get specific types. of input from the user, we can store those specific types of input inside of variables. Now I also want to show you guys one more thing we can do, which is we can actually get a string from the user. So in addition to getting numbers and a character, we could also get like a string of characters from the user.

And this is going to be a little bit different from doing like numbers and characters. So I want to kind of show you guys how we can do this really quick. So over here, let's create a variable, we'll just call it Let's just call it name.

So we'll be storing someone's name. And whenever we're creating a string of characters, we always need these open and close square brackets. And inside of here, I'm actually going to specify how many characters I want to be able to store inside this string of characters. So I'm just going to put 20. And in the past in this course, when we've been creating strings, we haven't put a number in there, we've just kind of said like, whatever, right, we've basically just given this a value straight away.

But in this particular situation, we're not going to be giving this variable of value right away. In other words, like I don't know what the user's name is going to be like, I don't know that right up front, they're going to tell us what that is. And so if I'm not going to give this a value right away, I do actually need to tell see how big I want this variable to be.

In other words, I need to tell see how many characters I want this variable to be able to store. And that way, see can go ahead and allocate enough memory for this variable. So I'm just going to put 20 and 20 basically means this will be able to store 20 characters. And I think that's enough for a name. So down here, we'll just say enter your name.

And I can use scan F in order to get input in the form of a string. But instead of saying percent C, I'm just going to say percent s. And over here, instead of saying ampersand grade, I'm just going to type out the name of the string of characters. So I don't actually need this ampersand here, I can just specify the name of the string. So down here, it says print f your grade is, and actually, let's just say your name is, I will just say percent s.

And then over here, we can print out the name. So this should work just like it did in the other cases. So let's go ahead and run this. And it says enter your name.

So my name is Mike. And you'll see it says your name is Mike. So that works out really well. But there is one problem when we're using scan F in order to get a string from the user. So for example, if I came in here, and I said, enter your name, and I said, My name was John Smith, when I click Enter, you'll notice that it's only saying your name is John, it's not including Smith over here.

And here's the problem. This is because whenever I use this scan F function, and I use it with a string, it's only going to grab the characters up to the first space. So once it sees this space, it's going to be like, okay, we're done getting characters, right? So that's kind of a problem.

And that's just kind of how scan F works. It's not really, you know, scan F's fault. There is a way that we can modify scan F in order to be able to get input with spaces. But there's another function, which I want to show you guys, which we can use to get a line of text from the user, and it's called F gets. And f gets is another function, it's similar to scan F.

But f gets is basically going to be more general. So f gets is essentially just going to grab like a whole line of text, it's not going to be able to grab it and store it inside of like an integer or a variable, or a character, it's just going to be able to store it inside of like a string of characters. So when we're using f gets, the first argument we want to give it is the name of the variable where we want to store the line of text. So in our case, it's just going to be this name variable.

The next thing we want to do is specify how many characters we want to be able to input from the user. So this will essentially limit the amount of characters that the user can input. And this is always a good idea when we're trying to get a string from the user.

Because one common problem is that the user could try to enter in like a million characters, and they would overflow the like the buffer. In other words, like C wouldn't be able to handle accepting that many characters and the program might break. So over here, with F guts, we can specify how many characters we want to be able to accept.

So I'm just going to say 20. Because that's how many characters we can store inside of this variable. And over here, I'm just going to say st di n. And this stands for standard in essentially, what we're doing over here is we're telling f gets where we want to get the information from and standard input is set as essentially just like that little console that we've been using. So now I'm using f gets and this is going to do exactly what scan F did.

So we'll be able to store the input inside of this name variable. But now we'll be able to store multiple words instead of just one single word. So let's run this program. And it says enter your name. So now if we entered in John Smith, you'll see it's able to enter in john Smith.

Now the one downside with just using this F gets is you'll notice that it printed out this new line over here. So let me actually demonstrate this a little bit. So if I was to print out like, you know, just some random text over here, whenever I get input using scan F, so if I entered in john Smith, when I click the Enter key, so when I'm entering this by clicking the Enter key, that's actually going to represent a new line character.

And that's going to get stored inside of the string. So when I click Enter, you'll notice it says your name is John Smith, then it prints out a new line character and it prints out this text. And so that's just something that you need to be aware of when you're using this f gets function. But for the most part, I would say if you're getting input from a user in the form of a string, you can use f gets. You could also use scan F and you could use multiple percent s's.

And I'm actually going to show you guys how we could do something like that in a future tutorial. But for the most part, I would say whenever you're getting a string from the user, you just want to use F gets. So that's the basics of getting input from the user.

Like I said, there's a couple other things that we can do. And I'm going to talk about at least one other way that we can get strings from the user in a future tutorial. But this is kind of the basics. And you can kind of play around with doing this stuff.

Transcript for:Lecture Notes: User Input in C

Transcript for:
Lecture Notes: User Input in C