WEBVTT

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Welcome back.

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In this lecture you're going to learn how to scan for input from the command-line you're going to build

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a program that counts the number of lines in the input.

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All right let's get started.

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First of all let's talk about what is scanning and how to scan for input.

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Simply put you can think of scanning as reading a stream of input data.

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Line by line into a byte buffer In Go

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You can use the buffer are your packages scanner type.

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To do that it scans an input stream such as a file line by line into a buffer.

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By the way the scanner can retrieve the input data from anywhere.

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For example you can receive the input stream data from the command line or a network source on the Internet

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or a file and so on by default the scanner stops scanning when it sees a new line character.

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This is its default behavior but you can set it to stop for anything.

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For now we're going to take a look at its default behavior.

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Let's say you have an input stream from a file like.

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So here is the first line from the file waiting to be scanned.

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You can call the scan method of the scanner to retrieve the first line.

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Like so the scanner puts the scanner data into an internal byte buffer when it puts the data into the

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buffer.

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You can retrieve it by calling the text method of the scanner like so it returns a string so you can

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work on it however you want.

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By the way the scanner buffers the data because it wants to read it only when the data is ready.

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This way it becomes more efficient.

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You had learned about using a buffer previously.

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It is the same thing here.

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OK.

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Now you need to call the scan and text methods repeatedly after it consumes all of the lines in the

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input stream like so. When there is an error or no more data to scan the scan method returns false

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so it can stop scanning for the input.

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So when there is more data it returns true instead.

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Okay now let's continue discussing the scanner in the coding.

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editor. You can create a scanner by using the NewScanner method of the bufio package like so. in means

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input stream.

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Okay let's take a look at its documentation.

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It accepts an IO.Reader interface and returns a pointer to a struct value.

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You don't have to know about interfaces and structs to use the scanner.

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For now let's pass it the standard in like so like everything in the Linux operating system standard.

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It is also a file and it supports the io.Reader interface.

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That's why I can pass it to the new scanner function here.

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You'll see how it works in a second.

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OK.

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Now the in variable contains a reference to a newly created scanner.

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Let's scan for the input stream by using the scan method.

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Like so when I run the program it waits for me to type something on the surface it looks like it doesn't

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do anything.

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However the scanner has buffered it (the data).

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The first line of the input to its own hidden buffer.

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Behind the scenes.

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So you need to get the data from it by using the text method.

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I'm going to call println "scanned text" in.Text.

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Okay.

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Let's take a look at its documentation.

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It just returns the scanned line as a string as you can see by using the text method you can retrieve

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the buffer data like so please note that it only puts the last scanned line into the buffer.

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So for example let me call the scan method a few times.

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Like so let me also print the buffered data a few times I have typed the lines and I pressed the Enter

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key three times.

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However in the end it only gets me the last line no matter how many times I call the text method.

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So let me take these back.

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I can also get the bytes of the input like so these may become handy when you are working with bytes

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only.

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For example I could have appended the returned bytes to a byte slice here

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if I wanted to. It print the bytes that I've typed.

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OK let me comment out it for now.

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Okay now let's say I want to read all the lines from the input data.

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To do that I'm going to use a loop like so I can do so.

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Because the scan method returns a bool value when the input data ends or there is an error.

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It returns false.

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So the loop eventually will quit as you can see whenever I hit the enter key it prints me

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what I've typed I am inside of the loop right now.

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So it keeps me asking for input to create from the loop.

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I can hit the Control+D keys together.

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If you are on Windows you may need to use Control+Z keys together.

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So now it quits from the loop because the scan method has returned false as the last example.

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Let's get the input from a file.

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I don't have to change my code to do that.

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I can simply redirect the standard in to the program like so cool I can read the file line by line.

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This works because the scanner doesn't care where it gets the input from here.

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It gets the input from the standard input.

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And by using this I redirected the standard input to the contents of the proverbs.txt file.

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This is called the command redirection and you can find more information about it in the resources of

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this lecture by the way you can find the proverbs file.

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Also in the resources of this lecture okay now I can count the lines in the file very simply I'm

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going to declare a variable here like so that I'm going to comment out the text call from here.

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You understand how it works. Instead, I'm going to increment the lines variable here because on each iteration

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the scanner returns a single line.

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Lastly I'm going to print the number of lines like so there are D lines in the file.

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I'm going to pass the lines variable here. Awesome!

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Congrats you have created a program that can count the lines from any input stream.

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There is one last thing though.

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What happens if there is an error when reading the file.

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While the scanner has an error method that returns an error instead of handling the error in the loop

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each time it allows you to handle it once it's because the scan method returns false when there is an

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error.

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Let's check for the error and print it.

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I'm going to close to standard input above like so.

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So it will return an error when I try to scan it.

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As you can see now it says that the file has already been closed.

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So now you have learned how to check for the errors with scanner as well.

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All right.

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That's all for now.

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See you in the next lecture bye!