Simple input and output

In the previous examples, we used the built-in print and say functions to print something to the console (speaking more strictly, to the standard output attached to the program). In this section, you will learn how to perform basic reading from the standard input. This is basically how the program gets what you type onto the console.

To read the input, there are a few functions that you may use directly without loading any modules. They are listed in the following table:

 

The get and line functions may be used when you need to parse the input data line by line. For example, call get as many times as you need if you know the structure of the input, or create a loop and iterate over the array that is returned by lines.

The slurp function does the job in one go. You can use it, for example, to copy everything from input to output. This is the program that does that:

print slurp;

There is another useful function: prompt. Use it to make two actions at once: the function prints a text message on the screen and returns the string that the user entered. This function blocks the execution of a program until the user finishes the input with a new line.

Let's demonstrate the work of the prompt function on an example program that calculates the circumference of a circle. The following program requests the radius from the user and then prints the result.

say 'The circumference is ', 
    tau * prompt 'Enter the radius > ';

The program first prints the prompt message Enter the radius > and waits until the user types a number and presses the Enter key. Then, the say function prints another message, The circumference is, and appends the value that it gets after multiplying the input value by τ, which, as we've seen in the Using Unicode section of this chapter, equates to 2π. We can see that in the following code:

$ perl6 circumference.pl
Enter the radius > 12
The circumference is 75.398223686155

We intentionally did not introduce any variables (we'll talk about them later in Chapter 3, Working with Variables and Built-in Data Types). Notice that the actual output happens from right to left: first, the message from prompt, then the text printed by say.

More sophisticated input and output, as well as working with files, are discussed in Chapter 9, Input and Output.