`Console`

Programs are often run with console input and output. We use the puts(), print() and p() methods in Ruby to display text (or other data types).

With stdin, we handle input from the console. Operators are helpful. And often a loop is used to create an interactive console program with a prompt.

Example

We begin with a simple program—here the puts() method writes a line to the console window. It writes each argument to a separate line.

Tip To combine multiple parts on a single line, we can concatenate strings. We use operators for this.

Also It sometimes is necessary to use the String constructor to convert integers to strings.

Note When displaying an array, puts will place each element onto a separate line.

value = 100

# Print values on separate lines.
# ... Parentheses are optional.
puts value
puts("FINISHED")

# Use << to append a string.
puts "VALUE " << String(value)

# Use + to append.
puts "VALUE " + String(value)

# Print all Array elements on separate lines.
elements = [10, 100, 1000]
puts elements100
FINISHED
VALUE 100
VALUE 100
10
100
1000

P method

Let us continue with the P method. This one is good for lazy programmers. It displays data in a literal way—it includes quotes around a string, for example.

And P displays an Array or Hash on a single line. This is convenient and often a good choice.

Tip As with other methods in Ruby, parentheses are optional. Usually "p" is used without parentheses.

# Use p method.
p "cat"

# Write an array.
items = [5, 55, 555]
p items

# Write a hash.
lookup = {"cat" => 4, "bird" => 2}
p lookup

# Use p to write two strings on one line.
part1 = "HELLO"
part2 = "WORLD"
p part1 << "... " << part2

# Nil is displayed as nil.
p(nil)
"cat"
[5, 55, 555]
{"cat"=>4, "bird"=>2}
"HELLO... WORLD"
nil

Print

This method appends no newlines to text. We can thus use many print statements, one after another, on a single line. But we must also print a newline manually.

Tip Print() is a good choice for lines that are built up in many steps. We can avoid concatenating strings ourselves.

# Print statements on the same line.
print "dog"
print " is cute"
print "\n"

# Print entire-line statements.
print "There are " << String(4) << " apples.\n"
print "I ate a lemon.\n"

# Print a multiline statement.
print "***\nYou are a winner!\n***\n"dog is cute
There are 4 apples.
I ate a lemon.
***
You are a winner!
***

Stdin

We can read a line from the console with $stdin.readline. This method returns a string. The stdin source can be configured but by default it is set to the keyboard on the console.

# Read line from console window.
line = $stdin.readline()

# Display the string.
puts "You typed: " << linecat
You typed: cat

Interactive

An interactive console program can be developed with $stdin.readline. First we enter a while-true loop—this continues indefinitely.

Note We use a begin, rescue, ensure construct to handle errors and run some code after the selection is made.

Next We convert the text entered by the user into an Integer. This makes it easier to test for values.

Finally The case statement tests the integer we received. We handle the values 0 through 3 in special ways.

while true
    print "Type a number: "
    line = $stdin.readline()
    begin
        # Convert string to integer.
        number = Integer(line)
        # Handle various cases.
        case number
            when 0
                puts "Zero"
            when 1, 2, 3
                puts "One to three"
            else
                puts "Other"
        end
    rescue
        # Let the loop continue.
        puts "Invalid"
    ensure
        puts "Done"
    end
endType a number: 0
Zero
Done
Type a number: 2
One to three
Done
Type a number: X
Invalid
Done

Summary

Ruby, as an interpreted language, is well-suited to developer or server tasks. For user programs, a compiled language is often a better choice.

But for developers, console programs are easy-to-maintain and efficient. With print(), puts() and the concise p() method, we easily output text to an output stream.