Ruby - Console: Puts, Print and stdin

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.

An 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.

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

Detail 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 elements

100 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: " << line

cat You typed: cat

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

while

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

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

Number

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

case

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 end

Type a number: 0 Zero Done Type a number: 2 One to three Done Type a number: X Invalid Done

A 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.