Ruby User's Guide

Back to the simple examples

Now let's take apart the code of some of our previous example programs.

The following appeared in the simple examples chapter.

def fact(n)
  if n == 0
    1
  else
    n * fact(n-1)
  end
end
puts fact(ARGV[0].to_i)

Because this is the first explanation, we examine each line individually.

Factorials

def fact(n)

In the first line, def is a statement to define a function (or, more precisely, a method; we'll talk more about what a method is in a later chapter). Here, it specifies that the function fact takes a single argument, referred to as n.

if n == 0

The if is for checking a condition. When the condition holds, the next bit of code is evaluated; otherwise whatever follows the else is evaluated.

1

The value of if is 1 if the condition holds.

else

If the condition does not hold, the code from here to end is evaluated.

n * fact(n-1)

If the condition is not satisfied, the value of if is the result of n times fact(n-1).

end

The first end closes the if statement.

end

The second end closes the def statement.

puts fact(ARGV[0].to_i)

This invokes our fact() function using a value specified from the command line, and prints the result.

ARGV is an array which contains command line arguments. The members of ARGV are strings, so we must convert this into a integral number by to_i. Ruby does not convert strings into integers automatically like perl does.

What would happen if we fed this program a negative number? Do you see the problem? Can you fix it?

Strings

Next we examine the puzzle program from the chapter on strings. As this is somewhat longer, we number the lines for reference.

01 words = ['foobar', 'baz', 'quux']
02 secret = words[rand(3)]
03
04 print "guess? "
05 while guess = STDIN.gets
06   guess.chop!
07   if guess == secret
08     puts "You win!"
09     break
10   else
11     puts "Sorry, you lose."
12   end
13   print "guess? "
14 end
15 puts "the word is ", secret, "."

In this program, a new control structure, while, is used. The code between while and its corresponding end will execute repeatedly as long as some specified condition remains true. In this case, guess=STDIN.gets is both an active statement (collecting a line of user input and storing it as guess), and a condition (if there is no input, guess, which repesents the value of the whole guess=STDIN.gets expression, has a nil value, causing while to stop looping).

STDIN is the standard input object. Usually, guess=gets does the same thing as guess=STDIN.gets.

rand(3) in line 2 returns a random number in the range 0 to 2. This random number is used to extract one of the members of the array words.

In line 5 we read one line from standard input by the method STDIN.gets. If EOF (end of file) occurs while getting the line, gets returns nil. So the code associated with this while will repeat until it sees ^D (try ^Z or F6 under DOS/Windows), signifying the end of input.

guess.chop! in line 6 deletes the last character from guess; in this case it will always be a newline character, gets includes that character to reflect the user's Return keystroke, but we're not interested in it.

In line 15 we print the secret word. We have written this as a puts (put string) statement with two arguments, which are printed one after the other; but it would have been equally effective to do it with a single argument, writing secret as #{secret} to make it clear that it is a variable to be evaluated, not a literal word to be printed:

puts "the word is #{secret}."

Many programmers feel this is a cleaner way to express output; it builds a single string and presents it as a single argument to puts.

Also, we are by now used to the idea of using puts for standard script output, but this script uses print instead, in lines 4 and 13. They are not quite the same thing. print outputs exactly what it is given; puts also ensures that the output line ends. Using print in lines 4 and 13 leaves the cursor next to what was just printed, rather than moving it to the beginning of the next line. This creates a recognizable prompt for user input. In general, the four output calls below are equivalent:

# newline is implicitly added by puts if there isn't one already:
puts  "Darwin's wife, Esmerelda, died in a fit of penguins."

# newline must be explicitly given to the print command:
print "Darwin's wife, Esmerelda, died in a fit of penguins.\n"

# you can concatenate output with +:
print 'Darwin's wife, Esmerelda, died in a fit of penguins.'+"\n"

# or concatenate by supplying more than one string:
print 'Darwin's wife, Esmerelda, died in a fit of penguins.', "\n"

One possible gotcha: sometimes a text window is programmed to buffer output for the sake of speed, collecting individual characters and displaying them only when it is given a newline character. So if the guessing game script misbehaves by not showing the prompt lines until after the user supplies a guess, buffering is the likely culprit. To make sure this doesn't happen, you can flush the output as soon as you have printed the prompt. It tells the standard output device (an object named STDOUT), "don't wait; display what you have in your buffer right now."

04 print "guess? "; STDOUT.flush
  ...
13 print "guess? "; STDOUT.flush

And in fact, we were more careful with this in the next script.

Regular expressions

Finally we examine this program from the chapter on regular expressions.

01 st = "\033[7m"
02 en = "\033[m"
03
04 puts "Enter an empty string at any time to exit."
05
06 while true
07   print "str> "; STDOUT.flush; str=gets.chop
08   break if str.empty?
09   print "pat> "; STDOUT.flush; pat=gets.chop
10   break if pat.empty?
11   re = Regexp.new(pat)
12   puts str.gsub(re, "#{st}\\&#{en}")
13 end

In line 6, the condition for while is hardwired to true, so it forms what looks like an infinite loop. However we put break statements in the 8th and 10th lines to escape the loop. These two breaks are also an example of "if modifiers." An if modifier executes the statement on its left hand side if and only if the specified condition is satisfied. This construction is unusual in that it operates logically from right to left, but it is provided because for many people it mimics a similar pattern in natural speech. It also has the advantage of brevity, as it needs no end statement to tell the interpreter how much of the following code is supposed to be conditional. An if modifier is conventionally used in situations where a statement and condition are short enough to fit comfortably together on one script line.

Note the difference in the user interface compared to the string-guessing script. This one lets the user quit by hitting the Return key on an empty line. We are testing for emptiness of the input string, not for its nonexistence.

In lines 7 and 9 we have a "non-destructive" chop; again, we're getting rid of the unwanted newline character we always get from gets. Add the exclamation point, and we have a "destructive" chop. What's the difference? In ruby, we conventionally attach '!' or '?' to the end of certain method names. The exclamation point (!, sometimes pronounced aloud as "bang!") indicates something potentially destructive, that is to say, something that can change the value of what it touches. chop! affects a string directly, but chop gives you a chopped copy without damaging the original. Here is an illustration of the difference.

ruby> s1 = "forth"
  "forth"
ruby> s1.chop!       # This changes s1.
  "fort"
ruby> s2 = s1.chop   # This puts a changed copy in s2,
  "for"
ruby> s1             # ... without disturbing s1.
  "fort"

You'll also sometimes see chomp and chomp! used. These are more selective: the end of a string gets bit off only if it happens to be a newline. So for example, "XYZ".chomp! does nothing. If you need a trick to remember the difference, think of a person or animal tasting something before deciding to take a bite, as opposed to an axe chopping indiscriminately.

The other method naming convention appears in lines 8 and 10. A question mark (?, sometimes pronounced aloud as "huh?") indicates a "predicate" method, one that can return either true or false.

Line 11 creates a regular expression object out of the string supplied by the user. The real work is finally done in line 12, which uses gsub to globally substitute each match of that expression with itself, but surrounded by ansi markups; also the same line outputs the results.

We could have broken up line 12 into separate lines like this:

highlighted = str.gsub(re,"#{st}\\&#{en}")
puts highlighted

or in "destructive" style:

str.gsub!(re,"#{st}\\&#{en}")
puts str

Look again at the last part of line 12. st and en were defined in lines 1-2 as the ANSI sequences that make text color-inverted and normal, respectively. In line 12 they are enclosed in #{} to ensure that they are actually interpreted as such (and we do not see the variable names printed instead). Between these we see \\&. This is a little tricky. Since the replacement string is in double quotes, the pair of backslashes will be interpreted as a single backslash; what gsub actually sees will be \&, and that happens to be a special code that refers to whatever matched the pattern in the first place. So the new string, when displayed, looks just like the old one, except that the parts that matched the given pattern are highlighted in inverse video.

Copyright (c) 2005-2008 Mark Slagell

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.

A copy of the license is included in the section entitled "GNU Free Documentation License."