User Guide #07 - Ruby - Back to the simple examples

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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
print fact(ARGV[0].to_i), "\n"

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

end

The first end closes the if statement.

end

The second end closes the def statement.

print fact(ARGV[0].to_i), "\n"

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.

Strings

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

words = ['foobar', 'baz', 'quux']
secret = words[rand(3)]

print "guess? "
while guess = STDIN.gets
  guess.chop!
  if guess == secret
    print "you win\n"
    break
  else
    print "you lose.\n"
  end
  print "guess? "
end
print "the word is ", secret, ".\n"

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.

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* (or *} Z under DOS), signifying the end of input.

guess.chop! in line 6 removes the last character from guess ; in this case it will always be a newline character.

In line 15 we print the secret word. We have written this as a print statement with three 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:

print "the word is #{secret}.\n"

Regular expressions

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

st = "\033[7m"
en = "\033[m"

while TRUE
  print "str> "
  STDOUT.flush
  str = gets
  break if not str
  str.chop!
  print "pat> "
  STDOUT.flush
  re = gets
  break if not re
  re.chop!
  str.gsub! re, "#{st}\\&#{en}"
  print str, "\n"
end
print "\n"

In line 4, 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 13th 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.

There is more to say about chop! (see lines 9 and 14). 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 with no exclamation point works on a copy. 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 will later come across method names that end in a question mark ( ? , sometimes pronounced aloud as "huh?"); this indicates a "predicate" method, one that can return either true of false.

Line 15 deserves some careful attention. First, notice that gsub is another so-called destructive method. It changes str by replacing everything matching the pattern re ( sub means substitute, and the leading

g