Question

I am reading “The Ruby Programming Language” and wanted to try the example provided in the book (Chapter 6, Altering Control Flow, catch and throw) for using catch/throw to break out of nested loops:

for matrix in data do 
  catch :missing_data do
    for row in matrix do 
      for value in row do
        throw :missing_data unless value
        puts value
      end
    end
  end
end

I defined data as an array of three matrices:

data
=> [[[2, 3, 7], [8, 9, 10], [0, 1, 4]], [[“a”, “b”, “c”], [“d”, false, “e”], [“f”, “g”, nil]], [[“abc”, “def”], [“ghi”, “jkl”]]]

However when I try the code in IRB (version 1.9.3p327), I get an “unexpected $end, expecting keyword_end” error. I’ve tried playing around with the code and have discovered that it works if I remove the outmost (i.e. for) loop, but with the loop I get the error. To double check, I created a simpler loop:

for i in [1,2,3] do
  catch :two do
    if i==2 then throw :two end
  end
end

Again, I get the same situation: error with the for loop, no error without the for loop (in fact, using IRB, I don’t get the chance to enter the last ‘end’, but get the error after entering the fourth line above).

Does anyone know why I am getting this error?

Answer 1

This appears to be build and/or environment-dependent (I lean towards environment):

• Original code works when I execute it from within Sublime Text 2
• Original code works when running via ruby from command line
• Original code does not work when I type it in to irb

Inside irb dropping the do in the outer for, or using {} for the catch, works:

$ rvm use 1.9.3-p327                                                                              Using /Users/Dave/.rvm/gems/ruby-1.9.3-p327
$ irb
irb(main):001:0> data = [[[2, 3, 7], [8, 9, 10], [0, 1, 4]], [["a", "b", "c"], ["d", false, "e"], ["f", "g", nil]], [["abc", "def"], ["ghi", "jkl"]]]
irb(main):002:0> for matrix in data do
irb(main):003:1*   catch(:missing_data) {
irb(main):004:2*     for row in matrix do 
irb(main):005:3*       for value in row do
irb(main):006:4*         throw :missing_data unless value
irb(main):007:4>         puts value
irb(main):008:4>       end
irb(main):009:3>     end
irb(main):010:2>   }
irb(main):011:1> end
2
3
7
8
9
10
0
1
4
a
b
c
d
abc
def
ghi
jkl