Hi,
I’ve just lost a few hours on this strange behaviour (bug ?).
Apparently it is caused by some kind of operator precedence
thingamagic. My original code was of course much more complicated, so
finding the problem wasn’t easy (I first thought my code was
buggy…). Here is a piece of minimal code that reproduces the
problem :
8<-------------8<-------------8<-------------8<-------------
C:\temp>ver
Microsoft Windows [version 6.0.6000]
C:\temp>ruby -v
ruby 1.8.6 (2007-03-13 patchlevel 0) [i386-mswin32]
C:\temp>type bug.rb
sum1 = 1 + 1 + 1
puts sum1
sum2 = (
1
puts sum2
C:\temp>ruby bug.rb
3
1
8<-------------8<-------------8<-------------8<-------------
From now on I promise I’ll remember that you cannot safely use multi-
line parenthesis expressions in Ruby ; I just would like to understand
what this code means to Ruby, if it’s not "give me the result of 1 + 1
The same takes place in irb :
C:\temp>irb
irb(main):001:0> sum1 = 1 + 1 + 1
=> 3
irb(main):002:0> sum2 = (
irb(main):003:1* 1
irb(main):004:1> + 1
irb(main):005:1> + 1
irb(main):006:1> )
=> 1
irb(main):007:0> op3 = (
irb(main):008:1* 1
irb(main):009:1> * 3
irb(main):010:1> * 9
irb(main):011:1> )
SyntaxError: compile error
(irb):9: syntax error, unexpected ‘\n’, expecting tCOLON2 or ‘[’ or
‘.’
from (irb):11
from :0
Duh ! This must mean something, but what ?
Regards,
Nicolas L.
On 8/6/07, Nicolas L. [email protected] wrote:
puts sum1
sum2 = (
1
this parses as:
(
1; +1; +1;
)
If you want 3, do:
1 +
1 +
1
which parses as 1 + 1 + 1
Nicolas L. wrote:
puts sum1
3
irb(main):008:1* 1
Regards,
Nicolas L.
+1 is a valid ruby statement by itself. In order for the interpreter to
realize that your statement continues onto another line, you must end
the line with an operator that begs for more. In your case, moving the
pluses up to the previous lines will work:
irb(main):003:0> (1 +
irb(main):004:1* 1 +
irb(main):005:1* 1)
=> 3
You get that error in irb with multiplication because * 3 is not a valid
ruby statement by itself (remember that + 1 is valid, meaning simply,
1).
Hope this helps!
Tom
OK, now I get it, pluses need to go at the end of the line. Thanks
Gregory and Tom !
Regards,
Nicolas
Tom W. wrote:
8<-------------8<-------------8<-------------8<-------------
C:\temp>ruby bug.rb
The same takes place in irb :
irb(main):007:0> op3 = (
Duh ! This must mean something, but what ?
the line with an operator that begs for more. In your case, moving the
pluses up to the previous lines will work:
irb(main):003:0> (1 +
irb(main):004:1* 1 +
irb(main):005:1* 1)
=> 3
You get that error in irb with multiplication because * 3 is not a valid
ruby statement by itself (remember that + 1 is valid, meaning simply, 1).
Are there any interesting uses of
+ x
as a statement on its own, or is it likely that
it’s a programmer error 99 times out 100? If the latter, perhaps it’s
worth
the ruby interpreter emitting a warning when it sees such constructs,
much
like the warning you get when you don’t use parantheses around argument
lists
in certain situations.
I’m happy to be told that there are valid uses of “+ x”. I’m still
learning something
new about ruby every day.
cheers,
mick
Michael H. wrote:
Are there any interesting uses of
as a statement on its own, or is it likely that
it’s a programmer error 99 times out 100?
Not that I’m recommending it, but just as an idea:
module Enumerable
def +@
inject {|s,x| s+(+x)}
end
end
class Object
def +x
x
end
end
p +[1,2,3] # => 6
sum = +[1, [2, 3], [4.0]]
p sum # => 10.0
Michael H. wrote:
like the warning you get when you don’t use parantheses around
I’ve seen it used in DSLs (domain specific languages) built with ruby.
It can be defined on your own classes like so:
class Foo
def initialize(x)
@x = x
end
def +@
@x.reverse
end
end
+Foo.new(‘foo’)
=> “oof”
I don’t think I’ve ever used it in any of my code, but that doesn’t mean
there aren’t legitimate uses for it. Anyone else used unary plus for
something useful?
Tom
2007/8/7, Michael H. [email protected]:
Are there any interesting uses of
+ x
as a statement on its own, or is it likely that
it’s a programmer error 99 times out 100?
I sometimes use +x side by side with -x. For example
UP = -1
DOWN = +1
Regards,
Pit