# Assigning to hash keys when there is a default value?

Can someone explain why there is a difference in the second line of
output for the two hashes:

h = Hash.new(5)

puts h[2]

h[2] ||= 10
p h

#----------
puts
#----------

h = Hash.new

puts h[2]

h[2] ||= 10
p h

—output:–
5
{}

nil
{2=>10}

Hi –

On Mon, 3 Sep 2007, 7stud – wrote:

#----------
—output:–
5
{}

nil
{2=>10}

x ||= y is, I think, always supposed to be exactly equivalent to
x = x || y, so that line in your first hash should be equivalent to:

h[2] = 5 || 10

which should assign 5 to h[2]. It looks to me like you’ve found a bug.
I can’t think of any reason (and I really hope there isn’t one,
because having an exception to that ||= rule would be very messy) why
using a default hash value would make any difference here. It’s still
5 || 10 on the rhs, and it’s still just an assignment.

David

unknown wrote:

x ||= y is, I think, always supposed to be exactly equivalent to
x = x || y, so that line in your first hash should be equivalent to:

h[2] = 5 || 10

which should assign 5 to h[2]. It looks to me like you’ve found a bug.
I can’t think of any reason (and I really hope there isn’t one,
because having an exception to that ||= rule would be very messy) why
using a default hash value would make any difference here. It’s still
5 || 10 on the rhs, and it’s still just an assignment.

I don’t think it’s a bug; the two examples are set forth on p. 276 of
“The Ruby Way (2nd ed)”. However, no explanation is given for why the
two examples work differently.

From: 7stud – [mailto:[email protected]]

# I don’t think it’s a bug;

dBlack was referring to this:

irb(main):001:0> h = Hash.new 5
=> {}
irb(main):002:0> h[2]
=> 5
irb(main):003:0> h
=> {}
irb(main):004:0> h[2] = h[2]
=> 5
irb(main):005:0> h
=> {2=>5}
irb(main):006:0> 5 || 10
=> 5
irb(main):007:0> h[3] = 5 || 10
=> 5
irb(main):008:0> h
=> {2=>5, 3=>5}
irb(main):009:0> h[4] = h[4] || 10
=> 5
irb(main):010:0> h
=> {2=>5, 3=>5, 4=>5}
irb(main):011:0> h[5] ||= 10
=> 5

after that step above, you must have h[5] or 5=>5

irb(main):012:0> h
=> {2=>5, 3=>5, 4=>5}

however, the ff works (wc is supposed to be the longcut version for h[5]
||= 10)

irb(main):013:0> h[5] = h[5] || 10
=> 5
irb(main):014:0> h
=> {5=>5, 2=>5, 3=>5, 4=>5}

# for why the two examples work differently.

they should be different (note, that has nothing to do w the bug dBlack
is pointing at) and the output should be,

5
{2=>5} #<—dblack is complaining here because the ruby output is {}

nil
{2=>10}

pls refer to the hash#new document.

## irb(main):015:0> system “qri hash.new” -------------------------------------------------------------- Hash::new Hash.new => hash Hash.new(obj) => aHash Hash.new {|hash, key| block } => aHash

`````` Returns a new, empty hash. If this hash is subsequently accessed
by a key that doesn't correspond to a hash entry, the value
returned depends on the style of new used to create the hash. In
the first form, the access returns nil. If obj is specified, this
single object will be used for all default values. If a block is
specified, it will be called with the hash object and the key, and
should return the default value. It is the block's responsibility
to store the value in the hash if required.

h = Hash.new("Go Fish")
h["a"] = 100
h["b"] = 200
h["a"]           #=> 100
h["c"]           #=> "Go Fish"
# The following alters the single default object
h["c"].upcase!   #=> "GO FISH"
h["d"]           #=> "GO FISH"
h.keys           #=> ["a", "b"]

# While this creates a new default object each time
h = Hash.new { |hash, key| hash[key] = "Go Fish: #{key}" }
h["c"]           #=> "Go Fish: c"
h["c"].upcase!   #=> "GO FISH: C"
h["d"]           #=> "Go Fish: d"
h.keys           #=> ["c", "d"]
``````

kind regards -botp

Hi –

On Mon, 3 Sep 2007, 7stud – wrote:

5 || 10 on the rhs, and it’s still just an assignment.

I don’t think it’s a bug; the two examples are set forth on p. 276 of
“The Ruby Way (2nd ed)”. However, no explanation is given for why the
two examples work differently.

It seems very bug-like to me. I don’t know what Hal’s take on it is.
Paging Hal…

David

PeÃ±a, Botp wrote:

they should be different (note, that has nothing to do w the bug dBlack
is pointing at) and the output should be,

5
{2=>5} #<—dblack is complaining here because the ruby output is {}

nil
{2=>10}

Yes. That is what I am complaining about too! Your are right: the
second line for each section of the output should be different–I
mispoke. What I want to know is why the the output is {} and not
{2=>5}.

The author of “The Ruby Way (2nd ed)” presents the two examples as well
as the different output for each example, yet he doesn’t explain the
reason for the different output. He suggests that you can use ||= to
selectively assign values only to keys that don’t exist. The point I
think he was trying to make with the two examples is that when you have
a default value for non-existent keys, all keys “exist”, so ||= will not
create new keys with the default value.

PeÃ±a, Botp wrote:

pls refer to the hash#new document.

I also have “Programming Ruby (2nd ed)”, and I read the documentation
therein for Hash.new many times before posting–which is the same
documentation that ri produces.

2007/9/3, [email protected] [email protected]:

x ||= y is, I think, always supposed to be exactly equivalent to
x = x || y, so that line in your first hash should be equivalent to:

h[2] = 5 || 10

which should assign 5 to h[2]. It looks to me like you’ve found a bug.
I can’t think of any reason (and I really hope there isn’t one,
because having an exception to that ||= rule would be very messy) why
using a default hash value would make any difference here. It’s still
5 || 10 on the rhs, and it’s still just an assignment.

I can’t point my finger on it but I believe x||=y is equivalent to
“x=y unless x” instead of “x=x||y”. It seems to be more reasonable to
skip the assignment altogether if the value is true equivalent
already. That would also explain behavior much better.

Note also:

\$ ruby -e ‘h=Hash.new 2;set_trace_func lambda {|*a| p a}; h[4]||=10’
[“line”, “-e”, 1, nil, #Binding:0x1002ff48, false]
[“c-call”, “-e”, 1, :[], #Binding:0x1002ff0c, Hash]
[“c-call”, “-e”, 1, :default, #Binding:0x1002fdf4, Hash]
[“c-return”, “-e”, 1, :default, #Binding:0x1002fcf0, Hash]
[“c-return”, “-e”, 1, :[], #Binding:0x1002fc00, Hash]

\$ ruby -e ‘h=Hash.new 2;set_trace_func lambda {|*a| p a}; h[4]=h[4]||10’
[“line”, “-e”, 1, nil, #Binding:0x1002fee4, false]
[“c-call”, “-e”, 1, :[], #Binding:0x1002fea8, Hash]
[“c-call”, “-e”, 1, :default, #Binding:0x1002fd90, Hash]
[“c-return”, “-e”, 1, :default, #Binding:0x1002fc8c, Hash]
[“c-return”, “-e”, 1, :[], #Binding:0x1002fb9c, Hash]
[“c-call”, “-e”, 1, :[]=, #Binding:0x1002faac, Hash]
[“c-return”, “-e”, 1, :[]=, #Binding:0x1002f9bc, Hash]

There is no assignment in the first piece.

Kind regards

robert

Hi –

On Mon, 3 Sep 2007, Robert K. wrote:

puts h[2]
puts h[2]

I can’t point my finger on it but I believe x||=y is equivalent to
“x=y unless x” instead of “x=x||y”. It seems to be more reasonable to
skip the assignment altogether if the value is true equivalent
already. That would also explain behavior much better.

I still don’t like it. I’m not sure about that “unless” thing… I
always thought x ||= y was strictly syntactic sugar for x = x || y.
I’m now thinking about how much fun it will be to explain to people
learning Ruby that that’s true, unless the lhs is a call to Hash#[] on
a hash with a default value with a boolean value of true, in which
case… What fun

[“line”, “-e”, 1, nil, #Binding:0x1002fee4, false]
[“c-call”, “-e”, 1, :[], #Binding:0x1002fea8, Hash]
[“c-call”, “-e”, 1, :default, #Binding:0x1002fd90, Hash]
[“c-return”, “-e”, 1, :default, #Binding:0x1002fc8c, Hash]
[“c-return”, “-e”, 1, :[], #Binding:0x1002fb9c, Hash]
[“c-call”, “-e”, 1, :[]=, #Binding:0x1002faac, Hash]
[“c-return”, “-e”, 1, :[]=, #Binding:0x1002f9bc, Hash]

There is no assignment in the first piece.

I wonder whether there’s some special optimization for hashes. I can’t
find any other example where the setter method isn’t called (except
attr_accessor-created setters, which I haven’t puzzled through yet). I
won’t paste the whole output but run this:

class C
def x; @x; end
def x=(x); @x = x; end
end

c = C.new

set_trace_func lambda {|*a| p a }
c.x ||= 1
puts
c.x = nil
puts
c.x = c.x || 1
puts

a = []
puts
a[1] ||= 1
puts
a[2] = a[2] || 1

As far as I can tell, the sugar version and the inlined version work
exactly the same, except for hashes.

My vote is still for bug

David

I don’t think this is a bug, kittens. since h[2] returns a value [even
though it’s not set], it causes h[2] to evaluate so the assignment
never happens. x ||= y just means give me x or set x to y if there’s
no value for x. h[2] does have a value if only a default one.

just my two cents.

unless i missed the point here, in which case i apologize for my hasty
conclusion. :))

RSL

unknown wrote:

The only possible explanation I can come up with, which I don’t like,
is that:

h[2] ||= 10

is being treated like:

5 = 5 || 10

which is just another can of worms.

I also considered that possibility. Does the trace output support that?
I don’t really know how to read the trace output, but it doesn’t look
like there are two calls to Hash#[] for each ||=.

Russell N. wrote:

x ||= y just means give me x or set x to y if there’s no value for x.

Maybe it does, but if we follow the logic that x op= y is a shortcut for
x = x op y, what it should mean is: set x to y if there’s no value for
x, otherwise set it to itself.

Hi –

On Mon, 3 Sep 2007, 7stud – wrote:

a default value for non-existent keys, all keys “exist”, so ||= will not
create new keys with the default value.

I don’t have the book at hand but the default value definitely doesn’t
mean that all keys exist (or even “exist”

irb(main):003:0> h = Hash.new(5)
=> {}
irb(main):004:0> h[1]
=> 5
irb(main):005:0> h.has_key?(1)
=> false

The only possible explanation I can come up with, which I don’t like,
is that:

h[2] ||= 10

is being treated like:

5 = 5 || 10

which is just another can of worms.

David

Hi –

On Mon, 3 Sep 2007, Russell N. wrote:

I don’t think this is a bug, kittens. since h[2] returns a value [even
though it’s not set], it causes h[2] to evaluate so the assignment
never happens. x ||= y just means give me x or set x to y if there’s
no value for x. h[2] does have a value if only a default one.

just my two cents.

unless i missed the point here, in which case i apologize for my hasty
conclusion. :))

The thing is, this:

x ||= y

always means (at least, so I’ve always been told):

x = x || y

which in the case of the hash with a default value of 5, would mean:

h[2] = 5 || 10

In other words: h.[]=(2, 5||10)

At that point, the default value is out of the picture. The default
value has no implications for the #[]= (writer) method; it’s only what
you get when you use the #[] (reader) method.

So what’s at stake here is the matter of x ||= y behaving predictably.
I have to say, it’s a case where I’m not even really concerned with
the implementation (e.g., whether it uses [] on its way to []=) but
just the semantics. I don’t like the idea that the syntactic sugar is
actually not a reliable drop-in replacement for the thing it’s
sugaring.

David

On Sep 3, 2007, at 7:37 AM, [email protected] wrote:

The only possible explanation I can come up with, which I don’t like,
is that:

h[2] ||= 10

is being treated like:

5 = 5 || 10

which is just another can of worms.

I don’t want to believe that because it raises the question of why

``````h[2] = 10
``````

isn’t treated as

``````5 = 10
``````

I mean, I simply can’t believe the Ruby interpreter would treat h[2]
as a simple rvalue in h[2] ||= 10.

Robert K. explanation is more believable although I find it
somewhat upsetting after years of believing x = y is just
syntactic sugar for x = x y. The optimization he describes
is a plausible one, and I can’t think of any other case where it
would backfire.

Regards, Morton

I learned that x ||= y means set x to y unless x, so I don’t see the
bug. and i don’t see this as being at all unpredictable. either way
you expect it to work, it either fails 100% or succeeds 100%. the
only problem is when you expect it to do something it doesn’t do,
which is tautologically what every problem is i guess.

RSL

Hi –

On Mon, 3 Sep 2007, Morton G. wrote:

which is just another can of worms.

I don’t want to believe that because it raises the question of why

h[2] = 10

isn’t treated as

5 = 10

I mean, I simply can’t believe the Ruby interpreter would treat h[2] as a
simple rvalue in h[2] ||= 10.

I agree – it definitely wouldn’t. It was a kind of reductio ad
absurdum.

David

Hi –

On Mon, 3 Sep 2007, Russell N. wrote:

I learned that x ||= y means set x to y unless x, so I don’t see the
bug. and i don’t see this as being at all unpredictable. either way
you expect it to work, it either fails 100% or succeeds 100%. the
only problem is when you expect it to do something it doesn’t do,
which is tautologically what every problem is i guess.

I guess one can just accomodate oneself to whatever happens, in which
case there are no bugs… but somehow it never seems to play out that
way Anyway, we’re going around in circles. I think I might hop
over to ruby-core and see what people think.

David

2007/9/3, [email protected] [email protected]:

I guess one can just accomodate oneself to whatever happens, in which
case there are no bugs… but somehow it never seems to play out that
way Anyway, we’re going around in circles. I think I might hop
over to ruby-core and see what people think.

Will be interesting to hear what they say.

In the meantime: I believe we’re discussing something that’s not an
issue most of the time, because even if there is no assignment with
the default value, querying will still return the same result. The
only possible issue here is that Hash#keys returns something different
depending on the implementation.

Why do I think there is no problem most of the time? Let’s look at
some typical idioms which work as we like them to work:

counters = Hash.new 0

counters[item] += 1

counters.each do |key,value|
printf “%5d %s\n”, value, key
end

lists = Hash.new {|h,k| h[k] = []}

lists[key] << item

lists.each do |key,list|
print key, " ", list.inspect, “\n”
end

I wonder where we would actually use ||= with a Hash. Is this a
realistic example?

found = Hash.new

found[item] ||= true

I don’t think so, because in that case I’d rather use

found[item] = true

Is ||= really used with Hash at all? I can’t think of a case ATM but
that might just be my limited fantasy.

Kind regards

robert

I always wondered why JavaScript is missing the ‘||=’
operator (I rarely need a “&&=” one). Maybe Brendan
was prescient enough to realize an r-value interpretation
would lead to ambiguity. Maybe, but I still say
it’s a bug. +1 for DavidB.

• Eric