# Hash default -- what's going on here? **URL:** \ **Category:** Ruby\ **Created:** [March 15, 2006, 8:10am UTC](https://www.ruby-forum.com/t/hash-default-whats-going-on-here/82114 "2006-03-15T08:10:27Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![unknown](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/4bbf92/32.png) [@unknown](https://www.ruby-forum.com/u/unknown)\ **Post date:** [March 15, 2006, 8:10am UTC](https://www.ruby-forum.com/t/hash-default-whats-going-on-here/82114/1 "2006-03-15T08:10:27Z")
I’ve hit what is either an egregious bug in ruby (1.8.4) or some subtlety that I haven’t grasped. [Most likely the latter… (:-)] Briefly I had two arrays of symbol strings of which I wanted to find which symbols occurred in both arrays, so I thought I’d use a hash with the key being the symbol and the value a two element array that would be used to flag which set(s) the symbol was found in. The following is a simplified (and ‘instrumented’) version of what I was trying to do: listA=[“line1”,“line2”,“line3”] listB=[“line5”,“line3”,“line4”] table={} table.default=[0,0] print "table default originally: ", table.default, “\n” listA.each {|a| t=table[a]; print a," – undef value: “,t; t[0]=1; print " new value: “,t,”\n”; table[a]=t} print "table default now: ", table.default, “\n” listB.each {|a| t=table[a]; print a," – undef value: “,t; t[1]=1; print " new value: “,t,”\n”; table[a]=t} print "table default now: ", table.default, “\ntable contents:\n” table.each {|k,v| print k, ": ", v[0], v[1], “\n”} Now, in the ‘each’ statements, the variable ‘a’ should (except in one case) not be in the hash yet, so I would expect the original provided default to be returned. However, here’s the printout I get: table default now: 00 line1 – undef value: 00 new value: 10 line2 – undef value: 10 new value: 10 line3 – undef value: 10 new value: 10 table default now: 10 line5 – undef value: 10 new value: 11 line3 – undef value: 11 new value: 11 line4 – undef value: 11 new value: 11 table default now: 11 table contents: line1: 11 line2: 11 line3: 11 line4: 11 line5: 11 Dunhh? Why is the default value getting changed? I can’t see that I’m resetting it anywhere. If I do the same sort of manipulations outside of a block (one at a time in irb for instance) things happen as I would expect – the default remains as set, and is returned for any undefined key. Illumination appreciated. – Pete – ---
**Author:** ![unknown](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/4bbf92/32.png) [@unknown](https://www.ruby-forum.com/u/unknown)\ **Post date:** [March 15, 2006, 8:19am UTC](https://www.ruby-forum.com/t/hash-default-whats-going-on-here/82114/2 "2006-03-15T08:19:35Z")
I’m guessing it’s got something to do with your assignment ``` table[a]=t ``` the array that is the hash’s default can be assigned to. I think that’s what you’re doing. If i did some testing I might be more sure 🙂 ;Daniel On 15/03/06, Pete G. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > do: > print "table default now: ", table.default, “\n” > case) > line4 – undef value: 11 new value: 11 > I’m resetting it anywhere. If I do the same sort of manipulations > outside of a block (one at a time in irb for instance) things happen > as I would expect – the default remains as set, and is returned for > any undefined key. > > Illumination appreciated. > – Pete – – Daniel B. [http://danielbaird.com](http://danielbaird.com) (TiddlyW;nks! :: Whiteboard Koala :: Blog :: Things That Suck) [[My webhost uptime is ~ 92%… if no answer pls call again later!]] ---
**Author:** ![unknown](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/4bbf92/32.png) [@unknown](https://www.ruby-forum.com/u/unknown)\ **Post date:** [March 15, 2006, 10:09am UTC](https://www.ruby-forum.com/t/hash-default-whats-going-on-here/82114/3 "2006-03-15T10:09:29Z")
In article [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid), Daniel B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > what you’re doing. If i did some testing I might be more sure 🙂 Heh – you can tell I’m new to ruby… (:-)) An hour after I posted I suddenly realized what is going on. When you assign an array, the array is _not_ copied! the assigned-to variable just gets a pointer to the other array. So when I did “t=table[a]” I was setting t to a pointer to the default! When I changed t[n], of course the default was actually what got changed… Sigh. Hardly a novel gotcha, but I fell right into it. Not sure of the best way to do what I want – maybe just use ‘has\_key?’ and then do the right thing. – Pete – ---
**Author:** ![unknown](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/4bbf92/32.png) [@unknown](https://www.ruby-forum.com/u/unknown)\ **Post date:** [March 15, 2006, 3:29pm UTC](https://www.ruby-forum.com/t/hash-default-whats-going-on-here/82114/4 "2006-03-15T15:29:44Z")
“Pete G.” [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote in message news:dv8lhf$18m$removed\_email\_address@domain.invalid… > Not sure of the best way to do what I want – maybe just use ‘has\_key?’ > and then do the right thing. The question is what exactly are you trying to do? ``` robert ``` ---
**Author:** ![unknown](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/u/4bbf92/32.png) [@unknown](https://www.ruby-forum.com/u/unknown)\ **Post date:** [March 15, 2006, 8:41am UTC](https://www.ruby-forum.com/t/hash-default-whats-going-on-here/82114/5 "2006-03-15T08:41:01Z")
2006/3/15, Pete G. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid): > listA=[“line1”,“line2”,“line3”] > listB=[“line5”,“line3”,“line4”] > table={} Here’s the problem: > table.default=[0,0] What you are looking for is: table = Hash.new { |h, k| h[k] = [0, 0] } otherwise you are always pointing to _the same_ object