# Data sctructure **URL:** \ **Category:** Ruby\ **Created:** [August 28, 2009, 12:25pm UTC](https://www.ruby-forum.com/t/data-sctructure/174540 "2009-08-28T12:25:05Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![Juliano\_SSS\_SSSS\_SS](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/77aa72/32.png) [@Juliano\_SSS\_SSSS\_SS](https://www.ruby-forum.com/u/Juliano_SSS_SSSS_SS)\ **Post date:** [August 28, 2009, 12:25pm UTC](https://www.ruby-forum.com/t/data-sctructure/174540/1 "2009-08-28T12:25:05Z")
Hello guys. I’ve just started using ruby and I come from Python… I’m still learning to think the Ruby way… Of course, any beginning is hard, but even so I’ve been having a lot of fun learning Ruby. The problem is that I still don’t know much about the tons of methods and I’m maybe letting something pass through my fingers. My problem is this: I have to read a file and set up a data structure of embedded dictionaries. In Python I can easily do that by means of “dict.defaultdict()”, but I didn’t understand how to do that in Ruby… I finally came up with the following: def preprocess(lines) d = {} lines.each do |line| c, s, fc, fl = line.split("\t") if !d.has\_key? c d[c] = {} end if !d[c].has\_key? s d[c][s] = {} end if !d[c][s].has\_key? fc d[c][s][fc] = [] end d[c][s][fc].push fl end return d end My question is: Is there an easier way to do this in Ruby as it is in Python? Same code in Python… ``` def preprocess(lines): d = {} for line in lines: c, s, fc, fl = line dc = d.setdefault(c, {}) ds = dc.setdefault(s, {}) dfc = ds.setdefault(fc, []) lfl = dfc.append(fl) return d ``` Thank you very much! 🙂 ---
**Author:** ![Juliano\_SSS\_SSSS\_SS](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/77aa72/32.png) [@Juliano\_SSS\_SSSS\_SS](https://www.ruby-forum.com/u/Juliano_SSS_SSSS_SS)\ **Post date:** [August 28, 2009, 1:02pm UTC](https://www.ruby-forum.com/t/data-sctructure/174540/2 "2009-08-28T13:02:33Z")
2009/8/28 David A. Black [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid): > > learning to think the Ruby way… > > dictionaries. In Python I can easily do that by means of > > if !d[c].has\_key? s > > My question is: Is there an easier way to do this in Ruby as it is in > > d[c][s] ||= {} > > There are also ways to set the default behavior of hashes, but I think > > the above is probably the most direct way to do what you want. You mean something like the more involved def preprocess(lines) d = Hash.new {|h,k| h[k] = Hash.new(&h.default\_proc)} lines.each do |line| c, s, fc, fl = line.split(“\t”) (d[c][s][fc] ||= []) \<\< fl end d end Kind regards robert ---
**Author:** ![Juliano\_SSS\_SSSS\_SS](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/77aa72/32.png) [@Juliano\_SSS\_SSSS\_SS](https://www.ruby-forum.com/u/Juliano_SSS_SSSS_SS)\ **Post date:** [August 28, 2009, 12:43pm UTC](https://www.ruby-forum.com/t/data-sctructure/174540/3 "2009-08-28T12:43:49Z")
Hi – On Fri, 28 Aug 2009, Juliano 준호 wrote: > Hello guys. > > I’ve just started using ruby and I come from Python… I’m still > learning to think the Ruby way… > Of course, any beginning is hard, but even so I’ve been having a lot > of fun learning Ruby. Welcome! I’m glad you’re enjoying it. > d = {} > end > d[c][s][fc].push fl > end > return d > end > > My question is: Is there an easier way to do this in Ruby as it is in > Python? The idiom for conditional assignment in Ruby is ||= (or-equals). You could do this: def preprocess(lines) d = {} lines.each do |line| c, s, fc, fl = line.split(“\t”) d[c] ||= {} d[c][s] ||= {} d[c][s][fc] ||= [] d[c][s][fc].push fl end return d end The only thing is, in cases where you want false or nil to be a hash value, ||= will over-eagerly do the assignment. But that’s not the case here. There are also ways to set the default behavior of hashes, but I think the above is probably the most direct way to do what you want. David – David A. Black / Ruby Power and Light, LLC / [http://www.rubypal.com](http://www.rubypal.com) September Ruby training in NJ has been POSTPONED. Stay tuned for updates! ---
**Author:** ![Juliano\_SSS\_SSSS\_SS](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/77aa72/32.png) [@Juliano\_SSS\_SSSS\_SS](https://www.ruby-forum.com/u/Juliano_SSS_SSSS_SS)\ **Post date:** [August 28, 2009, 1:16pm UTC](https://www.ruby-forum.com/t/data-sctructure/174540/4 "2009-08-28T13:16:01Z")
Hi – On Fri, 28 Aug 2009, Robert K. wrote: > > > I’ve just started using ruby and I come from Python… I’m still > > > I have to read a file and set up a data structure of embedded > > > end > > > > ``` > > d[c] ||= {} > > > > ``` > > ``` > (d[c][s][fc] ||= []) << fl > > ``` > > end > > d > end Yes, that’s what I meant. David ---
**Author:** ![Juliano\_SSS\_SSSS\_SS](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/77aa72/32.png) [@Juliano\_SSS\_SSSS\_SS](https://www.ruby-forum.com/u/Juliano_SSS_SSSS_SS)\ **Post date:** [August 28, 2009, 1:20pm UTC](https://www.ruby-forum.com/t/data-sctructure/174540/5 "2009-08-28T13:20:42Z")
At 2009-08-28 06:16AM, “Juliano ??” wrote: > I have to read a file and set up a data structure of embedded > dictionaries. In Python I can easily do that by means of > “dict.defaultdict()”, but I didn’t understand how to do that in > Ruby… I finally came up with the following: Referencing a hash element that doesn’t exist returns nil, so you might be looking for the ||= operator: ``` a ||= b # if a is false or nil, then set a=b, else return value of ``` a > ``` > if !d[c][s].has_key? fc > d[c][s][fc] = [] > end > d[c][s][fc].push fl > end > return d > > ``` > > end ``` def preprocess(lines) d = {} lines.each do |line| c,s,fc,fl = line.split("\t") d[c] ||= {} d[c][s] ||= {} d[c][s][fc] ||= [] d[c][s][fc] << fl end d end ```