Why does my Ruby 'ri' tool not return results in command prompt? [duplicate]

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ruby 1.9 ri problem

When I try to use Ruby's ri tool in a command prompt window to get help about classes, methods, etc. it seems to always fail. For example if I type:

ri Array

I get a message saying:

Updating class cache with 0 classes... Nothing known about Array

I am using Vista 64 with Ruby 1.9.1p243 (2009-07-16 revision 24175) [i386-mingw32] installed.

What should I do to configure ri to work?

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Although this question is old, nobody has yet provided a proper solution. I just ran into the same issue, and found the solution:

If you are using RVM:

rvm docs generate-ri # <- Just the ri docs, much faster
rvm docs generate    # <- Everything (rdoc + ri)

More info on managing RVM docs here:
https://rvm.io/rubies/docs

If not using RVM:

gem install rdoc-data

# Regenerate system docs
rdoc-data --install

# Regenerate all gem docs (rdoc + ri)
gem rdoc --all --overwrite 

# Regenerate all gem docs (ri only)
gem rdoc --all --overwrite --ri --no-rdoc

# Regenerate specific gem doc with specific version
gem rdoc gemname -v 1.2.3 --overwrite