How to count duplicate elements in a Ruby array

Question

I have a sorted array:

[   'FATAL <error title="Request timed out.">',   'FATAL <error title="Request timed out.">',   'FATAL <error title="There is insufficient system memory to run this query.">' ] 

I would like to get something like this but it does not have to be a hash:

[   {:error => 'FATAL <error title="Request timed out.">', :count => 2},   {:error => 'FATAL <error title="There is insufficient system memory to run this query.">', :count => 1} ] 

Answer 1

The following code prints what you asked for. I'll let you decide on how to actually use to generate the hash you are looking for:

# sample array a=["aa","bb","cc","bb","bb","cc"]  # make the hash default to 0 so that += will work correctly b = Hash.new(0)  # iterate over the array, counting duplicate entries a.each do |v|   b[v] += 1 end  b.each do |k, v|   puts "#{k} appears #{v} times" end 

Note: I just noticed you said the array is already sorted. The above code does not require sorting. Using that property may produce faster code.

Answer 2

You can do this very succinctly (one line) by using inject:

a = ['FATAL <error title="Request timed out.">',       'FATAL <error title="Request timed out.">',       'FATAL <error title="There is insufficient ...">']  b = a.inject(Hash.new(0)) {|h,i| h[i] += 1; h }  b.to_a.each {|error,count| puts "#{count}: #{error}" } 

Will produce:

1: FATAL <error title="There is insufficient ..."> 2: FATAL <error title="Request timed out.">