I have an associative array in ruby which I want to convert into a hash. This hash will represent the first values as key and sum of their second values as its value.
x = [[1,2],[1,3],[0,1],[0,2],[0,3],[1,5],[0,4],[1,6],[0,9],[1,9]]
How can I get a hash like the following from this associative array?
{
:0 => <sum_of_second_values_with_0_as_first_values>,
:1 => <sum_of_second_values_with_1_as_first_values>
}
It is not very beautiful but it works.
x = [[1,2],[1,3],[0,1],[0,2],[0,3],[1,5],[0,4],[1,6],[0,9],[1,9]]
p Hash[
x.group_by(&:first)
.map do |key, val|
[key,val.map(&:last).inject(:+)]
end
] # => {1=>25, 0=>19}
On second thought, this is simpler:
result = Hash.new(0)
x.each{|item| result[item.first] += item.last}
p result # => {1=>25, 0=>19}
An easy solution using reduce.
It starts with an empty Hash and iterates over all elements of x.
For each pair it adds its value to the hash element at key (if this index wasn't set before, default is 0). The last line sets the memory variable hash for the next iteration.
x.reduce(Hash.new(0)) { |hash, pair|
key, value = pair
hash[key] += value
hash
}
EDIT: set hash default at initialization
x = [[1,2],[1,3],[0,1],[0,2],[0,3],[1,5],[0,4],[1,6],[0,9],[1,9]]
arr_0,arr_1 = x.partition{|a,b| a==0 }
Hash[0,arr_0.map(&:last).inject(:+),1,arr_1.map(&:last).inject(:+)]
# => {0=>19, 1=>25}
or
x = [[1,2],[1,3],[0,1],[0,2],[0,3],[1,5],[0,4],[1,6],[0,9],[1,9]]
hsh = x.group_by{|a| a.first}.each_with_object(Hash.new(0)) do |(k,v),h|
h[k]=v.map(&:last).inject(:+)
end
hsh
# => {1=>25, 0=>19}
each_with_object also works
[[1,2],[1,3],[0,1],[0,2],[0,3],[1,5],[0,4],[1,6],[0,9],[1,9]].
each_with_object(Hash.new(0)) {|(first,last), h| h[first] += last }
# => {1=>25, 0=>19}
