SupremeA - 8 months ago 27

Ruby Question

I have an array of transactions. I need to group the transactions by name and then select the group with the highest amount & more than 1 instance.

For example, if I have 1 transaction named "car" with an amount of $3000, and 3 transactions with "boat" totaling $1800, and 4 transactions with "house" totaling $500, the method will select boat because it is the highest amount group with multiple transactions.

`@transactions =`

[{"amount"=>-3000, "name"=>"CAR"},

{"amount"=>-600, "name"=>"BOAT"},

{"amount"=>-600, "name"=>"BOAT"},

{"amount"=>-600, "name"=>"BOAT"},

{"amount"=>-125, "name"=>"HOUSE" },

{"amount"=>-125, "name"=>"HOUSE" },

{"amount"=>-125, "name"=>"HOUSE" },

{"amount"=>-125, "name"=>"HOUSE" }]

Right now I have this but it selects based on length of name.

`@transactions.group_by {|h| h['name'] }.max_by {|k, v| v.length }.first`

How can I group, then sum, then select by highest amount in a group with multiple transactions.

Answer

There are a lot of good answers here. I'd like to add that you can eliminate a lot of iteration by combining operations.

For example, rather than calculating the sums for each group in a second step, you can do that inside your `group_by`

block:

```
sums = Hash.new(0)
groups = transactions.group_by do |t|
sums[t["name"]] += t["amount"]
t["name"]
end
p groups
# => { "CAR" => [ { "amount" => -3000, "name" => "CAR" } ],
# "BOAT" => [ ... ],
# "HOUSE" => [ ... ] }
p sums
# => { "CAR" => -3000, "BOAT" => -1800, "HOUSE" => -500 }
```

Next instead of doing `groups.select`

to eliminate groups with only one member and then `min_by`

to get the final result, combine the former into the latter:

```
result = groups.min_by do |k,g|
g.size > 1 ? sums[k] : Float::INFINITY
end
p result[1]
# => [ { "amount" => -600, "name" => "BOAT" },
# { "amount" => -600, "name" => "BOAT" },
# { "amount" => -600, "name" => "BOAT" } ]
```

Because *everything* is smaller than `Float::INFINITY`

, those groups with only one member will never be selected (unless every group has only one member).

And so...

Putting it all together:

```
sums = Hash.new(0)
result =
transactions.group_by {|t|
sums[t["name"]] += t["amount"]
t["name"]
}.min_by {|k,g| g.size > 1 ? sums[k] : Float::INFINITY }[1]
p result
# => [ { "amount" => -600, "name" => "BOAT" },
# { "amount" => -600, "name" => "BOAT" },
# { "amount" => -600, "name" => "BOAT" } ]
```

You could also combine all of this into a single `reduce`

and iterate over the data only once, but it's not very Rubyish:

```
sums = Hash.new(0)
groups = Hash.new {|h,k| h[k] = [] }
min_sum = Float::INFINITY
result = transactions.reduce do |min_group, t|
name = t["name"]
sum = sums[name] += t["amount"]
(group = groups[name]) << t
if group.size > 1 && sum < min_sum
min_sum, min_group = sum, group
end
min_group
end
```

Note that you could move all of those outside variable declarations into, say, an array passed to `reduce`

(instead of `nil`

), but it would impact readability a lot.