如何在Ruby中获取数组第二列的最大值和总和

Question

for an array like

s = [[1,2],[4,6],[2,7]]

How i can select max and sum of 2nd column in each row in one statement

max= 7
sum= 15

I know, that

sum = 0
max = 0
s.each{ |a,b| sum+=b;if max<b then max = b end }

would work.

Answer 1

second_elements = s.map { |el| el[1] }
sum = second_elements.inject{|sum,x| sum + x }
max = second_elements.max

To be more clear: inject{|sum,x| sum + x } inject{|sum,x| sum + x } returns nil if array is empty, so if you want to get 0 for empty array then use inject(0, :+)

Answer 2

The transpose method is nice for accessing "columns":

s = [[1,2],[4,6],[2,7]]
col = s.transpose[1]
p col.max #=> 7
p col.inject(:+) #=> 15

Answer 3

s.max {|a| a[1]}[1]          # Max of elements at index 1
s.max {|a| a.last }.last     # Max of last elements
# => 7

To find the sum, if you use Ruby 2.4 or greater / if you are on Rails

s.sum {|a| a[1]}             # Sum of elements at index 1
s.sum(&:last)                # Sum of last elements
# => 15

else

s.inject(0) {|sum, a| sum+= a[1] }
# => 15

Answer 4

s.map{|e| e[1]}.max s.map{|e| e[1]}.max gives you max

s.map{|e| e[1]}.reduce(:+) s.map{|e| e[1]}.reduce(:+) gives you sum.

Answer 5

s = [[1,2],[4,6],[2,7]]
second_max = s.max_by(&:last).last 
# => 7
sum = s.reduce(0){|sum,a| sum + a.last}
# => 15