prabhu prabhu - 6 months ago 17
Ruby Question

Find if n exists as a sum of any 2 numbers in the given array

I am trying to find whether

n
exists as a sum of any two numbers in the passed array if so return
true
else
false
, the problem with my code is that
inject
is not iterating as I want it to. What am I doing wrong?

def sum_to_n?(array,n)
array.each do |i|
array.inject(i) do |memo,var|
if memo + var == n
return true
else
return false
end
end
end
end

puts sum_to_n?([1,2,3,4,5],9)

Answer

Here is an approach :

def sum_to_n?(a,n)
  !!a.find{|e| a.include?(n-e)}
end
a = [1,2,3,4,5]
sum_to_n?(a,9) # => true
sum_to_n?(a,11) # => false

If you want to get those 2 elements:

def sum_to_n?(a,n)
  num=a.find{|e| a.include?(n-e)}
  unless num
    puts "not exist"
  else
    p [n-num,num]
  end
end
a = [1,2,3,4,5]
sum_to_n?(a,9)
# >> [5, 4]
sum_to_n?(a,11)
# >> not exist

Logic

Enumerable#find method passing one array element per iteration.Now for any iteration,say I have an element e,and I subtracted it from n. Now I was just testing that (n-e) is present in the source array.If I found a match #find will stop finding,and immediately will return e.If not found,then it will go for next iteration. If #find completes its iteration,but didn't find (n-e),as per the documentation it will return nil.