Changing the order of my code result in error ? - ruby

Question

def stock_picker prices
min_day , max_day , profit = 0 , 0 , 0
i = 1
while i < prices.length
(0...i).each do |day|
if prices[i] - prices[day] > profit
min_day , max_day , profit = day , i , prices[i] - prices[day]
end
#i += 1
end
i += 1
end
return "[#{min_day}, #{max_day}]"
end
prices = [17,3,6,9,15,8,6,1,10]
puts stock_picker prices
My objective is to implement a method #stock_picker that takes in an array of stock prices, one for each hypothetical day. It should return a pair of days representing the best day to buy and the best day to sell. Days start at 0.
My question is why is it that this code wouldn't work if I remove line 11 and wrote it on line 9 instead. Which will then result in the error as follows :
**PS C:\Users\dlim\mystuff> ruby stockpicker.rb
stockpicker.rb:8:in `block in stock_picker': undefined method `-' for nil:NilClass (NoMethodError)
from stockpicker.rb:7:in `each'
from stockpicker.rb:7:in `stock_picker'
from stockpicker.rb:29:in `<main>'

Answer 1

You're basically trying to rewrite combination and max_by :
prices = [17, 3, 6, 9, 15, 8, 6, 1, 10]
days = (0...prices.size).to_a
p days.combination(2).max_by { |day1, day2| prices[day2] - prices[day1] }
# => [1,4]
If you want both the days and the corresponding prices :
[17,3,6,9,15,8,6,1,10].each.with_index.to_a.
combination(2).max_by{|(buy, day1), (sell, day2)|
sell-buy
}
# => [[3, 1], [15, 4]]

Answer 2

Where is this occurring?
Your error
stockpicker.rb:8:in `block in stock_picker': undefined method `-' for nil:NilClass (NoMethodError)
from stockpicker.rb:7:in `each'
from stockpicker.rb:7:in `stock_picker'
from stockpicker.rb:29:in `<main>'
Is occurring on 8th line
if prices[i] - prices[day] > profit
When it tries to access prices[i] when i = 9 and the prices returns nil,
which does not respond to the minus - operator.
Why is this occurring?
You are a loop within a loop
(0...i).each do |day|
if prices[i] - prices[day] > profit
min_day , max_day , profit = day , i , prices[i] - prices[day]
end
#i += 1
end
Increasing the i index counter variable here does not really make sense, since
the day is already iterating over the values in the Range (0...i) and
increasing i inside this loop means its comparing every value in the prices
array once against the next day value in the prices inner array, which
will only include the first three values of the prices (meaning the values at
the end of the prices array, like 1 and 10 will never be compared against
one another); e.g.
i = 1
prices = [17,3,6,9,15,8,6,1,10]
# iteration 0
if prices[i] - prices[day] > profit
# 3 - 17 > 0 # => false
# iteration 1
i += 1 # => 2
day # => 0
if prices[i] - prices[day] > profit
# 6 - 17 > 0 # => false
i += 1 # => 3
day # => 1
# iteration 2
if prices[i] - prices[day] > profit
# 9 - 3 > 0 # => true
min_day, max_day, profit = 1, 3, 6
i += 1 # => 4
day # => 0
# iteration 3
if prices[i] - prices[day] > profit
# 15 - 17 > 0 # => false
i += 1 # => 5
day # => 1
# iteration 4
if prices[i] - prices[day] > profit
# 8 - 3 > 0 # => true
min_day, max_day, profit = 1, 5, 5
i += 1 # => 6
day # => 2
# iteration 5
if prices[i] - prices[day] > profit
# 6 - 6 > 0 # => false
i += 1 # => 7
day # => 3
# iteration 6
if prices[i] - prices[day] > profit
# 1 - 9 > 0 # => false
i += 1 # => 8
day # => 0
# iteration 7
if prices[i] - prices[day] > profit
# 10 - 17 > 0 # => false
i += 1 # => 9
day # => 1
# iteration 8
if prices[i] - prices[day] > profit
# nil - 3 > 0 # => NoMethodError
At the 8th iteration, the outer loop has caused an out of bounds error when
accessing the prices array with prices[i], but is still iterating within the
second loop with the Range of (0...7) that was set after the 5th iteration, so
it is not reaching your while loop's escape clause/expression
while i < prices.length.
Possible solution:
You could keep your working solution or you could simplify your code by using
another Range as the outer loop
(1...prices.length).each do |i|
# ...
end
Instead of increasing an index counter variable within the while loop
i = 1
while i < prices.length
# ...
i +=1
end
Which would look like this
def stock_picker prices
min_day , max_day , profit = 0 , 0 , 0
(1...prices.length).each do |i|
(0...i).each do |day|
if prices[i] - prices[day] > profit
min_day , max_day , profit = day , i , prices[i] - prices[day]
end
end
end
return "[#{min_day}, #{max_day}]"
end
prices = [17,3,6,9,15,8,6,1,10]
puts stock_picker prices
And it will iterate over the following pairs of days, as per your requirements
[i, day]
# => [1, 0], [2, 0], [3, 0], [4, 0], [5, 0], [6, 0], [7, 0], [8, 0],
# [2, 1], [3, 1], [4, 1], [5, 1], [6, 1], [7, 1], [8, 1],
# [3, 2], [4, 2], [5, 2], [6, 2], [7, 2], [8, 2],
# [4, 3], [5, 3], [6, 3], [7, 3], [8, 3],
# [5, 4], [6, 4], [7, 4], [8, 4],
# [6, 5], [7, 5], [8, 5],
# [7, 6], [8, 6],
# [8, 7]
UPDATE:
You could also simplify it again with the Ruby Array combination method
(0...prices.length).to_a.combination(2)
To generate the same unique and non-duplicate pairs of days as iterating over the Ranges implied, which would look like this
def stock_picker prices
min_day , max_day , profit = 0 , 0 , 0
(0...prices.length).to_a.combination(2).each do |day, i|
if prices[i] - prices[day] > profit
min_day , max_day , profit = day , i , prices[i] - prices[day]
end
end
return "[#{min_day}, #{max_day}]"
end
prices = [17,3,6,9,15,8,6,1,10]
puts stock_picker prices
|day, i| will access the first and second variables in the array of day index pairs within array of combinations, whilst reusing the existing variable names you have used.