How to fix "String to Integer" error in Ruby - ruby-on-rails

Question

I need the shipping cost to be determined by the various rates.
I've tried a hodgepodge of things for the last 5 hours.
If I'm supposed to use .to_s .to_f, I've tried and done so incorrectly.
if (weight < 2)
rate = 0.10
elsif ((weight >= 2) or (weight < 10))
rate = 0.20
elsif ((weight >= 10) or (weight < 40))
rate = 0.30
elsif ((weight >= 40) or (weight < 70))
rate = 0.50
elsif ((weight >= 70) or (weight < 100))
rate = 0.75
else (weight >= 100)
rate = 0.90
end
rate = rate.to_i
ship_cost = weight * price * rate
ship_cost = ship_cost.to_i
The result is supposed to show a shipping cost after the rate is applied. I keep getting to the String to Integer error.

Answer 1

The problem is somehow one or more variables within the multiplication is a string, which results in the TypeError error you're getting, as in:
'a' * 'b' # '*': no implicit conversion of String into Integer (TypeError)
If you want to suppress the error, you can manually convert them into integers or floats. Meaning if the string doesn't have a numeric representation, it'll return 0:
'asd'.to_i # 0
'1'.to_i. # 1
'-9.9'.to_i # -9
'-9.9'.to_f # -9.9
Alternatively, you can handle the rate assignation by using a "dictionary" which holds the min and max value weight can be to return X. Creating a range from min to max and asking if it includes the value of weight you can get assign its value:
dict = {
[-Float::INFINITY, 2] => 0.10,
[2, 10] => 0.20,
[10, 40] => 0.30,
[40, 70] => 0.50,
[70, 100] => 0.75,
[100, Float::INFINITY] => 0.90
}
p dict.find { |(start, finish), _| (start...finish).include?(-42.12) }.last # 0.1
p dict.find { |(start, finish), _| (start...finish).include?(0) }.last # 0.1
p dict.find { |(start, finish), _| (start...finish).include?(1) }.last # 0.1
p dict.find { |(start, finish), _| (start...finish).include?(23) }.last # 0.3
p dict.find { |(start, finish), _| (start...finish).include?(101) }.last # 0.9

Answer 2

A less verbose and more idiomatically correct solution is to use a case statement with ranges:
def shipping_rate(weight)
case weight
when 0...2
0.10
when 2...10
0.20
when 10...40
0.30
when 40...70
0.50
when 70...100
0.75
when 100...Float::INFINITY
0.90
end
end
Declaring a range with ... excludes the end value. So (40...70).cover?(70) == false. That lets us avoid overlap issues.
require "minitest/autorun"
class TestShippingRate < Minitest::Test
def test_correct_rate
assert_equal 0.10, shipping_rate(1)
assert_equal 0.20, shipping_rate(3)
assert_equal 0.30, shipping_rate(39)
assert_equal 0.50, shipping_rate(40)
assert_equal 0.75, shipping_rate(70)
assert_equal 0.90, shipping_rate(101)
end
end
# Finished in 0.002255s, 443.3896 runs/s, 2660.3374 assertions/s.
# 1 runs, 6 assertions, 0 failures, 0 errors, 0 skips
If you want to use a dict like Sebastian Palma suggested you can do it with hash with ranges for keys instead:
def shipping_rate(weight)
{
0...2 => 0.10,
2...10 => 0.20,
10...40 => 0.30,
40...70 => 0.50,
70...100 => 0.75,
100...Float::INFINITY => 0.90
}.find { |k, v| break v if k.cover? weight }
end
Using case is a bit more flexible though as you can add a else condition or handle string arguments:
def shipping_rate(weight)
case weight
when 0...2
0.10
when 2...10
0.20
when 10...40
0.30
when 40...70
0.50
when 70...100
0.75
when 100...Float::INFINITY
0.90
# I'm not saying this is a good idea as the conversion should happen
# upstream. Its just an example of what you can do
when String
shipping_rate(weight.to_f) # recursion
else
raise "Oh noes. This should not happen."
end
end