How to fix "String to Integer" error in Ruby

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

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