Using Multiple Conditions in a Ruby IF statement

Question

I'm trying to use multiple conditions in an IF statement, to see if the number entered was 5, 10, or 15. If the input is not one of these numbers, then it should output an error message.

But even if 5, 10, and 15 are input, it always outputs the error message.

when 3
  print("Enter the discount percentage, must be (5, 10, or 15)")
  dis= gets.to_i
  if (dis != 5 || dis != 10 || dis != 15)
    puts("You entered an invalid discount")
  else
  end

The correct method to get an integer from the beginning of a string would be to_i, not _i. — Holger Just
– Holger Just, Commented Feb 11, 2017 at 19:46
p || q is the same as !(!p && !q) (from De Morgan's Law). So your expression is the same as !(dis == 5 && dis == 10 && dis == 15). The expression within the parentheses is obviously false for any value of dis, so !(false) #=> true, causing puts to be executed regardless of the value of dis. — Cary Swoveland
– Cary Swoveland, Commented Feb 11, 2017 at 20:06

Ursus · Accepted Answer · 2017-02-11 19:56:36Z

3

Correct logic for your condition would be

if dis != 5 && dis != 10 && dis != 15

Because you want to print the error if the number is not 5, nor 10, nor 15. A more cool way to write that is

if [5, 10, 15].all? { |i| dis != i }

edited Feb 11, 2017 at 19:56

answered Feb 11, 2017 at 19:42

Ursus

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3 Comments

Holger Just Over a year ago

or even if [5, 10, 15].none? { |i| dis == i }

Brian Mccomas Over a year ago

Thanks for showing me 2 different ways of writing this and explaining why. very much appreciate it.

Ursus Over a year ago

My pleasure Sir :)

Andrey Deineko · Accepted Answer · 2017-02-11 19:51:19Z

2

One could useEnumerable#grep:

puts ("You entered an invalid discount") unless [5,10,15].grep(dis).any?

edited Feb 11, 2017 at 19:51

answered Feb 11, 2017 at 19:44

Andrey Deineko

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3 Comments

Brian Mccomas Over a year ago

Thanks, haven't seen that one before as i am still real new to this. i will save that one for future use.

Andrey Deineko Over a year ago

@pjs actually without any? it will not work (since empty array is truthy), so thanks for noticing!

pjs Over a year ago

@AndreyDeineko with any? you don't need grep, and it should be faster for larger lists because it can short circuit the search.

Cary Swoveland · Accepted Answer · 2017-02-11 20:14:55Z

2

puts ("You entered an invalid discount") unless [5,10,15].include?(dis)

answered Feb 11, 2017 at 20:14

Cary Swoveland

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Comments

Sagar Pandya · Accepted Answer · 2017-02-12 01:01:55Z

1

Just another way using regex:

puts "Enter discount, must be 5,10 or 15"
puts gets[/\A1?[05]\n\z/] ? 'Discount applied' : 'Invalid discount'

edited Feb 12, 2017 at 1:01

answered Feb 11, 2017 at 21:21

Sagar Pandya

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