How to validate command line input from multiple choice prompt in Ruby?-CodePudding

I'm writing a multiple choice quiz where the user has to choose an answer by entering a letter from a to d. Each question prompt looks like this:

What is the last letter in alphabet?
(a) Y
(b) R
(c) Z
(d) Q

If the user enters anything else, I want to show a message and print the whole question again until the input is a, b, c or d.

Here's what I've tried: (simplified)

question = "What is the last letter in alphabet?\n(a) Y\n(b) R\n(c) Z\n(d) Q"

puts question
answer = gets.chomp.to_s
while answer > "d"
  puts "Please enter a, b, c, or d"
  puts
  puts question
  answer = gets.chomp.to_s
end

It works fine when entering e, f, g etc. but it doesn't catch input like ab, 1, Z, or when the user just presses enter.

CodePudding user response：

You have only four allowed inputs. Check for those directly. Even if as inelegantly as if !(answer == "a" || answer == "b" ...). Anything else will accept some invalid input. In particular, if answer > "d" allows "ab" as a valid answer.

CodePudding user response：

Your approach doesn't work because answer > "d" compares both strings character-wise using their (Unicode) code points. To understand what this means, take a look at the Basic Latin chart: (it's equivalent to an ASCII table)

Basic Latin chart

"d" has a codepoint of U 0064. Any smaller codepoint – i.e every character in the chart before "d" – is regarded smaller. This includes all (regular) digits, all (basic latin) uppercase letters and several symbols:

"0" > "d"  #=> false
"Z" > "d"  #=> false

You could add a lower bound like answer < "a" || answer > "d" but this would still allow all strings starting with one of the allowed characters, e.g.:

"apple" < "a" || "apple" > "d" #=> false

To actually limit the answer to the four allowed values, you have to compare the string to each of them. You could combine these comparisons:

answer == 'a' || answer == 'b' || answer == 'c' || answer == 'd'

use a loop over an array of allowed values:

['a', 'b', 'c', 'd'].any? { |letter| answer == letter }

check whether the array includes the answer:

['a', 'b', 'c', 'd'].include?(answer)

# or

%w[a b c d].include?(answer)

or use a regular expression to match a to d:

answer.match?(/\A[a-d]\z/)

Note that the examples above become true if answer is between a and d. You could either negate the condition via !(...):

while !(%w[a b c d].include?(answer))
  # ...
end

or use until instead of while:

until %w[a b c d].include?(answer)
  # ...
end