Cannot get the correct input using STDIN.gets.chomp() in ruby

Question

I am following the LearnRubyTheHardWay tutorial and having a difficulty in modifying the exercise 29. Everything works fine if I define the variables like (as in the tutorial):

people = 100000
cats = 34
dogs = 56

However, if I try to get the variables from STDIN like:

puts "How many people are here?"
people = STDIN.gets.chomp()
puts "How many cats?"
cats = STDIN.gets.chomp()
puts "And how many dogs?"
dogs = STDIN.gets.chomp()

The equality operators return false results as if they only do calculate the results using the first two digits of the numbers. So if I type 100000000 to people and 11, 12 or 13 to cats, the methods return "Too many cats.." If I type 150000000 to people and anything <15 to cats, they return "Not many cats.." Additionally I have to modify

dogs += 5

to

dogs += "5"

otherwise I get the following error: in `+': can't convert Fixnum into String (TypeError)

If I leave the double quotes in place and revert to people = 10000 stuff, I get the following error: in `+': String can't be coerced into Fixnum (TypeError)

So to say, I have no problem with the code in the tutorial, just try to learn what causes the errors introduced by the STDIN methods. I looked into the RubyDoc.org, to see whether it is an issue with fixnum, integer or string classes or anything related to chomp or gets methods but couldn't find the reason. I also tried to_i and to_s before or after but didn't get any result.

The full source code of the file is below:

puts "How many people are here?"
people = STDIN.gets
puts "How many cats?"
cats = STDIN.gets
puts "And how many dogs?"
dogs = STDIN.gets

#people = 100000
#cats = 34
#dogs = 56

puts "So, %d people, %d cats and %d dogs, huh?" % [people,cats,dogs]

if people < cats
  puts "Too many cats! The world is doomed!"
end

if people > cats
  puts "Not many cats! The world is saved!"
end

if people < dogs
  puts "The world is drooled on!"
end

if people > dogs
  puts "The world is dry!"
end

dogs += "5"
puts "Now there are #{dogs} dogs."

if people >= dogs
  puts "People are greater than or equal to dogs."
end

if people <= dogs
  puts "People are less than or equal to dogs."
end

if people == dogs
  puts "People are dogs."
end

Niklas B. · Accepted Answer · 2012-02-11 14:39:09Z

4

The problem is that STDIN.gets returns a string. All the comparison operations will therefore operate on strings. Example:

people = "100000000"
cats = "11"

puts people < cats  # => true!

This is because < will compare the strings lexicographically (and 1000... comes before 11 in the alphabet). There is actually one point in your example that makes it pretty obvious what's going on here:

dogs = STDIN.gets
dogs += "5"

If you input 7 here, it should print out 75. You see, it just concatenates the strings.

How to fix this? Easy, just convert the strings to integers:

puts "How many people are here?"
people = STDIN.gets.to_i
puts "How many cats?"
cats = STDIN.gets.to_i
puts "And how many dogs?"
dogs = STDIN.gets.to_i

answered Feb 11, 2012 at 14:39

Niklas B.

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3 Comments

barerd Over a year ago

Thank you, I tried people.to_i but now I see that I forgot to do it like "people = people.to_i". I haven't seen anything like STDIN.geti instead of gets. Is there any placeyou can suggest to study methods other than rubydoc.org?

Niklas B. Over a year ago

@barerd: There exists no such thing as STDIN.geti. You use gets.to_i, so the gets method is called on the STDIN file stream object, and then to_i is called on the resulting String object. If you want to get an overview over the available methods, you can use apidock, which is a bit more structered then rubydoc. However, it's probably better to get a basic understanding of the language first (tutorials are good for that).

barerd Over a year ago

Apidock is going to be a nice compliment to the tutorial. Thank you for that, too.

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