Test if string is a number in Ruby on Rails

I have the following in my application controller:

def is_number?(object)
  true if Float(object) rescue false
end

and the following condition in my controller:

if mystring.is_number?

end

The condition is throwing an undefined method error. I'm guessing I've defined is_number in the wrong place...?

Create is_number? Method.

Create a helper method:

def is_number? string
  true if Float(string) rescue false
end

And then call it like this:

my_string = '12.34'

is_number?( my_string )
# => true

Extend String Class.

If you want to be able to call is_number? directly on the string instead of passing it as a param to your helper function, then you need to define is_number? as an extension of the String class, like so:

class String
  def is_number?
    true if Float(self) rescue false
  end
end

And then you can call it with:

my_string.is_number?
# => true

class String
  def numeric?
    return true if self =~ /\A\d+\Z/
    true if Float(self) rescue false
  end
end  

p "1".numeric?  # => true
p "1.2".numeric? # => true
p "5.4e-29".numeric? # => true
p "12e20".numeric? # true
p "1a".numeric? # => false
p "1.2.3.4".numeric? # => false

Here's a benchmark for common ways to address this problem. Note which one you should use probably depends on the ratio of false cases expected.

1. If they are relatively uncommon casting is definitely fastest.
2. If false cases are common and you are just checking for ints, comparison vs a transformed state is a good option.
3. If false cases are common and you are checking floats, regexp is probably the way to go

If performance doesn't matter use what you like. :-)

Integer checking details:

# 1.9.3-p448
#
# Calculating -------------------------------------
#                 cast     57485 i/100ms
#            cast fail      5549 i/100ms
#                 to_s     47509 i/100ms
#            to_s fail     50573 i/100ms
#               regexp     45187 i/100ms
#          regexp fail     42566 i/100ms
# -------------------------------------------------
#                 cast  2353703.4 (±4.9%) i/s -   11726940 in   4.998270s
#            cast fail    65590.2 (±4.6%) i/s -     327391 in   5.003511s
#                 to_s  1420892.0 (±6.8%) i/s -    7078841 in   5.011462s
#            to_s fail  1717948.8 (±6.0%) i/s -    8546837 in   4.998672s
#               regexp  1525729.9 (±7.0%) i/s -    7591416 in   5.007105s
#          regexp fail  1154461.1 (±5.5%) i/s -    5788976 in   5.035311s

require 'benchmark/ips'

int = '220000'
bad_int = '22.to.2'

Benchmark.ips do |x|
  x.report('cast') do
    Integer(int) rescue false
  end

  x.report('cast fail') do
    Integer(bad_int) rescue false
  end

  x.report('to_s') do
    int.to_i.to_s == int
  end

  x.report('to_s fail') do
    bad_int.to_i.to_s == bad_int
  end

  x.report('regexp') do
    int =~ /^\d+$/
  end

  x.report('regexp fail') do
    bad_int =~ /^\d+$/
  end
end

Float checking details:

# 1.9.3-p448
#
# Calculating -------------------------------------
#                 cast     47430 i/100ms
#            cast fail      5023 i/100ms
#                 to_s     27435 i/100ms
#            to_s fail     29609 i/100ms
#               regexp     37620 i/100ms
#          regexp fail     32557 i/100ms
# -------------------------------------------------
#                 cast  2283762.5 (±6.8%) i/s -   11383200 in   5.012934s
#            cast fail    63108.8 (±6.7%) i/s -     316449 in   5.038518s
#                 to_s   593069.3 (±8.8%) i/s -    2962980 in   5.042459s
#            to_s fail   857217.1 (±10.0%) i/s -    4263696 in   5.033024s
#               regexp  1383194.8 (±6.7%) i/s -    6884460 in   5.008275s
#          regexp fail   723390.2 (±5.8%) i/s -    3613827 in   5.016494s

require 'benchmark/ips'

float = '12.2312'
bad_float = '22.to.2'

Benchmark.ips do |x|
  x.report('cast') do
    Float(float) rescue false
  end

  x.report('cast fail') do
    Float(bad_float) rescue false
  end

  x.report('to_s') do
    float.to_f.to_s == float
  end

  x.report('to_s fail') do
    bad_float.to_f.to_s == bad_float
  end

  x.report('regexp') do
    float =~ /^[-+]?[0-9]*\.?[0-9]+$/
  end

  x.report('regexp fail') do
    bad_float =~ /^[-+]?[0-9]*\.?[0-9]+$/
  end
end

As of Ruby 2.6.0, the numeric cast-methods have an optional exception-argument [1]. This enables us to use the built-in methods without using exceptions as control flow:

Float('x') # => ArgumentError (invalid value for Float(): "x")
Float('x', exception: false) # => nil

Therefore, you don't have to define your own method, but can directly check variables like e.g.

if Float(my_var, exception: false)
  # do something if my_var is a float
end

Relying on the raised exception is not the fastest, readable nor reliable solution.
I'd do the following :

my_string.should =~ /^[0-9]+$/

this is how i do it, but i think too there must be a better way

object.to_i.to_s == object || object.to_f.to_s == object

Tl;dr: Use a regex approach. It is 39x faster than the rescue approach in the accepted answer and also handles cases like "1,000"

def regex_is_number? string
  no_commas =  string.gsub(',', '')
  matches = no_commas.match(/-?\d+(?:\.\d+)?/)
  if !matches.nil? && matches.size == 1 && matches[0] == no_commas
    true
  else
    false
  end
end

--

The accepted answer by @Jakob S works for the most part, but catching exceptions can be really slow. In addition, the rescue approach fails on a string like "1,000".

Let's define the methods:

def rescue_is_number? string
  true if Float(string) rescue false
end

def regex_is_number? string
  no_commas =  string.gsub(',', '')
  matches = no_commas.match(/-?\d+(?:\.\d+)?/)
  if !matches.nil? && matches.size == 1 && matches[0] == no_commas
    true
  else
    false
  end
end

And now some test cases:

test_cases = {
  true => ["5.5", "23", "-123", "1,234,123"],
  false => ["hello", "99designs", "(123)456-7890"]
}

And a little code to run the test cases:

test_cases.each do |expected_answer, cases|
  cases.each do |test_case|
    if rescue_is_number?(test_case) != expected_answer
      puts "**rescue_is_number? got #{test_case} wrong**"
    else
      puts "rescue_is_number? got #{test_case} right"
    end

    if regex_is_number?(test_case) != expected_answer
      puts "**regex_is_number? got #{test_case} wrong**"
    else
      puts "regex_is_number? got #{test_case} right"
    end  
  end
end

Here is the output of the test cases:

rescue_is_number? got 5.5 right
regex_is_number? got 5.5 right
rescue_is_number? got 23 right
regex_is_number? got 23 right
rescue_is_number? got -123 right
regex_is_number? got -123 right
**rescue_is_number? got 1,234,123 wrong**
regex_is_number? got 1,234,123 right
rescue_is_number? got hello right
regex_is_number? got hello right
rescue_is_number? got 99designs right
regex_is_number? got 99designs right
rescue_is_number? got (123)456-7890 right
regex_is_number? got (123)456-7890 right

Time to do some performance benchmarks:

Benchmark.ips do |x|
 
  x.report("rescue") { test_cases.values.flatten.each { |c| rescue_is_number? c } }
  x.report("regex") { test_cases.values.flatten.each { |c| regex_is_number? c } }
 
  x.compare!
end

And the results:

Calculating -------------------------------------
              rescue   128.000  i/100ms
               regex     4.649k i/100ms
-------------------------------------------------
              rescue      1.348k (±16.8%) i/s -      6.656k
               regex     52.113k (± 7.8%) i/s -    260.344k

Comparison:
               regex:    52113.3 i/s
              rescue:     1347.5 i/s - 38.67x slower

no you're just using it wrong. your is_number? has an argument. you called it without the argument

you should be doing is_number?(mystring)

In rails 4, you need to put require File.expand_path('../../lib', __FILE__) + '/ext/string' in your config/application.rb

If you prefer not to use exceptions as part of the logic, you might try this:

class String
   def numeric?
    !!(self =~ /^-?\d+(\.\d*)?$/)
  end
end

Or, if you want it to work across all object classes, replace class String with class Object an convert self to a string: !!(self.to_s =~ /^-?\d+(\.\d*)?$/)

As Jakob S suggested in his answer, Kernel#Float can be used to validate numericality of the string, only thing that I can add is one-liner version of that, without using rescue block to control flow (which is considered as a bad practice sometimes)

  Float(my_string, exception: false).present?

use the following function:

def is_numeric? val
    return val.try(:to_f).try(:to_s) == val
end

so,

is_numeric? "1.2f" = false

is_numeric? "1.2" = true

is_numeric? "12f" = false

is_numeric? "12" = true

How dumb is this solution?

def is_number?(i)
  begin
    i+0 == i
  rescue TypeError
    false
  end
end