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Exceptions

Updated Aug 24, 2023 ·

Overview​

Errors help you identify problems and learn how to fix them.

  • Errors are called exceptions in Ruby
  • They happen when Ruby can’t run a line of code
  • Common causes include typos, wrong object types, or logical mistakes

TypeError​

This exception occurs when an object is not the expected type

  • Raised when operations receive incompatible data types
  • Ruby shows the line number and type of error

Example:

puts "4" + 3

Ruby expected a string for concatenation but received a number, so it raised a type error.

TypeError: no implicit conversion of Integer into String

Errors can also occur if you reverse values:

# puts "4" + 3
puts 3 + "4"

Output:

TypeError: String can't be coerced into Integer

Ruby reads left to right, expects addition with a number, but sees a string, and raises a type error.

NameError​

A name error exception happens when Ruby cannot find a name you reference in your program. This usually occurs with variables or methods that don’t exist or are misspelled.

Example:

puts favorite_food

Ruby cannot find favorite_food because it was never defined.

NameError: undefined local variable or method `favorite_food'

NoMethodError​

A NoMethodError happens when you try to call a method that an object does not have. This can occur due to typos or using a method on the wrong type of object.

Example:

str = "Hello"
puts str.lenght

Output:

undefined method 'lenght' for an instance of String (NoMethodError)
Did you mean?  length

The same error also occurs if the method does not exist for that object type. For example, using length with an integer:

num = 5
puts num.length

Output:

undefined method 'length' for an instance of Integer (NoMethodError)

ArgumentError​

This exception occurs when...

Consider the method below:

def dinosaur_name(name)
  puts "#{name} is powerful"
  puts "#{name} is legendary"
  puts "#{name} rules the park"
end

If you call the method without passing an argument:

dinosaur_name

This returns:

'dinosaur_name': wrong number of arguments (given 0, expected 1) (ArgumentError)

Similarly, if you provide too many arguments:

praise_dinosaur("Rex", "Blue")

Output:

'dinosaur_name': wrong number of arguments (given 2, expected 1) (ArgumentError)

Ruby’s Hint Feature​

Ruby sometimes suggests similar names that exist in the program to spot typos.

fav_food = "pizza"
puts fov_food

Output:

NameError: undefined local variable or method `fov_food'. Did you mean `fav_food`?

Using Comments​

The line of code causing the error can be commented out to handle the error temporarily.

# puts "4" + 3
puts 2 + 2

Output:

4

This allows the rest of the code to run without stopping due to an error.