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Data Types  ›  Capstone Project

Type Casting

Type Casting 15 minRuby converts types only when asked: to_i, to_f and to_s forgive bad input, while Integer() and Float() raise an error unless the whole value is valid.

Type Casting

A customer types "3" into Bean Cart's order form. The form hands your code the string "3", not the number 3, and 4.50 * "3" is an error. Converting between types, often called type casting, is a daily job. Ruby gives you two styles: a forgiving one and a strict one.

The rule

to_ methods forgive; capitalised Integer() and Float() complain.

Ruby rarely converts types behind your back. Mixing a number and a string raises a TypeError:

1 + "2"    # TypeError: String can't be coerced into Integer
"2" + 1    # TypeError: no implicit conversion of Integer into String

So you convert explicitly. The forgiving methods start with to_. They always return something, even if the input is nonsense:

"3".to_i        # => 3
"4.50".to_f     # => 4.5
"3 lattes".to_i # => 3   (reads leading digits, ignores the rest)
"latte".to_i    # => 0   (no digits at all)
12.to_s         # => "12"
3.to_f          # => 3.0
3.99.to_i       # => 3   (cuts off the decimals, no rounding)
:tea.to_s       # => "tea"
"tea".to_sym    # => :tea

The strict conversions are methods named like classes. They raise an error if the whole input is not a valid number:

Integer("3")        # => 3
Integer("3 lattes") # ArgumentError: invalid value for Integer()
Float("4.50")       # => 4.5
Float("")           # ArgumentError
Integer("3 lattes", exception: false)  # => nil

The last form returns nil instead of raising, which is handy for checking input.

When building text, you rarely need to_s at all: interpolation calls it for you, so "Cups: #{12}" works.

The shortcut

Ask "Do I trust this input?" Values your own code produced, like a count you store as a string: to_i and to_f are fine. Values a person typed or a file supplied: use Integer() or Float(), so bad input fails loudly instead of turning into a silent 0.

Reading a Bean Cart order quantityStep 1 of 4

  1. The input

    The customer typed "three" in the quantity box.

Drills

Answer fast — trust the rule.

quick drill

What does "12 cups".to_i return?

quick drill

What does Integer("12 cups") do?

quick drill

What is 4.99.to_i?

quick drill

What happens with "Total: " + 9.0?

Remember

  • to_ methods forgive; Integer() and Float() complain about bad input.
  • Ask "do I trust this input?" Untrusted input gets the strict version.
  • Trap: "abc".to_i is a silent 0, and to_i truncates rather than rounds.

Practice — level up

0/1 solved

Write your answer, then compare it with a rubric and a model answer.

ExplainReading a quantity safely

A customer types a quantity into Bean Cart's form and your code receives a string. Compare using to_i with using Integer() to convert it, using the inputs "3" and "three", and say which you would choose and why.