Type Conversion

Type conversion means turning a value of one type into another — most often text from the user into a number.

Lenient conversion: to_i, to_f, to_s

p "42".to_i
p "3.5".to_f
p 42.to_s
p "42abc".to_i
p "abc".to_i
p 3.99.to_i

42
3.5
"42"
42
0
3

The to_* methods never fail. They read as much as they can and give up quietly — "abc".to_i is 0, and 3.99.to_i cuts off the decimal (it doesn't round). That's convenient but can hide bad input.

Strict conversion: Integer() and Float()

Integer(...) and Float(...) raise an error when the text isn't a valid number:

p Integer("42")
p Float("3.5")

42
3.5

But Integer("abc") stops the program with an error:

invalid value for Integer(): "abc" (ArgumentError)

Use the strict versions for user input you want to validate; the Basic Error Handling lesson shows how to catch the error and ask again. Pass exception: false to get nil instead of an error:

p Integer("abc", exception: false)

nil

Common mistakes

• Trusting to_i on user input: someone types ten, and your program silently treats it as 0.
• Expecting to_i to round. Use 3.99.round for rounding.

Practice

Convert "19.99" to a number, add 5, and print 24.99.

price = Float("19.99")
puts price + 5

24.99