Number Parsing in Crystal

Parsing numbers from strings is a basic but common task in many programs; here’s how to do it in Crystal.

# The built-in String class provides number parsing methods.

# With `to_f`, this parses the string to a Float64.
f = "1.234".to_f
puts f

# For integer parsing, we can use `to_i`.
# By default, it assumes base 10.
i = "123".to_i
puts i

# We can specify a different base for integer parsing.
d = "0x1c8".to_i(base: 16)
puts d

# There's also a method for parsing unsigned integers.
u = "789".to_u64
puts u

# Crystal doesn't have a direct equivalent to Go's Atoi,
# but we can use `to_i` for basic base-10 int parsing.
k = "135".to_i
puts k

# Parse methods raise an exception on bad input.
begin
  "wat".to_i
rescue ex
  puts ex
end

To run the program, save it as number_parsing.cr and use the crystal command:

$ crystal number_parsing.cr
1.234
123
456
789
135
Invalid Int32: wat

Crystal’s number parsing is somewhat different from Go’s:

1. Instead of a separate package, Crystal’s number parsing methods are part of the String class.
2. Crystal doesn’t require specifying bit precision for floating-point parsing; to_f returns a Float64 by default.
3. For integer parsing, Crystal’s to_i method assumes base 10 by default, but allows specifying a different base.
4. Crystal raises exceptions for parsing errors instead of returning error values.
5. There’s no direct equivalent to Go’s ParseUint, but Crystal provides to_u8, to_u16, to_u32, and to_u64 for parsing unsigned integers of different sizes.

Crystal’s approach to number parsing is more object-oriented and integrated into the language’s type system, reflecting its Ruby-inspired design.

Next, we’ll look at another common parsing task: URLs.