Scanf

This is a Crystal wrapper around the C function scanf, specifically sscanf. It reads numeric and string data types from other strings. Usefully for Crystal users, it returns scanned data as a tuple with specific types, rather than an array of union types.

Installation

Add scanf to your dependencies in your shard.yml file:

dependencies:
  scanf:
    github: tenebrousedge/crscanf
    branch: master

Then run shards install.

Usage

This provides a global scanf macro, which takes a format string as an input, and returns a lambda. The lambda takes a string input, and the return type is dependent on the format string. Some examples:

a = scanf("%3s %d").call("abc 123")
puts a #=> {"abc", 123}
puts typeof(a) #=> Tuple(String, Int32)

lam = scanf("%f -- %c")
b = lam.call("1234 -- q2")
puts b #=> {1234.0, "q"}
puts typeof(b) #=> Tuple(Float32, String)

The format string specification is documented in a number of places, here being one example.

Some specifiers were not implemented. The p specifier does not work, and it probably does not make sense to try to make it work. The j, z, and t length specifiers are not implemented, and they should not be necessary. The L length specifier is not implemented, because Crystal doesn't have Float128. The c specifier is treated exactly the same as s.

Various versions of the C scanf function may be used to read directly from IO objects. This is deliberately not implemented.

Warning

scanf is a dangerous tool. It can easily result in undefined behavior. Test your format strings carefully. This implementation has at least as many bugs as the C version, and likely more.