Arithmetics

Now that we know how to use expressions and bindings and have basic idea of how function types work, we can look at the arithmetics.

As we’ve seen earlier, integer and floating point numbers are distinct types in OCaml. The character type is not a numeric type and cannot be used in arithmetic expressions.

An unusual feature of OCaml is that it uses different sets of arithmetic functions for integers and floating point numbers. The reason for it is that otherwise the language would require either support for ad hoc polymorphism, which would ruin the decidable type inference without any type annotations; or magical overloading specially for arithmetics. The language designers sacrificed some convenience for consistency.

The integer operators look as usual: +, -, *, /. The floating point operators have a dot at the end: +., -., *., /..

Remember to always use dotted operators with floating point numbers, and write integer numbers with a dot at the end like 4. (or use 4.0) to let the compiler know they are supposed to be floats:

let a = 4 + 2 (* good *)
let b = 4.0 *. 3.5 (* good *)

let c = (float 4) +. 2. (* good, integer is converted to float *)

Bad examples:

let d = 4.0 + 2.0 (* bad, using integer addition with floats *)
let e = 4 +. 2 (* bad using floating point addition with integers *)
let f = 4.0 + 2 (* bad, mixing floats with integers *)

Now let’s write a program that takes temperature in Celsius from the standard input and converts it to Kelvin.

let celsius = read_float ()

let kelvin = celsius +. 273.15

let () = print_float kelvin; print_newline ()

Let’s verify that it works:

$ ocamlopt -o kelvin ./kelvin.ml 

$ ./kelvin
20
293.15

Exercises

Write a program that takes an integer from the standard input and prints its square. Use read_int function for reading and print_int for writing.

Write a program that takes a floating point number representing temperature in Celsius from the standard input and converts it to Fahrenheit.