Expressions

An expression defines a value simply by a literal or by combining multiple other expressions. For example, we can multiply a quantity of 4 millimeters with a factor 5 and assign it to a constant v with the following code:

v = 5 * 4.0mm;

std::debug::assert_eq([v, 20mm]);

The result of this expression would be 20mm like the test (see assert_eq) proves. In this form, the computed value is not yet used for anything, so the examples above will not produce any visible output or effect.

Expression result types

In the above example v is of type Length because the expression 5 * 4.0mm is the multiplication of a factor (without unit) and a length (in mm).

Scalar & Quantity results

In general units of quantities are calculated according to the operator and the units of the operands in an expression like the following examples show.

use std::debug::assert_eq;

// an integer multiplied with another one remains a integer
assert_eq([5 * 4, 20]);

// a scalar multiplied with another scalar or integer results in a scalar
assert_eq([5.5 * 4.5, 24.75]);
assert_eq([5.1 * 4, 20.4]);
assert_eq([5 * 4.1, 20.5]);

// a scalar multiplied with a length is a length
assert_eq([5 * 4mm, 20mm]);

// two length multiplied with each another is an area
assert_eq([5mm * 4mm, 0.2cm²]);

// dividing an area by a length is a length
assert_eq([20mm² / 4mm, 5mm]);

Boolean results

Boolean operations (e.g. !, >, == or &&) lead to a boolean result.

// Using logical operators lead to a boolean result
std::debug::assert_eq([5mm > 4mm, true]);

Even when using them with models:

// Using logical operators between models too
std::debug::assert_eq([std::geo2d::Circle(1cm) == std::geo2d::Circle(10mm), true]);

Only Boolean expressions (expressions with a boolean result) can be used to define conditions (see if statement).