For loops

for repeats a block of statements:

for i in 0..4 {
    place Pad at (i * 100um, 0)
}

0..4 is a range of integers from 0 up to, but not including, 4, so the loop runs with i = 0, 1, 2 and 3. A loop can also walk through a list: for x in [0, 30um, 70um] { ... }.

Loops go wherever statements go: into place { } blocks for instances and into layer blocks for shapes. A shape defined inside a loop is a new shape in every round, so each one can be placed:

layer si {
    for i in 0..3 {
        shape rect with 1um, 1um as dot
        place dot at (i * 2um, 0)
    }
}

In the same way, a name given with as inside a loop belongs to its own round.

Instances in a regular grid, like the ones a loop tends to produce, are written to GDSII as a single array reference.