Conditions

if runs a block only when a condition holds. else adds an alternative, and else if chains more conditions:

if i % 3 == 0 {
    place Pad at (i * pitch, 0)
} else if large {
    place Pad with side = 120um at (i * pitch, 0)
} else {
    place Pad with side = 40um at (i * pitch, 0)
}

Conditions are built from comparisons (==, !=, <, <=, >, >=) and combined with && (and), || (or) and ! (not); a bool parameter such as large above can be used directly. % is the remainder of an integer division, handy for patterns that repeat.

Loops nest, so two loops walk through a grid:

for i in 0..columns {
    for j in 0..rows {
        ...
    }
}