Figure 1: Order of elements in a 2-dimensional array ‘A’The third type of Cicada variable is an array variable, which is simply N copies of some other variable type. To define an array we write N inside square brackets before the type we want to copy. Multi-dimensional arrays are just arrays of arrays. Here are some examples.
numDays :: [12] int
checkerboard :: [8][8] bool
coordinates :: [] { x :: y :: double }
Notice that coordinates was defined without a specific array size, so it will start off as a size-0 array until it is manually resized. Arrays defined using empty brackets (or equivalently with the [*] symbol) are given extra storage so that elements can be added easily. Use empty brackets for arrays that will be resized often.
We access array elements using square brackets after the array name, just as in C. The last element can be accessed using the keyword top. To access a range of elements a through b, write [<a, b>]. To access all elements we can simply write [*] or [].
numDays[1] = 31 | assign a single element
numDays[<2, 4>] = { 31, 31, 30 } | assign 3 elements
checkerboard[1][1] = checkerboard[1][3] = true | assign 2 elements
checkerboard[3] = checkerboard[1] | assign a whole row of elements
checkerboard[top] = checkerboard[top-2] | ditto for the 6th and 8th rows
coordinates[*].x = coordinates[*].y | assign all 'x's
coordinates[*].y = -1 | assign all 'y's (scalar-to-array copying)
Cicada has a technical restriction on the elements we can access simultaneously in multi-dimensional arrays: their memory has to be contiguous. Internally, the last index counts consecutive elements in memory; then the next-to-last index increments; etc. So the elements of some 2-dimensional array A elements are [1][1], [1][2], ..., [2][1], [2][2], ..., etc as shown in Figure 1. Here are some legal and illegal expressions involving A.
A[<2, 3>] | legal
A[<2, 3>][<1, 4>] | legal
A[<2, 3>][2] | will cause error
A[2][<2, 3>] | legal
A[1][2] | legal
A[<2, 3>][*].x | legal -- x is stored separately from other members
Figure 1: Order of elements in a 2-dimensional array ‘A’When we address part of an array using two or more [*] operators, Cicada loses track of how many elements were in each dimension and pretends that it is looking at a one-dimensional list. For example, if we define:
grid :: [5][5] int
then both grid and grid[*] refer to a 2-dimensional array, but grid[*][*] is effectively a single 1-dimensional list of 25 elements. This is another technical limitation of Cicada, and it may change in the future.
Last update: May 11, 2017