How do you create multi-dimensional arrays?

Swift version: 5.6

Paul Hudson @twostraws May 28th 2019

Creating an array that holds strings is as simple as writing [String], but what about creating an array where each item is an array of strings – an array of arrays? This is called a multi-dimensional array, because if you were to draw its contents on paper it would look like a grid rather than a line.

In Swift, creating a multi-dimensional array is just a matter of adding another set of brackets. For example, to turn our [String] array into an array of arrays, you would just write [[String]]. But be careful: arrays are value types, which means if you add an array to another array, then modify it, only one of the two copies will be changed.

The code below demonstrates creating an array of arrays, where each inner array holds strings. It also demonstrates the value type situation that might catch you out:

// this is our array of arrays
var groups = [[String]]()

// we create three simple string arrays for testing
var groupA = ["England", "Ireland", "Scotland", "Wales"]
var groupB = ["Canada", "Mexico", "United States"]
var groupC = ["China", "Japan", "South Korea"]

// then add them all to the "groups" array
groups.append(groupA)
groups.append(groupB)
groups.append(groupC)

// this will print out the array of arays
print("The groups are:", groups)

// we now append an item to one of the arrays
groups[1].append("Costa Rica")
print("\nAfter adding Costa Rica, the groups are:", groups)

// and now print out groupB's contents again
print("\nGroup B still contains:", groupB)

That last print() statement will still print out "Canada, Mexico, United States" because of array's value type behavior: we modified the copy of the array that was inside groups, not groupB.