A slicing operation is a simple way to extract a range of elements from a container. The boo compiler supports native slicing operations on lists, arrays and strings. Support for user defined slicing operations is planned but currently not implemented.

General Syntax

A slicing operation is applied to a container through the following sintax:

range = container[<firstIndexWanted> : <firstIndexNotWanted> : <step>]

When firstIndexWanted is omitted it is assumed to be 0.

When firstIndexNotWanted is omitted it is assumed to be equals to len(container)+1.

When step is omitted it is assumed to be 1.

List Slicing

l = [1, 2, 3, 4]
assert [1] == l[0]
assert [1] == l[0:1]
assert [1, 2] == l[0:2]
assert [2, 3] == l[1:3]
assert [1, 2, 3, 4] == l[:] // easy way to clone a list

Array Slicing

a = (1, 2, 3, 4)
assert (1,) == a[0:1]
assert (1, 2, 3) == a[:3]
assert (1, 2) == a[:2]
assert (1, 3) == a[::2]
assert (4, 3) == a[-2:-1:-1]

String Slicing

s = "bamboo"
assert "b" == a[0:1]
assert "boo" == a[3:]
assert "bo" == a[3:-1]

Considerations and differences between slices types

If you read the examples with some attention, you saw that [] is used to create slices of lists - Boo.Lang.List or System.Collections.IList, to be more specific -, and using () you create slices of arrays. Consider the following example:

print([1, 2, 3].GetType()) // will print "Boo.Lang.List"
print((4, 5, 6).GetType()) // will print "System.Int32[]"
print(("1", "2", 3).GetType()) // will print "System.Object[]"
print(("a", "b").GetType()) // will print "System.String[]"
print(["foo", "bar"].GetType()) // will print "Boo.Lang.List"

So, having that in mind, you must specify the desired slice type when using it with methods:

// This code will NOT compile
class Test:
    def example(itens):
        i = 0
        itensLen = len(itens)
        while i < itensLen:
            print (itens[i])
            ++i

t = Test()
t.example([1, 2, 3, 4, 5])

the above code will result on compile time error The type 'System.Object' does not support splicing'. This is because the boo compiler cannot predict that you want to pass a collection to the method, and since the type Object is neither a collection or array, we got the error. To handle that, explicitly tell the compiler that you will pass a collection to the method:

// This code WILL compile
import System.Collections

class Test:
    def example(itens as IList):
        i = 0
        itensLen = len(itens)
        while i < itensLen:
            print (itens[i])
            ++i
t = Test()
t.example([1, 2, 3, 4, 5])