11.2.2 Creation of vectors and access to elements
There are several ways to create collections. One way is to create a collection by using make. For example, here we create a vector that contains two elements:
? define variable *my-vector* = make(<vector>, size: 2);
We can change the first and second elements:
? *my-vector*[0] := 5; 5 ? *my-vector*[1] := 3; 3 ? *my-vector*; #[5, 3]
If you want to create a sequence of a certain size, with every element having the same value, you can specify a fill keyword argument to make. The default value for the fill keyword parameter is #f. Thus, if you had read an element of *my-vector* before you wrote numbers into it, you would have received #f.
We can create and initialize a vector to different values all at once by using a built-in constructor. A constructor is a function that creates an instance; using it is a shorthand for calling make. Here, we use the vector constructor function to create a vector and to initialize it with data.
? define variable *my-vector* = vector(5, 3); ? *my-vector*; #[5, 3]
As we saw in Section 11.2.1, certain collections have a literal syntax that enables you to specify a particular data structure as part of the program:
? define variable *my-vector* = #[5, 3]; ? *my-vector*; #[5, 3]
Figure 11.2 shows how you can picture the vector that we just created.
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You might think that *my-vector* is a direct instance of <vector>, but it is not: The <vector> class is abstract, but instantiable. When you use the vector function, or use make with <vector>, the result is a general instance of <simple-object-vector>. You specify the size of a <simple-object-vector> when you create one, and you cannot change that size later. If you need a vector that can change size, use the <stretchy-vector> class. See Section 16.1.1, page 246, for an example that uses stretchy vectors.