src/doc/book/src/ch08-01-vectors.md

   1 ## Storing Lists of Values with Vectors
   2
   3 The first collection type we’ll look at is `Vec<T>`, also known as a *vector*.
   4 Vectors allow you to store more than one value in a single data structure that
   5 puts all the values next to each other in memory. Vectors can only store values
   6 of the same type. They are useful when you have a list of items, such as the
   7 lines of text in a file or the prices of items in a shopping cart.
   8
   9 ### Creating a New Vector
  10
  11 To create a new, empty vector, we can call the `Vec::new` function, as shown in
  12 Listing 8-1.
  13
  14 ```rust
  15 {{#rustdoc_include ../listings/ch08-common-collections/listing-08-01/src/main.rs:here}}
  16 ```
  17
  18 <span class="caption">Listing 8-1: Creating a new, empty vector to hold values
  19 of type `i32`</span>
  20
  21 Note that we added a type annotation here. Because we aren’t inserting any
  22 values into this vector, Rust doesn’t know what kind of elements we intend to
  23 store. This is an important point. Vectors are implemented using generics;
  24 we’ll cover how to use generics with your own types in Chapter 10. For now,
  25 know that the `Vec<T>` type provided by the standard library can hold any type,
  26 and when a specific vector holds a specific type, the type is specified within
  27 angle brackets. In Listing 8-1, we’ve told Rust that the `Vec<T>` in `v` will
  28 hold elements of the `i32` type.
  29
  30 In more realistic code, Rust can often infer the type of value you want to
  31 store once you insert values, so you rarely need to do this type annotation.
  32 It’s more common to create a `Vec<T>` that has initial values, and Rust
  33 provides the `vec!` macro for convenience. The macro will create a new vector
  34 that holds the values you give it. Listing 8-2 creates a new `Vec<i32>` that
  35 holds the values `1`, `2`, and `3`. The integer type is `i32` because that’s
  36 the default integer type, as we discussed in the [“Data Types”][data-types]<!--
  37 ignore --> section of Chapter 3.
  38
  39 ```rust
  40 {{#rustdoc_include ../listings/ch08-common-collections/listing-08-02/src/main.rs:here}}
  41 ```
  42
  43 <span class="caption">Listing 8-2: Creating a new vector containing
  44 values</span>
  45
  46 Because we’ve given initial `i32` values, Rust can infer that the type of `v`
  47 is `Vec<i32>`, and the type annotation isn’t necessary. Next, we’ll look at how
  48 to modify a vector.
  49
  50 ### Updating a Vector
  51
  52 To create a vector and then add elements to it, we can use the `push` method,
  53 as shown in Listing 8-3.
  54
  55 ```rust
  56 {{#rustdoc_include ../listings/ch08-common-collections/listing-08-03/src/main.rs:here}}
  57 ```
  58
  59 <span class="caption">Listing 8-3: Using the `push` method to add values to a
  60 vector</span>
  61
  62 As with any variable, if we want to be able to change its value, we need to
  63 make it mutable using the `mut` keyword, as discussed in Chapter 3. The numbers
  64 we place inside are all of type `i32`, and Rust infers this from the data, so
  65 we don’t need the `Vec<i32>` annotation.
  66
  67 ### Dropping a Vector Drops Its Elements
  68
  69 Like any other `struct`, a vector is freed when it goes out of scope, as
  70 annotated in Listing 8-4.
  71
  72 ```rust
  73 {{#rustdoc_include ../listings/ch08-common-collections/listing-08-04/src/main.rs:here}}
  74 ```
  75
  76 <span class="caption">Listing 8-4: Showing where the vector and its elements
  77 are dropped</span>
  78
  79 When the vector gets dropped, all of its contents are also dropped, meaning
  80 those integers it holds will be cleaned up. This may seem like a
  81 straightforward point but can get a bit more complicated when you start to
  82 introduce references to the elements of the vector. Let’s tackle that next!
  83
  84 ### Reading Elements of Vectors
  85
  86 Now that you know how to create, update, and destroy vectors, knowing how to
  87 read their contents is a good next step. There are two ways to reference a
  88 value stored in a vector. In the examples, we’ve annotated the types of the
  89 values that are returned from these functions for extra clarity.
  90
  91 Listing 8-5 shows both methods of accessing a value in a vector, either with
  92 indexing syntax or the `get` method.
  93
  94 ```rust
  95 {{#rustdoc_include ../listings/ch08-common-collections/listing-08-05/src/main.rs:here}}
  96 ```
  97
  98 <span class="caption">Listing 8-5: Using indexing syntax or the `get` method to
  99 access an item in a vector</span>
 100
 101 Note two details here. First, we use the index value of `2` to get the third
 102 element: vectors are indexed by number, starting at zero. Second, the two ways
 103 to get the third element are by using `&` and `[]`, which gives us a reference,
 104 or by using the `get` method with the index passed as an argument, which gives
 105 us an `Option<&T>`.
 106
 107 Rust has two ways to reference an element so you can choose how the program
 108 behaves when you try to use an index value that the vector doesn’t have an
 109 element for. As an example, let’s see what a program will do if it has a vector
 110 that holds five elements and then tries to access an element at index 100, as
 111 shown in Listing 8-6.
 112
 113 ```rust,should_panic,panics
 114 {{#rustdoc_include ../listings/ch08-common-collections/listing-08-06/src/main.rs:here}}
 115 ```
 116
 117 <span class="caption">Listing 8-6: Attempting to access the element at index
 118 100 in a vector containing five elements</span>
 119
 120 When we run this code, the first `[]` method will cause the program to panic
 121 because it references a nonexistent element. This method is best used when you
 122 want your program to crash if there’s an attempt to access an element past the
 123 end of the vector.
 124
 125 When the `get` method is passed an index that is outside the vector, it returns
 126 `None` without panicking. You would use this method if accessing an element
 127 beyond the range of the vector happens occasionally under normal circumstances.
 128 Your code will then have logic to handle having either `Some(&element)` or
 129 `None`, as discussed in Chapter 6. For example, the index could be coming from
 130 a person entering a number. If they accidentally enter a number that’s too
 131 large and the program gets a `None` value, you could tell the user how many
 132 items are in the current vector and give them another chance to enter a valid
 133 value. That would be more user-friendly than crashing the program due to a typo!
 134
 135 When the program has a valid reference, the borrow checker enforces the
 136 ownership and borrowing rules (covered in Chapter 4) to ensure this reference
 137 and any other references to the contents of the vector remain valid. Recall the
 138 rule that states you can’t have mutable and immutable references in the same
 139 scope. That rule applies in Listing 8-7, where we hold an immutable reference to
 140 the first element in a vector and try to add an element to the end, which won’t
 141 work if we also try to refer to that element later in the function:
 142
 143 ```rust,ignore,does_not_compile
 144 {{#rustdoc_include ../listings/ch08-common-collections/listing-08-07/src/main.rs:here}}
 145 ```
 146
 147 <span class="caption">Listing 8-7: Attempting to add an element to a vector
 148 while holding a reference to an item</span>
 149
 150 Compiling this code will result in this error:
 151
 152 ```console
 153 {{#include ../listings/ch08-common-collections/listing-08-07/output.txt}}
 154 ```
 155
 156 The code in Listing 8-7 might look like it should work: why should a reference
 157 to the first element care about what changes at the end of the vector? This
 158 error is due to the way vectors work: adding a new element onto the end of the
 159 vector might require allocating new memory and copying the old elements to the
 160 new space, if there isn’t enough room to put all the elements next to each
 161 other where the vector currently is. In that case, the reference to the first
 162 element would be pointing to deallocated memory. The borrowing rules prevent
 163 programs from ending up in that situation.
 164
 165 > Note: For more on the implementation details of the `Vec<T>` type, see [“The
 166 > Rustonomicon”][nomicon].
 167
 168 ### Iterating over the Values in a Vector
 169
 170 If we want to access each element in a vector in turn, we can iterate through
 171 all of the elements rather than use indices to access one at a time. Listing
 172 8-8 shows how to use a `for` loop to get immutable references to each element
 173 in a vector of `i32` values and print them.
 174
 175 ```rust
 176 {{#rustdoc_include ../listings/ch08-common-collections/listing-08-08/src/main.rs:here}}
 177 ```
 178
 179 <span class="caption">Listing 8-8: Printing each element in a vector by
 180 iterating over the elements using a `for` loop</span>
 181
 182 We can also iterate over mutable references to each element in a mutable vector
 183 in order to make changes to all the elements. The `for` loop in Listing 8-9
 184 will add `50` to each element.
 185
 186 ```rust
 187 {{#rustdoc_include ../listings/ch08-common-collections/listing-08-09/src/main.rs:here}}
 188 ```
 189
 190 <span class="caption">Listing 8-9: Iterating over mutable references to
 191 elements in a vector</span>
 192
 193 To change the value that the mutable reference refers to, we have to use the
 194 dereference operator (`*`) to get to the value in `i` before we can use the
 195 `+=` operator. We’ll talk more about the dereference operator in the
 196 [“Following the Pointer to the Value with the Dereference Operator”][deref]
 197 section of Chapter 15.
 198
 199 ### Using an Enum to Store Multiple Types
 200
 201 At the beginning of this chapter, we said that vectors can only store values
 202 that are the same type. This can be inconvenient; there are definitely use
 203 cases for needing to store a list of items of different types. Fortunately, the
 204 variants of an enum are defined under the same enum type, so when we need to
 205 store elements of a different type in a vector, we can define and use an enum!
 206
 207 For example, say we want to get values from a row in a spreadsheet in which
 208 some of the columns in the row contain integers, some floating-point numbers,
 209 and some strings. We can define an enum whose variants will hold the different
 210 value types, and then all the enum variants will be considered the same type:
 211 that of the enum. Then we can create a vector that holds that enum and so,
 212 ultimately, holds different types. We’ve demonstrated this in Listing 8-10.
 213
 214 ```rust
 215 {{#rustdoc_include ../listings/ch08-common-collections/listing-08-10/src/main.rs:here}}
 216 ```
 217
 218 <span class="caption">Listing 8-10: Defining an `enum` to store values of
 219 different types in one vector</span>
 220
 221 Rust needs to know what types will be in the vector at compile time so it knows
 222 exactly how much memory on the heap will be needed to store each element. A
 223 secondary advantage is that we can be explicit about what types are allowed in
 224 this vector. If Rust allowed a vector to hold any type, there would be a chance
 225 that one or more of the types would cause errors with the operations performed
 226 on the elements of the vector. Using an enum plus a `match` expression means
 227 that Rust will ensure at compile time that every possible case is handled, as
 228 discussed in Chapter 6.
 229
 230 When you’re writing a program, if you don’t know the exhaustive set of types
 231 the program will get at runtime to store in a vector, the enum technique won’t
 232 work. Instead, you can use a trait object, which we’ll cover in Chapter 17.
 233
 234 Now that we’ve discussed some of the most common ways to use vectors, be sure
 235 to review the API documentation for all the many useful methods defined on
 236 `Vec<T>` by the standard library. For example, in addition to `push`, a `pop`
 237 method removes and returns the last element. Let’s move on to the next
 238 collection type: `String`!
 239
 240 [data-types]: ch03-02-data-types.html#data-types
 241 [nomicon]: ../nomicon/vec.html
 242 [deref]: ch15-02-deref.html#following-the-pointer-to-the-value-with-the-dereference-operator