Motivating Iterators

If you want to iterate over the contents of an array, you'll need to define:

• Some state to keep track of where you are in the iteration process, e.g. an index.
• A condition to determine when iteration is done.
• Logic for updating the state of iteration each loop.
• Logic for fetching each element using that iteration state.

In a C-style for loop you declare these things directly:

for (int i = 0; i < array_len; i += 1) {
    int elem = array[i];
}

In Rust we bundle this state and logic together into an object known as an "iterator".

• This slide provides context for what Rust iterators do under the hood. We use the (hopefully) familiar construct of a C-style for loop to show how iteration requires some state and some logic, that way on the next slide we can show how an iterator bundles these together.

• Rust doesn't have a C-style for loop, but we can express the same thing with while:

  rust,editableCopy

  # // Copyright 2024 Google LLC
  # // SPDX-License-Identifier: Apache-2.0
  #
  let array = [2, 4, 6, 8];
  let mut i = 0;
  while i < array.len() {
      let elem = array[i];
      i += 1;
  }
  

More to Explore

There's another way to express array iteration using for in C and C++: You can use a pointer to the front and a pointer to the end of the array and then compare those pointers to determine when the loop should end.

for (int *ptr = array; ptr < array + len; ptr += 1) {
    int elem = *ptr;
}

If students ask, you can point out that this is how Rust's slice and array iterators work under the hood (though implemented as a Rust iterator).