WEBVTT 00:00:00.700 --> 00:00:06.055 This lesson turns a few pieces of information into a small, readable data model. 00:00:06.205 --> 00:00:11.453 We will keep a user record, count a visit, extend a list, and inspect a range. 00:00:11.603 --> 00:00:15.059 Before executing each example, predict the result. 00:00:15.209 --> 00:00:21.502 HHY chooses value types at runtime, but clear names and simple checks still matter. 00:00:21.652 --> 00:00:28.581 A record named user tells the reader more than a variable named data, especially once a script grows 00:00:28.581 --> 00:00:30.121 beyond a few lines. 00:00:32.270 --> 00:00:34.190 Let introduces a binding. 00:00:34.340 --> 00:00:39.140 Add mut when the binding needs reassignment, as our visit counter does. 00:00:39.290 --> 00:00:44.133 The update reads the old counter, adds one, and stores the new value. 00:00:44.283 --> 00:00:49.595 A plain let binding communicates that the name will keep its assigned value. 00:00:49.745 --> 00:00:53.606 Start with the simplest binding that matches your intention. 00:00:53.756 --> 00:01:01.484 Mutability of a binding and the behavior of collection operations are separate ideas; we will inspect 00:01:01.484 --> 00:01:05.831 collection results explicitly instead of assuming in place updates. 00:01:07.980 --> 00:01:11.991 Here are a Boolean, an integer, a floating point number, and null. 00:01:12.141 --> 00:01:17.538 Null represents absence; it is different from the text null and different from zero. 00:01:17.688 --> 00:01:24.024 The type function helps inspect a value, while is type checks a named type and returns a Boolean. 00:01:24.174 --> 00:01:28.419 These functions are useful when learning or handling external data. 00:01:28.569 --> 00:01:34.948 Prefer an explicit check when a later operation depends on a value being a particular kind. 00:01:37.100 --> 00:01:41.047 A list stores values in order and can be kept for reuse. 00:01:41.197 --> 00:01:46.061 Append returns an extended list, so assign that result to a name. 00:01:46.211 --> 00:01:53.339 Printing both names demonstrates the distinction: original still contains two entries, while extended has 00:01:53.339 --> 00:01:53.848 three. 00:01:53.998 --> 00:01:57.134 Length tells us how many entries are present. 00:01:57.284 --> 00:02:04.995 This is a helpful habit for unfamiliar APIs: keep the input, capture the output, and observe both before 00:02:04.995 --> 00:02:07.993 building more logic on top of them. 00:02:10.140 --> 00:02:12.636 A map associates keys with values. 00:02:12.786 --> 00:02:18.055 This example uses named fields so the source reads like a compact record. 00:02:18.205 --> 00:02:20.936 Dot access reads the known name field. 00:02:21.086 --> 00:02:26.377 Get is useful when the key is chosen as a string, and a missing key returns null. 00:02:26.527 --> 00:02:31.476 Put produces an updated map; capture that result as we did with append. 00:02:31.626 --> 00:02:36.405 Do not treat a missing key as proof that a numeric field contains zero. 00:02:36.555 --> 00:02:39.477 Decide how your application handles absence. 00:02:41.630 --> 00:02:46.963 A range describes a sequence of numbers without spelling each number in a list. 00:02:47.113 --> 00:02:54.717 Run this example and inspect its endpoint behavior: zero, one, and two are printed, while three is the 00:02:54.717 --> 00:02:55.561 exclusive end. 00:02:55.711 --> 00:02:58.783 This distinction prevents off by one mistakes. 00:02:58.933 --> 00:03:01.600 A for loop can visit the range directly. 00:03:01.750 --> 00:03:08.723 In a later lesson we will convert ranges into streams when we want the lazy transformation operators 00:03:08.723 --> 00:03:10.774 rather than an explicit loop. 00:03:12.920 --> 00:03:19.683 HHY also has literal forms for quantities such as duration, size, and percentage. 00:03:19.833 --> 00:03:23.822 They express intent more clearly than unexplained numbers. 00:03:23.972 --> 00:03:31.763 In the tested runtime, five seconds prints as five billion nanoseconds, and ten mebibytes prints as ten 00:03:31.763 --> 00:03:36.345 million four hundred eighty five thousand seven hundred sixty bytes. 00:03:36.495 --> 00:03:39.525 The displayed unit can differ from the source spelling. 00:03:39.675 --> 00:03:46.888 These literals will become useful for timeouts and size filters; do not replace them blindly with plain 00:03:46.888 --> 00:03:47.312 integers. 00:03:49.460 --> 00:03:55.540 This deliberately broken example reassigns a binding that was not declared mutable. 00:03:55.690 --> 00:04:01.493 Run the separate error file to see the diagnostic, then compare it with the working counter. 00:04:01.643 --> 00:04:07.957 The repair is to declare the counter with let mut when repeated updates are part of the design. 00:04:08.107 --> 00:04:12.651 Do not change every binding to mutable just to silence one error. 00:04:12.801 --> 00:04:19.820 Keep the lesson small: identify the name being reassigned and explain why that name needs to change. 00:04:21.970 --> 00:04:28.545 For practice, create another user, add a visit count, and print selected fields rather than depending on 00:04:28.545 --> 00:04:30.866 the display order of map keys. 00:04:31.016 --> 00:04:34.707 Then create a list of two user maps and check its length. 00:04:34.857 --> 00:04:42.040 You now have the basic shapes needed for real scripts: single values, stable and mutable bindings, 00:04:42.040 --> 00:04:44.734 ordered lists, keyed maps, and ranges. 00:04:44.884 --> 00:04:51.711 Next we will put decisions and repeated work around those values using functions and control flow.