ITEC 380 2017fall ibarland

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union-of-structs
union-of-structs

Review (optional): video: a struct for DVDs (6m50s)

Another example of using structs to represent DVDs (and practice using the design recipe). The dvd struct will be used, along with book, in the following examples.
code: The initial version to follow along and type in yourself (starting around line 80, the data def'n for dvd); and, the completed code

Making a lib-item: video: union-of-structs (35m30s)

In the template for a union-type, if we are in a branch where we know we have a struct, let's go ahead and pull out the fields of that struct right here in that cond-branch.
code: The initial version to follow along and type in yourself (starting around line 100, the data def'n for lib-item); and, the completed code

Practice for class-discussion: htdp: spiders, dillos, and more

Note:“girth” means circumference

But don't nest other templates; call a helper-function rather than process a union-type inside a struct-type-handling-function. video (23m53s)

• [0:00,3:30) has the main point: call helpers, rather than nest templates (beyond union-of-structs mentioned above)
• [3:30,6:44) gives a data-definition for collectors-edition. Example: struct-of-unions (collectors-edition); its template is merely the struct template (and you'll call helper functions).
• [6:44,12:10) (skimmable) practice writing a functions for collectors-editions, ripoff. Including calling helper-functions.
• [12:10,14:20) (skimmable) illustrates debugging an error in the function (part 1): when you construct objects with the fields swapped — hard to track down! We pointing out that the pink-highlighted code is where the error was triggered, and black-highlighted code isn't any error — it's code that was never run (a code-coverage indicator).
• [14:20,17:40) (skimmable) write the function verify-title; point out that when we call a helper function on one of our fields, that helper function could merely be the getter.
• [17:40,23:53) (skimmable) More involved debugging for a real-world issue: (because an earlier function had an error, but that earlier function's test-cases had been wrong!) Handy keyboard shortcut illustrated at 22:35 (ESC-Ctrl-t to transpose two expressions).
  You can see how to have Racket enforce contracts for constructor-calls.

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©2017, Ian Barland, Radford University Last modified 2017.Oct.16 (Mon)

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