Year #3, Week #4 💻 ⚾️

New stuff we learned this week: 🤔

SQL Joins

  • A SQL JOIN statement allows you to query more than one table at once, merging the results into one set. You can sort of think of it like mixing paint colors, you take a little blue and a little green, and make purple! Same with joins - you pluck some stuff from one table, some stuff for another, and return a record set that is the combination result.
  • For the database to know how to join the tables, you have to tell it how to connect them with an ON ... clause:
SELECT name FROM orders
LEFT JOIN order_items
ON order_items.order_id = orders.id
  • You can alias columns to make them make more sense in your joined result, because id is a perfectly good column name for a single table, but if you’re joining two tables, you might want both id columns with them named more clearly:
SELECT orders.id as order_id, order_items.id as order_item_id
FROM orders
LEFT JOIN order_items
ON order_items.order_id = orders.id
  • You can alias tables as well, if it’s helpful:
-- a bit cryptic, but hey, it works!
SELECT o.id as order_id, i.id as order_item_id
FROM orders AS o
LEFT JOIN order_items AS i
ON i.order_id = o.id
  • The different types of JOINs have to do with how the database engine handles partial matches. Left join is more permissive, it supplies NULL where it can’t make a match with the second table, where as an INNER join won’t return a result when it can’t perform the join.

C: Functions that take pointers

  • A pointer is a variable containing a memory address.
  • You can get a pointer for any variable by using the address of operator &.
int my_number = 3;
int *pointer_to_my_number = &my_number;
  • A pointer is a type in it’s own right. In modern languages with generics, this is expressed as something like: Pointer<Int>. But in C, you add a * after the type instead, so it’s int *:
// `p_int` is a variable of type: "pointer to int"
int *p_int;

// `p_char` is a variable of type: "pointer to char"
char *p_char;
  • Once you have a pointer, you very often need to “follow the treasure map” to the area of memory pointed to, to look up and use the value there. To do this, you use the * symbol AGAIN (called the indirection operator), but this time, before the value:
int three = 3;
int *p_three = &three;

// here we'll use the value POINTED TO by `p_three`:
// the `*` here is NOT part of the type, it rather
// is the "INDIRECTION OPERATOR"
int sum = *p_three + 2; // = `5`
  • Probably the most confusing part about pointers in C is that you use * in the type, and you also use it in the indirection operator. For the type, it comes AFTER the type (like int *x;), and when using just as a the indirection operator, it just comes before the variable with no type (like *x;)
  • Pointers have many uses, one of them is that it solves a problem with C, that functions can not return more than one value. Imagine for some reason that you wanted a function that you could pass two integers to, and have it return the sum AND the product of both. There’s no way to return a tuple in C, so one way you can handle this is by passing pointers to two variables:
void sum_and_product(int x, int y, int* sum, int *product) {
  *sum = x + y;
  *product = x * y;
}
  • You would use the sum_and_product() function like so:
int x = 5;
int y = 3;
int sum, product;

sum_and_product(x, y, &sum, &product);
  • Notice how in the above snippet we pass the x and y ints normally, but pass pointers two two other variables. The function will then mutate these variables by following the memory address and storing the sum and product in each. This pattern should be familiar, because it’s exactly how scanf works. It takes multiple pointers so that it can give the calling code multiple results, which would otherwise be impossible in C.

Homework Plan (2 weeks)

  • 1 day Monkey Assignment #12
  • 2 day review all flashcards in your app.
  • 2 day touch typing practice
  • 1 day King C reading assignment (chapter 5)
  • 1 day King C exercizes
  • 1 day MySQL/Flashcards homework
  • 1 day CS50 “Credit” homework
  • watch Lecture 2 of CS50, up until 35:30
  • 6 days Execute Program homework

    • King C Chapter 5 Projects 👑

    • First, make sure you’ve slowly and carefully read all of chapter 5 of King C. assignment.
    • Second, make sure you’ve read the C “New Stuff” above, slowly and carefully.
    • While you’re working on the exercizes listed below, keep in mind that I want you at least once to extract a helper function that takes one or more pointers, as demonstrated in the example functions in new stuff. Try to find a spot where it would actually help you accomplish the task. You can do it more than once, but I want you to do it at least once.
    • Merge your king-c repo branch from last week, and CREATE A NEW BRANCH.
    • Complete the following programming projects (not exercizes) from chapter 3: 1, 2, 4, 5, 7, 9, 19, 11. Make a new file for every one.
    • commit your work, and push up a MR on GitLab, then slack me the URL.

    Monkey #12 (Prefix Operators) 🐒

    • Address all feedback from prior MR’s, and merge.
    • Create a new branch.
    • Start in the middle of section 2.6, in the section labeled Prefix Operators. Work through that section, and stop when you get to the section labeled Infix Operators.
    • As always, try to do as much as you can on your own, leaning on the videos when you get stuck. But also, make sure to watch the videos and update your code accordingly.
    • Video link
    • Submit a MR, slack the MR url

    CS50 Credit Assignment

    • Before you do this assignment, make sure that you have completed:
      • the King ch. 5 reading
      • the King ch. 5 programming projects
    • Merge your cs50 MR from 2 weeks ago, then CREATE A NEW BRANCH called credit.
    • Before you read the assignment, there are two variations of requirements. Kiah Credit: ✨ is to do the assignment exactly as written, supporting credit card numbers of various length. For everyone else, you can complete the assignment only supporting 16 digit card numbers, which makes it a chunk easier.
    • Also, same as in the King assignment this week, I want you to extract one function. You can decide if it will accept pointers or not, whichever seems most helpful.
    • Slowly and carefully read the assignment here.
    • Complete the assignment in a file called credit.c.
    • Be sure to check some of the card numbers shown at the bottom of the page (only do the 16 digit numbers, unless you’re doing the Kiah credit).
    • Commit your work, submit a MR, slack me the URL.

    Personal Project Homework

    • Refer to your work plan you created a few weeks ago, and select the next item on your list. If you’re ahead or behind of where you thought you would be, make any modifications you think appropriate, then Slack me your goal for this week by WEDNESDAY at 9AM!!!! 📅 👋
    • Make sure you’ve addressed all of my feedback from last week, merge your MR, connect with vscode, pull from origin, and create a new branch.
    • Implement the feature or chunk of work you planned.
    • When you think you’re done, check things like:
      • did you leave in any console.log()s?
      • does it look good at all screen sizes?
      • do your storybook stories work and cover your components (if you’re using storybook)
      • are your components and variables named well?
      • is there anything you want to clean up, refactor, or DRY up before you submit?
    • when you’re happy with the code, build your site, submit a MR, and Slack both the URLs.
    • after I review, address any feedback I give you.

    MySQL / Flashcards Assignment

    • Slowly and carefully review this weeks New Stuff^^^ on SQL joins.
    • Fire up your flashcards dev app, make sure it’s running and talking to your API without any problems. If something’s broken or error-ing, fix it. :)
    • CREATE A NEW BRANCH and switch to it.
    • Make sure you did the previous step.
    • Harriet, did you do the previous step?
    • Next, (referring to last weeks new stuff, if necessary), make a backup of your prod database using mysqldump, just in case you mess something up horribly working on this assignment, you can always restore it from that.
    • At the end of last week’s assignment, we got a foreign key relationship set up between your cards table and the new categories table. But we left in the old column called type (or maybe something else), that is now redundant — it is taken over by the name column of the categories table now. Connect with Sequel Pro, find the column, and then execute a MySQL query to delete that column.
    • next, in Sequel Pro still, practice writing a SQL statement including a JOIN that retrieves the name or type of the category by joining with the categories table. If you craft the query just write, you won’t have to change any of your typescript code to accommodate the change in database schema. But, if you end up with different field names from your join, you can also adjust the typescript code to match. I would advise trying to do the former.
    • once you’ve got the query returning the right results by joining the two tables, replace the query in your server typescript code, and verify that everything still works.
    • Creating a new card might now be broken, that’s OK - as long as selecting/fetching the cards works, and your app still functions to review the cards, that’s enough. We’ll fix the other parts of the app in some future homework.
    • Commit your work, submit a MR, slack the URL.
    ← All homework