Bags I

Loops really start to shine when you combine it with repeated data: a bag full of values. They’re like twins. A loop repeats code, the bag provides a new element every iteration.

A bag is nothing more than a group of booleans, numbers or strings. It contains multiple “literals” under one name. You might also call it a list, and everything inside an element.

Yes, a shopping list is probably a good first example. Maybe you’re coding a web shop. When the customer checks out, you want to create a list of everything in their shopping cart.

You could do this with what I taught so far.

But this feels repetitive, doesn’t it? And I repeat: we—coders—do not like repeating ourselves!

Additionally, you don’t know how many things people will put in their cart. Maybe you only coded 10 items, do you really want to push away customers that want to buy more things from you?

Enter the bag! It holds as many or as few items as you want, under a single name.

To create a new bag, use bag NAME holds, then type each element on a new line after it.

Values

 => 

Bag

Try it! Add some more elements to the shopping list, or remove a few. It doesn’t matter: that one name (shoppingList) will hold everything for us.

Getting the size

Okay, we’ve grouped it, nice. But we want to know how many items are in the shopping cart. So we can display that on the web site, like most shops do.

To get a bag’s size, use BAGNAME's size

Bag

 => 

Number

Yes, the same syntax and word that you used for Strings. In that chapter, I told you that strings were basically lists of characters. In other words, a string is a bag of symbols, right? Most languages, therefore, use the same syntax for getting a string size as for getting a list size.

It also helps, in my view, to think of them as similar. A string is much more like a bag, than a number or boolean.

Getting an element

Okay, we have a bag and we know how big it is. But now we want to check one specific element. Maybe we’re running a discount on pizzas and want to check if somebody has a pizza in their shopping cart.

The following principle is good to remember when it comes to any programming.

Lists, loops, counters, everything starts at 0. (Such a position of an element is called its index.)

There are many good reasons for this. They’re outside of the scope of this tutorial—look it up if you’re interested. Just remember that everything starts at 0, not 1.

This means we can get the first element of a bag by requesting index 0. The second one by requesting index 1. And so forth.

To get an element from a bag, use BAGNAME's INDEX

Bag & Number

 => 

Value

Let’s see if we can get that discount going.

Hmm. It doesn’t say anything. Why?

Ah yes, we only check the first element. But pizza is the third ( = index 2).

No, we don’t want to repeat that if statement for all values. Remember the two rules:

• Don’t Repeat Yourself
• Bags and loops are twins

Maybe we can … use a loop!

Looping a bag

What do we want? We want to check for every element if it’s a pizza. This means we need one general block of code that we can repeat for every element.

Let’s consider our data transformations again.

• Input = a bag and current iteration of the loop
• Change = turn that into the element name
• Output = whether the element name is equal to Pizza

Try writing this code.

Ah, that’s better. And all that with just a few lines of code! This works for any bag. Add more elements. Add more pizzas, if you want, or remove them altogether. The only thing that changes is the bag. The other code will continue to work, whatever is inside, no matter how big it is.

That’s the power of loops and bags. Given any data, of any size, it will continue to work. It just repeats the same code as much as needed.

For that to work, though, the code needs to be general. That’s one of the steps that makes programming hard—a puzzle, an exercise in “problem solving”. Because you need to figure out how to solve a problem for any input. How to solve it in a general way.

Exercise 1

Rewrite the code above to give a discount on anything but a smartphone.

Remember those “guard clauses” from previous chapter.

Bag & Number

 => 

Bool & Output

Exercise 2

Pad the shopping list with a few extra items. Now rewrite the code to give a discount on every third element. (Like some “2 + 1 free” action.)

Remember the modulo operator I explained in the chapters on Loops.

Bag & Numbers

 => 

Bool & Output

Conclusion

Bags (or lists/arrays) are powerful. With little code, you can make the computer do a lot. I have only given one very simple example. In the upcoming chapters, you’ll see way more practical use cases for bags.

But most of all, there’s another thing you can do with elements in the real world. Instead of numbering them … you might want to name them.