Lists and loops

By the end of this step you can keep many values together and do something with each one: the move that makes programs worth writing.

Lists: many values, in order

One variable holds one value. A list holds many, in order, inside square brackets:

Counting starts at 0, not 1. sizes[0] is "S", sizes[1] is "M". Every developer trips on this at first.

Records: named values together

A product has several facts: a name, a price, a stock count. A record keeps them together, each with a name, inside curly braces. Python calls it a dictionary; JavaScript calls it an object.

A list of records is how most business data looks: a spreadsheet, really. Each record is a row; each field is a column.

Loops: do it for every item

A loop repeats some lines once for each item in a list.

for size in ["S", "M", "L"]: print("We stock size", size)

Output

Press Run (or Ctrl+Enter) to see the output here.

Read it as: for each size in this list, print "We stock size" and the size. The indented line is the body of the loop: it runs once per item. In Python the indent (4 spaces) is what marks the body, so it matters. JavaScript uses { } instead, and the indent is just for readability.

Try it

This puts it together: lists, records, a loop, and a running total.

# A list holds several values, in order, inside [ ]. sizes = ["S", "M", "L", "XL"] print(sizes[0]) # the first item is number 0, not 1 print(len(sizes)) # len() = how many items # A loop does the same thing for every item in the list. # The indented lines are the "body": they run once per item. for size in sizes: print("We stock size", size) # A record groups named values inside { }. Python calls it a dictionary. tent = {"name": "Trail Tent", "price": 249.99} print(tent["name"], "costs", tent["price"]) # A list of records: this is how most business data looks. products = [ {"name": "Trail Tent", "price": 249.99}, {"name": "Camp Mug", "price": 14.50}, {"name": "Headlamp", "price": 39.00}, ] total = 0 for item in products: total = total + item["price"] print("Everything together:", total) # Your turn: add a fourth product, run it again, # and check the total changed by the right amount.

Output

Press Run (or Ctrl+Enter) to see the output here.

// A list (JavaScript calls it an array) holds several values, in order, inside [ ]. const sizes = ["S", "M", "L", "XL"]; console.log(sizes[0]); // the first item is number 0, not 1 console.log(sizes.length); // how many items // A loop does the same thing for every item in the list. // The lines inside { } are the "body": they run once per item. for (const size of sizes) { console.log("We stock size", size); } // A record groups named values inside { }. JavaScript calls it an object. const tent = { name: "Trail Tent", price: 249.99 }; console.log(tent.name, "costs", tent.price); // A list of records: this is how most business data looks. const products = [ { name: "Trail Tent", price: 249.99 }, { name: "Camp Mug", price: 14.5 }, { name: "Headlamp", price: 39.0 }, ]; let total = 0; // "let" instead of "const": this value will change for (const item of products) { total = total + item.price; } console.log("Everything together:", total); // Your turn: add a fourth product, run it again, // and check the total changed by the right amount.

Output

Press Run (or Ctrl+Enter) to see the output here.

Look closely at the total. It starts at 0. Each time round the loop, total = total + item["price"] takes the old total, adds this item's price, and stores the result back in the box. After the last item, the box holds the sum. That pattern (start empty, add as you go) is everywhere.

Your turn

Add a fourth product to the list (a name and a price you choose), run it, and check the total went up by exactly that price.

Show a hint (1 of 3)