141 lines
4.7 KiB
Zig
141 lines
4.7 KiB
Zig
//
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// Loop bodies are blocks, which are also expressions. We've seen
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// how they can be used to evaluate and return values. To further
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// expand on this concept, it turns out we can also give names to
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// blocks by applying a 'label':
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//
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// my_label: { ... }
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//
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// Once you give a block a label, you can use 'break' to exit
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// from that block.
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//
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// outer_block: { // outer block
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// while (true) { // inner block
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// break :outer_block;
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// }
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// unreachable;
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// }
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//
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// As we've just learned, you can return a value using a break
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// statement. Does that mean you can return a value from any
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// labeled block? Yes it does!
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//
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// const foo = make_five: {
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// const five = 1 + 1 + 1 + 1 + 1;
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// break :make_five five;
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// };
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//
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// Labels can also be used with loops. Being able to break out of
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// nested loops at a specific level is one of those things that
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// you won't use every day, but when the time comes, it's
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// incredibly convenient. Being able to return a value from an
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// inner loop is sometimes so handy, it almost feels like cheating
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// (and can help you avoid creating a lot of temporary variables).
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//
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// const bar: u8 = two_loop: while (true) {
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// while (true) {
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// break :two_loop 2;
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// }
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// } else 0;
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//
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// In the above example, the break exits from the outer loop
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// labeled "two_loop" and returns the value 2. The else clause is
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// attached to the outer two_loop and would be evaluated if the
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// loop somehow ended without the break having been called.
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//
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// Finally, you can also use block labels with the 'continue'
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// statement:
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//
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// my_while: while (true) {
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// continue :my_while;
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// }
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//
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const print = @import("std").debug.print;
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// As mentioned before, we'll soon understand why these two
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// numbers don't need explicit types. Hang in there!
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const ingredients = 4;
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const foods = 4;
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const Food = struct {
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name: []const u8,
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requires: [ingredients]bool,
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};
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// Chili Macaroni Tomato Sauce Cheese
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// ------------------------------------------------------
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// Mac & Cheese x x
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// Chili Mac x x
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// Pasta x x
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// Cheesy Chili x x
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// ------------------------------------------------------
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const menu: [foods]Food = [_]Food{
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Food{
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.name = "Mac & Cheese",
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.requires = [ingredients]bool{ false, true, false, true },
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},
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Food{
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.name = "Chili Mac",
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.requires = [ingredients]bool{ true, true, false, false },
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},
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Food{
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.name = "Pasta",
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.requires = [ingredients]bool{ false, true, true, false },
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},
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Food{
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.name = "Cheesy Chili",
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.requires = [ingredients]bool{ true, false, false, true },
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},
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};
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pub fn main() void {
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// Welcome to Cafeteria USA! Choose your favorite ingredients
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// and we'll produce a delicious meal.
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//
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// Cafeteria Customer Note: Not all ingredient combinations
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// make a meal. The default meal is macaroni and cheese.
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//
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// Software Developer Note: Hard-coding the ingredient
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// numbers (based on array position) will be fine for our
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// tiny example, but it would be downright criminal in a real
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// application!
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const wanted_ingredients = [_]u8{ 0, 3 }; // Chili, Cheese
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// Look at each Food on the menu...
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var meal = food_loop: for (menu) |food| {
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// Now look at each required ingredient for the Food...
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for (food.requires) |required, required_ingredient| {
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// This ingredient isn't required, so skip it.
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if (!required) continue;
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// See if the customer wanted this ingredient.
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// (Remember that want_it will be the index number of
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// the ingredient based on its position in the
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// required ingredient list for each food.)
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var found = for (wanted_ingredients) |want_it| {
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if (required_ingredient == want_it) break true;
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} else false;
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// We did not find this required ingredient, so we
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// can't make this Food. Continue the outer loop.
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if (!found) continue :food_loop;
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}
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// If we get this far, the required ingredients were all
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// wanted for this Food.
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//
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// Please return this Food from the loop.
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break food;
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} else menu[0];
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// ^ Oops! We forgot to return Mac & Cheese as the default
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// Food when the requested ingredients aren't found.
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print("Enjoy your {s}!\n", .{meal.name});
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}
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// Challenge: You can also do away with the 'found' variable in
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// the inner loop. See if you can figure out how to do that!
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