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140 | //
// Loop bodies are blocks, which are also expressions. We've seen
// how they can be used to evaluate and return values. To further
// expand on this concept, it turns out we can also give names to
// blocks by applying a 'label':
//
// my_label: { ... }
//
// Once you give a block a label, you can use 'break' to exit
// from that block.
//
// outer_block: { // outer block
// while (true) { // inner block
// break :outer_block;
// }
// unreachable;
// }
//
// As we've just learned, you can return a value using a break
// statement. Does that mean you can return a value from any
// labeled block? Yes it does!
//
// const foo = make_five: {
// const five = 1 + 1 + 1 + 1 + 1;
// break :make_five five;
// };
//
// Labels can also be used with loops. Being able to break out of
// nested loops at a specific level is one of those things that
// you won't use every day, but when the time comes, it's
// incredibly convenient. Being able to return a value from an
// inner loop is sometimes so handy, it almost feels like cheating
// (and can help you avoid creating a lot of temporary variables).
//
// const bar: u8 = two_loop: while (true) {
// while (true) {
// break :two_loop 2;
// }
// } else 0;
//
// In the above example, the break exits from the outer loop
// labeled "two_loop" and returns the value 2. The else clause is
// attached to the outer two_loop and would be evaluated if the
// loop somehow ended without the break having been called.
//
// Finally, you can also use block labels with the 'continue'
// statement:
//
// my_while: while (true) {
// continue :my_while;
// }
//
const print = @import("std").debug.print;
// As mentioned before, we'll soon understand why these two
// numbers don't need explicit types. Hang in there!
const ingredients = 4;
const foods = 4;
const Food = struct {
name: []const u8,
requires: [ingredients]bool,
};
// Chili Macaroni Tomato Sauce Cheese
// ------------------------------------------------------
// Mac & Cheese x x
// Chili Mac x x
// Pasta x x
// Cheesy Chili x x
// ------------------------------------------------------
const menu: [foods]Food = [_]Food{
Food{
.name = "Mac & Cheese",
.requires = [ingredients]bool{ false, true, false, true },
},
Food{
.name = "Chili Mac",
.requires = [ingredients]bool{ true, true, false, false },
},
Food{
.name = "Pasta",
.requires = [ingredients]bool{ false, true, true, false },
},
Food{
.name = "Cheesy Chili",
.requires = [ingredients]bool{ true, false, false, true },
},
};
pub fn main() void {
// Welcome to Cafeteria USA! Choose your favorite ingredients
// and we'll produce a delicious meal.
//
// Cafeteria Customer Note: Not all ingredient combinations
// make a meal. The default meal is macaroni and cheese.
//
// Software Developer Note: Hard-coding the ingredient
// numbers (based on array position) will be fine for our
// tiny example, but it would be downright criminal in a real
// application!
const wanted_ingredients = [_]u8{ 0, 3 }; // Chili, Cheese
// Look at each Food on the menu...
const meal = food_loop: for (menu) |food| {
// Now look at each required ingredient for the Food...
for (food.requires, 0..) |required, required_ingredient| {
// This ingredient isn't required, so skip it.
if (!required) continue;
// See if the customer wanted this ingredient.
// (Remember that want_it will be the index number of
// the ingredient based on its position in the
// required ingredient list for each food.)
const found = for (wanted_ingredients) |want_it| {
if (required_ingredient == want_it) break true;
} else false;
// We did not find this required ingredient, so we
// can't make this Food. Continue the outer loop.
if (!found) continue :food_loop;
}
// If we get this far, the required ingredients were all
// wanted for this Food.
//
// Please return this Food from the loop.
break;
};
// ^ Oops! We forgot to return Mac & Cheese as the default
// Food when the requested ingredients aren't found.
print("Enjoy your {s}!\n", .{meal.name});
}
// Challenge: You can also do away with the 'found' variable in
// the inner loop. See if you can figure out how to do that!
|
