Fix 076, add 077,078 sentinels and many-item pointers

This commit is contained in:
Dave Gauer 2021-05-08 18:51:08 -04:00
parent 831ee03e32
commit d4f5684450
7 changed files with 126 additions and 35 deletions

View file

@ -381,7 +381,15 @@ const exercises = [_]Exercise{
},
.{
.main_file = "076_sentinels.zig",
.output = "Array:123056. Many-pointer:123.",
.output = "Array:123056. Many-item pointer:123.",
},
.{
.main_file = "077_sentinels2.zig",
.output = "Weird Data!",
},
.{
.main_file = "078_sentinels3.zig",
.output = "Weird Data!",
},
};

View file

@ -29,11 +29,11 @@
// Slice 'b' is only allowed to point to zero-terminated arrays
// but otherwise works just like a normal slice.
//
// Pointer 'c' is exactly like the many-pointers we learned about
// in exercise 054, but it is guaranteed to end in 0. Because of
// this guarantee, we can safely find the end of this
// many-pointer without knowing its length. (We CAN'T do that
// with regular many-pointers!).
// Pointer 'c' is exactly like the many-item pointers we learned
// about in exercise 054, but it is guaranteed to end in 0.
// Because of this guarantee, we can safely find the end of this
// many-item pointer without knowing its length. (We CAN'T do
// that with regular many-item pointers!).
//
const print = @import("std").debug.print;
@ -41,24 +41,25 @@ pub fn main() void {
// Here's a zero-terminated array of u32 values:
var nums = [_:0]u32{ 1, 2, 3, 4, 5, 6 };
// And here's a zero-terminated many-pointer:
// And here's a zero-terminated many-item pointer:
var ptr: [*:0]u32 = &nums;
// For fun, let's replace the value at position 3 with the
// sentinel value 0. This seems kind of naughty.
nums[3] = 0;
// So now we have a zero-terminated array and a many-pointer
// that reference the same data: a sequence of numbers that
// both ends in and CONTAINS the sentinal value.
// So now we have a zero-terminated array and a many-item
// pointer that reference the same data: a sequence of
// numbers that both ends in and CONTAINS the sentinal value.
//
// Attempting to loop through and print both of these should
// demonstrate how they are similar and different.
//
// (It turns out that the array prints completely, including
// the sentinel 0 in the middle. The many-pointer must stop
// at the first sentinel value. The difference is simply that
// arrays have a known length and many-pointers don't.)
// the sentinel 0 in the middle. The many-item pointer must
// stop at the first sentinel value. The difference is simply
// that arrays have a known length and many-item pointers
// don't.)
printSequence(nums);
printSequence(ptr);
@ -86,7 +87,7 @@ fn printSequence(my_seq: anytype) void {
.Pointer => {
// Check this out - it's pretty cool:
const my_sentinel = my_type.Pointer.sentinel;
print("Many-pointer:", .{});
print("Many-item pointer:", .{});
// Loop through the items in my_seq until we hit the
// sentinel value.
@ -100,22 +101,3 @@ fn printSequence(my_seq: anytype) void {
}
print(". ", .{});
}
//
// ------------------------------------------------------------
// TOP SECRET TOP SECRET TOP SECRET TOP SECRET TOP SECRET
// ------------------------------------------------------------
//
// Are you ready for the THE TRUTH about Zig string literals?
//
// You've earned it. Here it is:
//
// @TypeOf("foo") == *const [3:0]u8
//
// Zig's string literals are constant pointers to zero-terminated
// (or "null-terminated") arrays of u8.
//
// Now you know. Welcome to the secret club!
//
// ------------------------------------------------------------
// TOP SECRET TOP SECRET TOP SECRET TOP SECRET TOP SECRET
// ------------------------------------------------------------

View file

@ -0,0 +1,66 @@
//
// ------------------------------------------------------------
// TOP SECRET TOP SECRET TOP SECRET TOP SECRET TOP SECRET
// ------------------------------------------------------------
//
// Are you ready for the THE TRUTH about Zig string literals?
//
// Here it is:
//
// @TypeOf("foo") == *const [3:0]u8
//
// Which means a string literal is a "constant pointer to a
// zero-terminated (null-terminated) fixed-size array of u8".
//
// Now you know. You've earned it. Welcome to the secret club!
//
// ------------------------------------------------------------
//
// Why do we bother using a zero/null sentinel to terminate
// strings in Zig when we already have a known length?
//
// Versatility! Zig strings are compatible with C strings (which
// are null-terminated) AND can be coerced to a variety of other
// Zig types:
//
// const a: [5]u8 = "array".*;
// const b: *const [16]u8 = "pointer to array";
// const c: []const u8 = "slice";
// const d: [:0]const u8 = "slice with sentinel";
// const e: [*:0]const u8 = "many-item pointer with sentinel";
// const f: [*]const u8 = "many-item pointer";
//
// All but 'f' may be printed. (A many-item pointer without a
// sentinel is not safe to print because we don't know where it
// ends!)
//
const print = @import("std").debug.print;
const WeirdContainer = struct {
data: [*]const u8,
length: usize,
};
pub fn main() void {
// WeirdContainer is an awkward way to house a string.
//
// Being a many-item pointer (with no sentinel termination),
// the 'data' field "loses" the length information AND the
// sentinel termination of the string literal "Weird Data!".
//
// Luckily, the 'length' field makes it possible to still
// work with this value.
const foo = WeirdContainer {
.data = "Weird Data!",
.length = 11,
};
// How do we get a printable value from 'foo'? One way is to
// turn it into something with a known length. We do have a
// length... You've actually solved this problem before!
//
// Here's a big hint: do you remember how to take a slice?
const printable = ???;
print("{s}\n", .{printable});
}

View file

@ -0,0 +1,27 @@
//
// We were able to get a printable string out of a many-item
// pointer by using a slice to assert a specific length.
//
// But can we ever GO BACK to a sentinel-terminated pointer
// after we've "lost" the sentinel in a coercion?
//
// Yes, we can. Zig's @ptrCast() builtin can do this. Check out
// the signature:
//
// @ptrCast(comptime DestType: type, value: anytype) DestType
//
// See if you can use it to solve the same many-item pointer
// problem, but without needing a length!
//
const print = @import("std").debug.print;
pub fn main() void {
// Again, we've coerced the sentinel-terminated string to a
// many-item pointer, which has no length or sentinel.
const data: [*]const u8 = "Weird Data!";
// Please cast 'data' to 'printable':
const printable: [*:0]const u8 = ???;
print("{s}\n", .{printable});
}

View file

@ -1,8 +1,8 @@
82c82
83c83
< for (???) |s| {
---
> for (my_seq) |s| {
94c94
95c95
< while (??? != my_sentinel) {
---
> while (my_seq[i] != my_sentinel) {

View file

@ -0,0 +1,4 @@
63c63
< const printable = ???;
---
> const printable = foo.data[0..foo.length];

View file

@ -0,0 +1,4 @@
24c24
< const printable: [*:0]const u8 = ???;
---
> const printable: [*:0]const u8 = @ptrCast([*:0]const u8, data);