Added ex 48, additional comment on 46

README.md

@@ -127,7 +127,7 @@ Planned exercises:
 * [x] Structs
 * [x] Pointers
 * [x] Optionals
-* [ ] Struct methods
+* [x] Struct methods
 * [ ] Slices
 * [ ] Multi pointers
 * [ ] Unions

build.zig

@@ -250,8 +250,16 @@ const exercises = [_]Exercise{
         .output = "5 aliens. 4 aliens. 1 aliens. 0 aliens. Earth is saved!",
         .hint = "Use the heat ray. And the method!",
     },
-    // use struct method for elephant tails
-    // quiz: add elephant trunk (like tail)!
+    .{
+        .main_file = "48_methods2.zig",
+        .output = "Elephant A (U). Elephant B (U). Elephant C (U).",
+        .hint = "This just needs one little fix."
+    },
+    // 48 use struct method for elephant tails
+    // 49 quiz: add elephant trunk (like tail)!
+    // 50 null vs undefined
+    // 51 pass-by-value and const fn params
+    // 52 slices!
 };
 
 /// Check the zig version to make sure it can compile the examples properly.

exercises/46_optionals2.zig

@@ -5,11 +5,23 @@
 // linked to the first elephant. This is because we had NO CONCEPT
 // of a tail that didn't point to another elephant!
 //
+// We also introduce the handy ".?" shortcut:
+//
+//     const foo = bar.?;
+//
+// is the same as
+//
+//     const foo = bar orelse unreachable;
+//
+// See if you can find where we use this shortcut below.
+//
+// Now let's make those elephant tails optional!
+//
 const std = @import("std");
 
 const Elephant = struct {
     letter: u8,
-    tail: *Elephant = null, // <---- make this optional!
+    tail: *Elephant = null, // Hmm... tail needs something...
     visited: bool = false,
 };
 

exercises/48_methods2.zig

@@ -0,0 +1,68 @@
+//
+// Now that we've seen how methods work, let's see if we can help
+// our elephants out a bit more with some Elephant methods.
+//
+const std = @import("std");
+
+const Elephant = struct {
+    letter: u8,
+    tail: ?*Elephant = null,
+    visited: bool = false,
+
+    // New Elephant methods!
+    pub fn getTail(self: *Elephant) *Elephant {
+        return self.tail.?; // Remember, this is means "orelse unreachable"
+    }
+
+    pub fn hasTail(self: *Elephant) bool {
+        return (self.tail != null);
+    }
+
+    pub fn visit(self: *Elephant) void {
+        self.visited = true;
+    }
+
+    pub fn print(self: *Elephant) void {
+        // Prints elephant letter and (V)isited or (U)nvisited.
+        var v: u8 = if (self.visited) 'V' else 'U';
+        std.debug.print("Elephant {u} ({u}). ", .{ self.letter, v});
+    }
+};
+
+pub fn main() void {
+    var elephantA = Elephant{ .letter = 'A' };
+    var elephantB = Elephant{ .letter = 'B' };
+    var elephantC = Elephant{ .letter = 'C' };
+
+    // Link the elephants so that each tail "points" to the next.
+    elephantA.tail = &elephantB;
+    elephantB.tail = &elephantC;
+
+    visitElephants(&elephantA);
+
+    std.debug.print("\n", .{});
+}
+
+// This function visits all elephants once, starting with the
+// first elephant and following the tails to the next elephant.
+fn visitElephants(first_elephant: *Elephant) void {
+    var e = first_elephant;
+
+    while (true) {
+        e.print();
+        e.visit();
+
+        // Get the next elephant or stop.
+        if (e.hasTail()) {
+            e = e.???; // Which method do we want here?
+        } else {
+            break;
+        }
+    }
+}
+
+// Bonus: Zig's enums can also have methods! Can you find
+// one in the wild? If you can, mention it along with your
+// name or alias in a comment below this one and make a
+// pull request on GitHub for a piece of eternal Ziglings
+// glory. The first five (5) PRs will be accepted!

patches/patches/46_optionals2.patch

@@ -1,8 +1,21 @@
-12c12
-<     tail: *Elephant = null, // <---- make this optional!
+8,19d7
+< // We also introduce the handy ".?" shortcut:
+< //
+< //     const foo = bar.?;
+< //
+< // is the same as
+< //
+< //     const foo = bar orelse unreachable;
+< //
+< // See if you can find where we use this shortcut below.
+< //
+< // Now let's make those elephant tails optional!
+< //
+24c12
+<     tail: *Elephant = null, // Hmm... tail needs something...
 ---
 >     tail: ?*Elephant = null, // <---- make this optional!
-42c42
+54c42
 <         if (e.tail == null) ???;
 ---
 >         if (e.tail == null) break;

patches/patches/48_methods2.patch

@@ -0,0 +1,4 @@
+57c57
+<             e = e.???; // Which method do we want here?
+---
+>             e = e.getTail(); // Which method do we want here?