Add ex 35,36 enums; updated README

I'm changing the order of some more topics. Trying to explain the value
of pointers when we're mostly dealing with stack-sized values like
integers feels convoluted. So I'm starting with enums (which also has a
nice segue from an earlier "switch" exercise). Then structs. Then unions
(just in keeping with the order of these items on ziglearn.org) and THEN
pointers and multi-pointers and slices.

1 files changed, 49 insertions, 0 deletions

diff --git a/35_enums.zig b/35_enums.zig
new file mode 100644
index 0000000..cf455a4
--- /dev/null
+++ b/35_enums.zig
@@ -0,0 +1,49 @@
+//
+// Remember that little mathematical virtual machine we made using the
+// "unreachable" statement?  Well, there were two problems with the
+// way we were using op codes:
+//
+//   1. Having to remember op codes by number is no good.
+//   2. We had to use "unreachable" because Zig had no way of knowing
+//      how many valid op codes there were.
+// 
+// An "enum" is a Zig construct that lets you give names to numeric
+// values and store them in a set. They look a lot like error sets:
+//
+//     const Fruit = enum{ apple, pear, orange };
+//
+//     const my_fruit = Fruit.apple;
+//
+// Let's use an enum in place of the numbers we were using in the
+// previous version!
+// 
+const std = @import("std");
+
+// Please complete the enum!
+const Ops = enum{ ??? };
+
+pub fn main() void {
+    const operations = [_]Ops{
+        Ops.inc,
+        Ops.inc,
+        Ops.inc,
+        Ops.pow,
+        Ops.dec,
+        Ops.dec
+    };
+
+    var current_value: u32 = 0;
+
+    for (operations) |op| {
+        switch (op) {
+            Ops.inc  => { current_value += 1; },
+            Ops.dec  => { current_value -= 1; },
+            Ops.pow  => { current_value *= current_value; },
+            // No "else" needed! Why is that?
+        }
+
+        std.debug.print("{} ", .{current_value});
+    }
+
+    std.debug.print("\n", .{});
+}