Added ex 21-26 for error handling

21_errors.zig

@@ -0,0 +1,46 @@
+//
+// Believe it or not, sometimes things to wrong in programs.
+//
+// In Zig, an error is a value. Errors are named so we can identify
+// things that can go wrong. Errors are created in "error sets", which
+// are just a collection of named errors.
+// 
+// We have the start of an error set, but we're missing the condition
+// "TooSmall". Please add it where needed!
+const MyNumberError = error{
+    TooBig,
+    ???,
+    TooFour,
+};
+
+const std = @import("std");
+
+pub fn main() void {
+    var nums = [_]u8{2,3,4,5,6};
+
+    for (nums) |n| {
+        std.debug.print("{}", .{n});
+
+        const number_error = numberFail(n);
+        
+        if (number_error == MyNumberError.TooBig) {
+            std.debug.print(">4. ", .{});
+        }
+        if (???) {
+            std.debug.print("<4. ", .{});
+        }
+        if (number_error == MyNumberError.TooFour) {
+            std.debug.print("=4. ", .{});
+        }
+    }
+
+    std.debug.print("\n", .{});
+}
+
+// Notice how this function can return any member of the MyNumberError
+// error set.
+fn numberFail(n: u8) MyNumberError {
+    if(n > 4) return MyNumberError.TooBig;
+    if(n < 4) return MyNumberError.TooSmall; // <---- this one is free!
+    return MyNumberError.TooFour;
+}