Solve Exercise 4-14: Swap Macro

Thanks to ##c on Freenode for helping me understand what the text
didn't clarify.

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diff --git a/ch4/4-14_swap-macro.c b/ch4/4-14_swap-macro.c
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+#include <stdio.h>
+
+/* The C Programming Language: 2nd Edition
+ *
+ * Exercise 4-14: Define a macro swap(t,x,y) that interchanges two arguments
+ * of type t. (Block structure will help.)
+ *
+ * Answer: Macros are read and processed before syntax is, so you can turn
+ * a macro into shorthand code; more shorthand than functions sometimes.
+ *
+ * This macro assumes x and y are of t type. If the types don't match up,
+ * it'll (probably) break.
+ */
+
+#define swap(t, x, y) {\
+	t z;\
+	z = x;\
+	x = y;\
+	y = z;\
+}
+
+int main() {
+	int a = 3;
+	int b = 7;
+
+	printf("a is %d, b is %d\n", a, b);
+	swap(int, a, b);
+	printf("Now swap, and... a is %d and b is %d!\n", a, b);
+	return 0;
+}