Solve Exercise 7-3: minprintf()

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diff --git a/ch7/7-03_minprintf.c b/ch7/7-03_minprintf.c
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+#include <stdio.h>
+#include <stdarg.h>
+
+/* The C Programming Language: 2nd Edition
+ *
+ * Exercise 7-3: Revise `minprintf` to handle more of the other facilities
+ * of `printf`.
+ *
+ * Notes: It doesn't specify *which* facilities, or how many, so we'll choose
+ * three:
+ *
+ *     o - octal
+ *     x - hexadecimal
+ *     c - unsigned char
+ *
+ * Since we're passing to printf(), this was easy. Doing it ourselves would
+ * take some more work. But we'll skirt by with the minimum.
+ */
+
+void minprintf(char *fmt, ...) {
+	va_list ap;
+	char *p, *sval;
+	int ival;
+	double dval;
+	va_start(ap, fmt);
+	for (p = fmt; *p; p++) {
+		if (*p != '%') {
+			putchar(*p);
+			continue;
+		}
+		switch (*++p) {
+			case 'd':
+				ival = va_arg(ap, int);
+				printf("%d", ival);
+				break;
+			case 'f':
+				dval = va_arg(ap, double);
+				printf("%f", dval);
+				break;
+			case 's':
+				for (sval = va_arg(ap, char *); *sval; sval++) {
+					putchar(*sval);
+				}
+				break;
+			case 'c':
+				ival = va_arg(ap, int);
+				printf("%c", ival);
+				break;
+			case 'x':
+				ival = va_arg(ap, int);
+				printf("0x");
+				printf("%x", ival);
+				break;
+			case 'o':
+				ival = va_arg(ap, int);
+				printf("0o");
+				printf("%o", ival);
+				break;
+			default:
+				putchar(*p);
+				break;
+		}
+	}
+	va_end(ap);
+}
+
+int main(int argc, char *argv[]) {
+	/* Should output 0o144 */
+	minprintf("100 = %o\n", 100);
+	/* Should output 0x64 */
+	minprintf("100 = %x\n", 100);
+	/* Should output a semicolon */
+	minprintf("char(0x59) = %c\n", 59);
+	return 0;
+}