Solve Exercise 7-5: RPN calculator with scanf()

This exercise was deceptively simple, and ended up simpler than the
original! The *scanf series of functions is very useful, but you also
have to be careful or you'll run into segfaults.

1 files changed, 114 insertions, 0 deletions

diff --git a/ch7/7-05_postfix-calc-scanf.c b/ch7/7-05_postfix-calc-scanf.c
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+++ b/ch7/7-05_postfix-calc-scanf.c
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+#include <stdio.h>
+#include <stdlib.h>
+#include <ctype.h>
+#include <math.h>
+#include <string.h>
+
+/* The C Programming Language: 2nd Edition
+ *
+ * Exercise 7-5: Rewrite the postfix calculator of Chapter 4 to use scanf
+ * and/or sscanf to do the input and number conversion.
+ *
+ * Notes:
+ * scanf wants allocated memory of some sort, meaning you're either going
+ * to pass it a pointer (with malloc()'d memory) or you're passing a variable
+ * with the unary & (address-of) operator. Failing to use scanf correctly
+ * _will_ create runtime problems! gcc won't tell you what happens without
+ * -Wall and -Wextra, but scanf segfaults by trying to access an uninitialized
+ * pointer.
+ *
+ * Thus there are two ways to use scanf:
+ *
+ * int foo = 0;
+ * scanf("%d", &foo);
+ * // do stuff
+ *
+ * -- or --
+ *
+ * int *foo = malloc(sizeof(int));
+ * scanf("%d", foo);
+ * // do stuff
+ * free(foo);
+ *
+ * For the sake of sanity in an easy exercise, I'll be using the former form
+ * since a few values on the stack won't hurt anything.
+ */
+
+#define MAXOP    100
+#define MAXVAL   100
+#define BUFSIZE  100
+
+void push(double);
+double pop(void);
+
+int sp = 0; // Next free stack position
+double val[MAXVAL]; // Value stack
+char buf[BUFSIZE]; // buffer
+int bufp = 0; // next free position in buf
+double vars[27];
+
+/* Reverse Polish calculator:
+ *
+ * Binary operations (+-*\)
+ *     operand operand operator
+ *
+ * Example: 6 minus 2 in Reverse Polish Notation is "6 2 -"
+ */
+int main() {
+	char *c;
+	double a = 0, op2;
+	char s[MAXOP];
+	char ltr = '\0';
+	while (scanf("%s%c", s, &ltr) == 2) {
+		if (sscanf(s, "%lf", &a) == 1) {
+			push(a);
+		} else if (sscanf(s, "%s", buf)) {
+			for (c = buf; *c; c++) {
+			switch (*c) {
+				case '+':
+					push(pop() + pop());
+					break;
+				case '*':
+					push(pop() * pop());
+					break;
+				case '-':
+					op2 = pop();
+					push(pop() - op2);
+					break;
+				case '/':
+					op2 = pop();
+					if (op2 != 0.0) {
+						push(pop() / op2);
+					} else {
+						printf("Error: Cannot divide by zero.\n");
+					}
+					break;
+				default:
+					printf("Error: Unknown command %s\n", s);
+					break;
+			}
+			}
+			if (ltr == '\n') {
+				printf("%.8f\n", pop());
+			}
+		}
+	}
+	return 0;
+}
+
+void push(double f) {
+	if (sp < MAXVAL) {
+		val[sp++] = f;
+	} else {
+		printf("Error: Stack full. Cannot push %g\n", f);
+	}
+}
+
+double pop(void) {
+	if (sp > 0) {
+		return val[--sp];
+	} else {
+		printf("Error: Stack empty.\n");
+		return 0.0;
+	}
+}