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-rw-r--r--ch4/4-03_calc-modulus.c147
1 files changed, 147 insertions, 0 deletions
diff --git a/ch4/4-03_calc-modulus.c b/ch4/4-03_calc-modulus.c
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+++ b/ch4/4-03_calc-modulus.c
@@ -0,0 +1,147 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <ctype.h>
+
+/* The C Programming Language: 2nd Edition
+ *
+ * Exercise 4-3: Given the basic framework, it's straightforward to extend the
+ * calculator. Add the modulus (%) operator and provisions for negative
+ * numbers.
+ *
+ * Answer: Supporting modulus is indeed rather easy. The modulus operator
+ * divides the first operand by the second and returns the remainder, and C
+ * supports this out of the box, so there's no need to implement much math.
+ * Note that the modulus operator does integer division to get its remainder.
+ * Doing a "proper" modulus operator would require more work, and floats can
+ * be tricky. I might implement this sometime. For now, I typecasted.
+ *
+ * For negative numbers, you just need to make sure you allow the dash symbol
+ * in two key places.
+ */
+
+#define MAXOP 100
+#define NUMBER '0'
+#define MAXVAL 100
+#define BUFSIZE 100
+
+int getop(char []);
+void push(double);
+double pop(void);
+int getch(void);
+void ungetch(int);
+
+int sp = 0; // Next free stack position
+double val[MAXVAL]; // Value stack
+char buf[BUFSIZE]; // buffer for ungetch
+int bufp = 0; // next free position in buf
+
+/* Reverse Polish calculator:
+ *
+ * Binary operations (+-*\%)
+ * operand operand operator
+ *
+ * Example: 6 minus 2 in Reverse Polish Notation is "6 2 -"
+ */
+int main() {
+ int type;
+ double op2;
+ char s[MAXOP];
+
+ while ((type = getop(s)) != EOF) {
+ switch (type) {
+ case NUMBER:
+ push(atof(s));
+ break;
+ 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;
+ /* Yay for modulus! */
+ case '%':
+ op2 = pop();
+ if (op2 != 0.0) {
+ push((int)pop() % (int)op2);
+ } else {
+ printf("Error: Cannot modulo by zero.\n");
+ }
+ break;
+ case '\n':
+ printf("\t%.8g\n", pop());
+ break;
+ default:
+ printf("Error: Unknown command %s\n", s);
+ break;
+ }
+ }
+ 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;
+ }
+}
+
+int getop(char s[]) {
+ int i, c;
+
+ while ((s[0] = c = getch()) == ' ' || c == '\t') {
+ }
+ s[1] = '\0';
+ /* The final check is for negative numbers. */
+ if (!isdigit(c) && c != '.' && c != '-') {
+ return c;
+ }
+ i = 0;
+ /* The second half of this if-statement accounts for negatives */
+ if (isdigit(c) || (i == 0 && c == '-')) {
+ while (isdigit(s[++i] = c = getch())) {
+ }
+ }
+ if (c == '.') {
+ while (isdigit(s[++i] = c = getch())) {
+ }
+ }
+ s[i] = '\0';
+ if (c != EOF) {
+ ungetch(c);
+ }
+ return NUMBER;
+}
+
+int getch(void) {
+ return (bufp > 0) ? buf[--bufp] : getchar();
+}
+
+void ungetch(int c) {
+ if (bufp >= BUFSIZE) {
+ printf("ungetch: Too many characters.\n");
+ } else {
+ buf[bufp++] = c;
+ }
+}