1 files changed, 113 insertions, 0 deletions
diff --git a/ex4-3.c b/ex4-3.c
new file mode 100644
index 0000000..47689f4
--- /dev/null
+++ b/ex4-3.c
@@ -0,0 +1,113 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <ctype.h>
+
+#define NUMBER '0'
+#define MAXOP 100
+#define MAXVAL 100
+
+int getop(char[]);
+void push(double);
+double pop(void);
+
+/**
+ * Reverse polish notation calculator
+ */
+int main(void) {
+    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: zero division attempted!\n");
+                break;
+            case '\n':
+                printf("\t%.8g\n", pop());
+                break;
+            default:
+                printf("unknown command %s\n", s);
+                break;
+        }
+    }
+    return 0;
+}
+
+
+int sp; // Stack position
+double val[MAXVAL]; // Value stack
+void push(double f) {
+    if (sp < MAXVAL) {
+        val[sp++] = f;
+    } else {
+        printf("error, stack full\n");
+    }
+}
+
+double pop(void) {
+    if (sp >= 0) {
+        return val[--sp];
+    } else {
+        printf("error, empty stack\n");
+        return 0.0;
+    }
+}
+
+
+int getch(void);
+void ungetch(int);
+
+int getop(char s[]) {
+    int i,c;
+    // Skip to teh first non-space or tab character
+    while ((s[0] = c = getch()) == ' ' || c == '\t');
+    // Then add a null for some reason?
+    s[1] = '\0';
+    // if it's not a part of a number, it's an operation
+    if (!isdigit(c) && c != '.') 
+        return c;
+    // Integer part
+    i = 0;
+    if (isdigit(c)) 
+        while (isdigit(s[++i] = c = getch()));
+    // fraction part
+    if (c == '.')
+        while (isdigit(s[++i] = c = getch()));
+    s[i] = '\0';
+    if (c != EOF) 
+        ungetch(c);
+    return NUMBER;
+}
+
+#define BUFSIZE 100
+
+char buf[BUFSIZE]; // buffer for ungetch
+int bufp = 0; // next free in buf
+
+int getch(void) {
+    return (bufp > 0) ? buf[--bufp] : getchar();
+}
+
+void ungetch(int c) {
+    if (bufp >= BUFSIZE) 
+        printf("ungetch: buffer full\n");
+    else
+        buf[bufp++] = c;
+}
+\ No newline at end of file