1 files changed, 111 insertions, 0 deletions
diff --git a/4/3.c b/4/3.c
new file mode 100644
index 0000000..55ef9f9
--- /dev/null
+++ b/4/3.c
@@ -0,0 +1,111 @@
+#include <ctype.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+#define MAXOP 100  /* max size of operand or operator */
+#define NUMBER '0' /* signal that a number was found */
+#define MAXVAL 100 /* max depth of val stack */
+#define BUFSIZE 100
+
+int sp = 0;         /* next free stack position */
+double val[MAXVAL]; /* value stack */
+
+int bufp = 0;      /* next free position in buf */
+char buf[BUFSIZE]; /* buffer for ungetch */
+
+/* get next operator or numeric operand */
+int getop(char[]);
+/* push f onto value stack */
+void push(double);
+/* pop and return top value from stack */
+double pop();
+
+/* reverse Polish calculator */
+int main(int argc, char *argv[]) {
+  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: division by zero\n");
+      break;
+    case '\n':
+      printf("\t%.8g\n", pop());
+      break;
+    default:
+      printf("error: unknown command %s\n", s);
+    }
+  }
+
+  return 0;
+}
+
+void push(double f) {
+  if (sp < MAXVAL)
+    val[sp++] = f;
+  else
+    printf("error: stack empty\n");
+}
+
+double pop() {
+  if (sp > 0)
+    return val[--sp];
+  else {
+    printf("error: stack empty\n");
+    return 0.0;
+  }
+}
+
+int getch() { return (buf > 0) ? buf[--bufp] : getchar(); }
+
+void ungetch(int c) {
+  if (bufp >= BUFSIZE)
+    printf("ungetch: too many characters\n");
+  else
+    buf[bufp++] = c;
+}
+
+int getop(char s[]) {
+  int i, c;
+
+
+  while ((s[0] = c = getch()) == ' ' || c == '\t')
+    ;
+  s[1] = 0;
+
+  if (!isdigit(c) && c != '.')
+    return c;
+
+  i = 0;
+  if (isdigit(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;
+}
+\ No newline at end of file