From cc66f02855428590fa4172aa6a8b88fa0a8109bf Mon Sep 17 00:00:00 2001
From: Martin Ashby <martin@ashbysoft.com>
Date: Mon, 21 Nov 2022 21:09:30 +0000
Subject: have just a single char pushback

---
 ex4-3.c | 34 +++++++++++++++++++++++++++-------
 1 file changed, 27 insertions(+), 7 deletions(-)

diff --git a/ex4-3.c b/ex4-3.c
index 7de79fb..ca5739a 100644
--- a/ex4-3.c
+++ b/ex4-3.c
@@ -7,8 +7,11 @@
 #define MAXVAL 100
 
 int getop(char[]);
+
 void push(double);
 double pop(void);
+void clear(void);
+double peek(void);
 
 // Naïve implementation of remainder operation
 double frem(double f1, double f2) {
@@ -51,8 +54,10 @@ int main(void) {
                 else 
                     printf("error: zero division attempted!\n");
                 break;
+            case '~':
+                clear();
             case '\n':
-                printf("\t%.8g\n", pop());
+                printf("\t%.8g\n", peek());
                 break;
             default:
                 printf("unknown command %s\n", s);
@@ -62,7 +67,6 @@ int main(void) {
     return 0;
 }
 
-
 int sp; // Stack position
 double val[MAXVAL]; // Value stack
 void push(double f) {
@@ -82,6 +86,18 @@ double pop(void) {
     }
 }
 
+double peek(void) {
+    if (sp >= 0) {
+        return val[sp-1];
+    } else {
+        printf("error, empty stack\n");
+        return 0.0;
+    }   
+}
+
+void clear(void) {
+    sp = 0;
+}
 
 int getch(void);
 void ungetch(int);
@@ -110,16 +126,20 @@ int getop(char s[]) {
 
 #define BUFSIZE 100
 
-char buf[BUFSIZE]; // buffer for ungetch
-int bufp = 0; // next free in buf
+int buf = -2;
 
 int getch(void) {
-    return (bufp > 0) ? buf[--bufp] : getchar();
+    if (buf != -2) {
+        int z = buf;
+        buf = -2;
+        return z;
+    }
+    return getchar();
 }
 
 void ungetch(int c) {
-    if (bufp >= BUFSIZE) 
+    if (buf != -2) 
         printf("ungetch: buffer full\n");
     else
-        buf[bufp++] = c;
+        buf = c;
 }
\ No newline at end of file
-- 
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