diff --git a/k&r/04-funcs-and-prog-struct/04-calc.c b/k&r/04-funcs-and-prog-struct/04-calc.c
index c290c25..839595d 100644
--- a/k&r/04-funcs-and-prog-struct/04-calc.c
+++ b/k&r/04-funcs-and-prog-struct/04-calc.c
@@ -37,15 +37,19 @@ double val[MAXVAL];
 /* length: current length of stack */
 #define length() sp
 /* push: push f onto value stack */
-#define push(x) ((sp < MAXVAL) ? val[sp++] = (x) : printerr(FULL_STACK_ERR, ((double) (x))))
+#define push(x) \
+    ((sp < MAXVAL) ? val[sp++] = x : printerr(FULL_STACK_ERR,  (double) (x)))
 /* pop: pop and return top value from stack */
-#define pop() (sp ? val[--sp] : (printerr(EMPTY_STACK_ERR), 0))
+double pop(void)
+{
+    return sp ? val[--sp] : (printerr(EMPTY_STACK_ERR), 0);
+}
 /* peek: return top value from stack, leave stack unchanged */
 #define peek() (sp ? val[sp-1] : (printerr(EMPTY_STACK_ERR), 0))
 /* clear: clear stack */
 #define clear() (sp = 0)
 /* dup: duplicate top element */
-#define dup() push(peek())
+#define dup() do { double tmp = peek(); push(tmp); } while (0) 
 /* swap: swap top two elements */
 #define swap() \
    do { if (sp < 2) printerr(SHORT_STACK_ERR); else { \
@@ -203,7 +207,7 @@ int main()
     int t_len;
     
     printf(INITIAL_PROMPT_F);
-    while (t_len = gettoken(token, MAXTOKEN)) {
+    while ((t_len = gettoken(token, MAXTOKEN))) {
         if (t_len < 0) { /* newline token */
             if (length())
                 print_pop();