The entire code below for reference:
Whenever I try out a code sample from a book like this, I always make it a point to break it. The "MAXOP" defined in the book was 100. So for grins I changed it to 5 and tried using operands of more than 5 digits. Notice the other line I highlighted in red where I print out the array for inspection. This array is supposedly 5 char max, and yet I am able to get it to work with like ~17-digit operands and even print all of them back as part of the string/array. After 17/18 digits, it stops printing... but still, shouldn't it stop printing (and give me an error before that) at 5?
The second issue, is that the author puts a BUFSIZE of 100 on his ungetch() buffer... but as far as I can tell there is no way to get more than 1 character full. No matter what you do, it will only fill one character up, and that character will always empty before another one has a chance to go in. Am I missing something, or is the BUFSIZE of 100 totally unnecessary? Is it even possible to see the "ungetch: too many characters" error message?
Code:#include <stdio.h> #include <stdlib.h> /* for atof() */ #define MAXOP 5 /* max size of operand or operator */ #define NUMBER '0' /* signal that a number was found */ int getop(char []); void push(double); double pop(void); /* reverse Polish calculator */ main() { int type; double op2; char s[MAXOP]; while ((type = getop(s)) != EOF) { switch (type) { case NUMBER: printf("s = %s\n", s); 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 divisor\n"); break; case '\n': printf("\t%.8g\n", pop()); break; default: printf("error: unknown command %s\n", s); break; } } return 0; } #define MAXVAL 100 /* maximum depth of val stack */ int sp = 0; /* next free stack position */ double val[MAXVAL]; /* value stack */ /* push: push f onto value stack */ void push(double f) { if (sp < MAXVAL) val[sp++] = f; else printf("error: stack full, can't push %g\n", f); } /* pop: pop and return top value from stack */ double pop(void) { if (sp > 0) return val[--sp]; else { printf("error: stack empty\n"); return 0.0; } } #include <ctype.h> int getch(void); void ungetch(int); /* getop: get next character or numeric operand */ int getop(char s[]) { int i, c; while ((s[0] = c = getch()) == ' ' || c == '\t') ; s[1] = '\0'; if (!isdigit(c) && c != '.') return c; /* not a number */ i = 0; if (isdigit(c)) /* collect integer part */ while (isdigit(s[++i] = c = getch())) ; if (c == '.') /* collect fraction part */ 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 position in buf */ int getch(void) /* get a (possibly pushed-back) character */ { return (bufp > 0) ? buf[--bufp] : getchar(); } void ungetch(int c) /* push character back on input */ { if (bufp >= BUFSIZE) printf("ungetch: too many characters\n"); else buf[bufp++] = c; }
Originally Posted by The111
