Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c > #163727

on ``warning: array subscript has type ‘char’''

Show all headers | View raw

I can't explain this.

%make warning
gcc -Wall -x c -g -std=c99 -pedantic-errors    warning.c   -o warning
In file included from warning.c:2:
warning.c: In function ‘getop’:
warning.c:38:27: warning: array subscript has type ‘char’ [-Wchar-subscripts]
   38 |     while (isdigit(s[++i] = c = getch()))
      |                    ~~~~~~~^~~~~~~~~~~~~

I understand the warning is trying to tell me that a char is not a good
idea as an array subscript as a char could be negative.  However, the
array subscript is /i/, not /c/.  So I'm puzzled.  Thank you!

This is essentially the reverse-polish calculator by Brian Kernighan in
chapter 4 of K&R.  The relevant code is the following.  (See full source
of the program warning.c below.)

int getch(void) { /* get a (possibly pushed-back) character */
  if (p > 0) 
    return buf[--p];
  else 
    return getchar();
}

int getop(char s[]) {
  int i, c;

  while ((s[0] = c = getch()) == ' ' || c == '\t')
    ; /* skip white space */

  s[1] = '\0'; 

  if (!isdigit(c)) {
    return c; /* not a digit, not an integer */
  }

  i = 0;
  if (isdigit(c)) /* read the integer into s until its end */
    while (isdigit(s[++i] = c = getch()))
      ;
  s[i] = '\0'; /* close the string */

  if (c != EOF)
    ungetch(c); 

  return NUMBER;
}

--8<---------------cut here---------------start------------->8---
--8<---------------cut here---------------end--------------->8---

(*) Full source code

#include <stdio.h>
#include <ctype.h>

#define NUMBER '\0'

#define BUFSIZE 100
char buf[BUFSIZE]; /* a buffer for ungetch() */
int p = 0; /* the next free position in buf */

int getch(void) { /* get a (possibly pushed-back) character */
  if (p > 0) 
    return buf[--p];
  else 
    return getchar();
}

void ungetch(int c) {
  if (p >= BUFSIZE)
    printf("ungetch: too many characters; buffer full\n");
  else
    buf[p++] = c;
}

int getop(char s[]) {
  int i, c;

  while ((s[0] = c = getch()) == ' ' || c == '\t')
    ; /* skip white space */

  s[1] = '\0'; 

  if (!isdigit(c)) {
    return c; /* not a digit, not an integer */
  }

  i = 0;
  if (isdigit(c)) /* read the integer into s until its end */
    while (isdigit(s[++i] = c = getch()))
      ;
  s[i] = '\0'; /* close the string */

  if (c != EOF)
    ungetch(c); 

  return NUMBER;
}

int main(void) {
  char buf[100]; int i;
  ungetch('1');
  i = getop(buf); printf("i = %d, buf = %s\n", i, buf);

  return 0;
}

Back to comp.lang.c | Previous | Next — Next in thread | Find similar | Unroll thread

Thread

on ``warning: array subscript has type ‘char’'' Meredith Montgomery <mmontgomery@levado.to> - 2021-12-06 17:40 -0300
  Re: on ``warning: array subscript has type ‘char’'' Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-12-06 13:12 -0800
    Re: on ``warning: array subscript has type ‘char’'' Meredith Montgomery <mmontgomery@levado.to> - 2021-12-07 15:11 -0300

csiph-web