C言語で文字列が自然数かどうかを判定する関数

is_natural.c

#include <stdio.h>

const int TRUE = 1;
const int FALSE = 0;

int is_natural(char *s) {
  if (s[0] == '\0' || s[0] == '0') return FALSE;

  for (int pos = 0; TRUE; pos++) {
    char ch = s[pos];
    if ('0' <= ch && ch <= '9') continue;
    if (ch == '\0') break;
    return FALSE;
  }
  return TRUE;
}

void assert_eq(int want, int got) {
  if (want == got) {
    printf("PASS\n");
    return;
  }
  printf("FAIL\n");
}

int main(void) {
  assert_eq(TRUE, is_natural("1"));
  assert_eq(TRUE, is_natural("9"));
  assert_eq(TRUE, is_natural("10"));
  assert_eq(TRUE, is_natural("19"));
  assert_eq(TRUE, is_natural("99"));
  assert_eq(TRUE, is_natural("65536"));
  assert_eq(FALSE, is_natural("0"));
  assert_eq(FALSE, is_natural("01"));
  assert_eq(FALSE, is_natural("-1"));
  assert_eq(FALSE, is_natural(""));
  assert_eq(FALSE, is_natural("a"));
  assert_eq(FALSE, is_natural("123a"));
}

z_result.md

動作確認

gcc is_natural.c
./a.out

実行結果

PASS
PASS
PASS
PASS
PASS
PASS
PASS
PASS
PASS
PASS
PASS
PASS