如果输入一个a，那么iCheck不是等于0吗，这样子的话输出就是错的了，为什么？

Question

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

int main()
{
char cLetter;
int iInRange, iCheck, i, iLetter;
i = 1;
while (i)
{
printf("输入一个字母, 判断元音与辅音. 如果想退出,请输入“0”。\n");
scanf("%c", &cLetter);
getchar();
printf("----------------------------------------------------------------------\n");
iLetter = (int)cLetter;
iCheck = isalpha(iLetter);
if (iCheck)
{
iInRange = (cLetter == 'a') || (cLetter == 'e') || (cLetter == 'i') || (cLetter == 'o') || (cLetter == 'u') || (cLetter == 'A') || (cLetter == 'E') || (cLetter == 'I') || (cLetter == 'O') || (cLetter == 'U');
if (iInRange)
{
printf("字母 %c 是元音字母!\n", cLetter);
printf("----------------------------------------------------------------------\n");
continue;
}
else
{
printf("字母 %c 是辅音字母!\n", cLetter);
printf("----------------------------------------------------------------------\n");
continue;
}
}
else if (iCheck == 0)
{
if (iLetter != 48)
{
printf("Error input!\n");
printf("----------------------------------------------------------------------\n");
continue;
}
else if ((int)cLetter == 48)
{
printf("Bye bye~~~!\n");
printf("----------------------------------------------------------------------\n");
i = (int)cLetter - 48;
}
}
}
return 0;
}

Answer 1

iCheck = isalpha(iLetter);
a是英文字母，返回的结果是1，为什么会等于0？