为什么 程序无法显示

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

int main(void)
{
    FILE *fp;
    char string[81];
    int bigLetter = 0, smallLetter = 0, space = 0, number = 0, elseChar = 0, i;
    fp = fopen("essay.txt", "r");
    if (fp == NULL) exit(0);
    while (fgets(string,81,fp) != NULL)
    {
        for (i = 0; i < 81; i++)
        {
            if (string[i] >= 'A' && string[i] <= 'Z')
                bigLetter++;
            else if (string[i] >= 'a' && string[i] <= 'Z')
                smallLetter++;
            else if (string[i] == ' ')
                space++;
            else if (string[i] >= '0' && string[i] <= '9')
                number++;
            else
                elseChar++;
        }
    }
    printf("大写字母%d个，小写字母%d个，空格%d个，数字%d个，其他%d个\n", bigLetter, smallLetter, space, number, elseChar);
    fclose(fp);

    system("pause");
    return 0; 
}

错的地方有：
1：用exit(0);需包含

#include "stdlib.h" 

2:对小写字母统计条件语句里，后面大写Z应改为小写z

else if (string[i] >= 'a' && string[i] <= 'Z')
//to
else if (string[i] >= 'a' && string[i] <= 'z')

3:由于是大小写及数字空格统计
请确定存在这个essay.txt文件
不然将顺着exit(0);退出