C语言小题-字母大小写转换，保存到文件中

1 题目要求

2 代码

/*
* Author: @utah
* Date:   2023-12-7
* Subject: NUll
*/

#define _CRT_SECURE_NO_WARNINGS 1

#include<stdio.h>
#include<string.h>

int main()
{
	FILE* fp;
	char str[100];
	char filename[20];
	int i = 0;
	gets(filename);
	if (fp = fopen(filename,"w") == null) {
		printf("无法打开这个文件\n");
		exit(0);
	}

	printf("输入准备转换的字符\n");
	gets(str);

	while (str[i]) {
		if (str[i] >= 'a' && str[i] <= 'z') {
			str[i] = str[i] + 32; //小写转大写
		}
		fputc(str[i], fp);
		i++;
	}

	fclose(fp);

	fp = fopen(filename,"r");
	fgets(str, 100, fp);
	printf("%s\n", str);
	fclose(fp);

	return 0;
}