Using file and loop in c - c

Question

In this program, how i do at the same time convert the uppercase letters to lowercase letters and lowercase letters to uppercase letter?
I have tried many times but it is not working.
My expectations, Suppose for example
Input:
from read.txt(orginal contant of the file:
Hello World)
Output
hELLO wORLD
This is my code....
(I can only convert from uppercase to lowercase. At the same time I could not convert from uppercase to lowercase and lowercase to uppercase).
#include<stdio.h>
#include<stdlib.h>
int main()
{
FILE* file;
char ch;
file = fopen("read.txt","r");
while (ch != EOF)
{
ch = toupper(ch);
printf("%c", ch);
ch = fgetc(file);
}
fclose(file);
return 0;
}

Answer 1

you have plenty errors here.
your first check the not initialized ch variable, you use it and try to print, then you read it. The order has to be right opposite.
ch has to be of type int to accommodate EOF
you need to check if the fopen was successful
int main()
{
FILE* fptr;
int ch;
fptr = fopen("read.txt","r");
if(fptr)
{
while ((ch = fgetc(fptr)) != EOF)
{
ch = toupper(ch);
printf("%c", ch);
}
fclose(fptr);
}
return 0;
}

Answer 2

// Note that UPPER and lower chars differ by bit 5 (value 0x20).
// If you want to switch case for any [A-Za-z] in one step,
// an exclusive-OR (^ bitwise operator) can be used:
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#define TOGGLE_CASE(c) ((c) ^ 0x20)
int
main(void)
{
FILE *file;
char ch;
if ((file = fopen("read.txt", "r")) == NULL) {
dprintf(2, "fopen error\n");
return (1);
}
while ((ch = fgetc(file)) != EOF)
printf("%c", isalpha(ch) ? TOGGLE_CASE(ch) : ch);
fclose(file);
return (0);
}