Reading character by character from a file in C - c

Question

I wrote the following program :
int main(){
char str[500],c;
FILE *f1=fopen("input.txt","r");
FILE *f2=fopen("output.txt","w");
while(c=fgetc(f1)!=EOF)
fputc(toupper(c),f2);
fclose(f1);
}
I was not getting the desired result though.
I rewrote the code using a do while loop.
int main(){
char str[500];
FILE *f1=fopen("input.txt","r");
FILE *f2=fopen("output.txt","w");
char c;
do
{
fputc(toupper(c),f2);
c=fgetc(f1);
}while(c!=EOF);
}
I figured out that the reason the first code fails is because in the while loop
while(c=fgetc(f1)!=EOF), we cannot guarantee that the left part of != is evaluated first and hence the results are not proper. Is this correct the explanation?

Answer 1

Yes you are correct; in your first code your while loop is written wrongly:
while(c=fgetc(f1)!=EOF)
Should be:
while((c=fgetc(f1))!=EOF)
// ^ ^ added parenthesis
Because the precedence of operator != is greater than = operator in conditional expression c=fgetc(f1)!=EOF, the first returned the result of comparing the value from fgetc() with EOF (either 0 or 1) and assigned that to c. (That means simply c=fgetc(f1)!=EOF expression is equivalent to c=(fgetc(f1)!=EOF) and this is not what you need.)
You need () to overwrite precedence as I suggested.
But you have second thing to improve that is c variable must be an int (not char) in order to hold an EOF-value.
A very good Read: Definition of EOF and how to use it effectively

Answer 2

I will add a little why c should be int, not char. Suppose you write
#include <stdio.h>
#include <ctype.h>
#include <locale.h>
int main(){
setlocale(LC_ALL,"");
FILE *f1=fopen("input.txt","r");
FILE *f2=fopen("output.txt","w");
char c;
while(EOF != (c=fgetc(f1))){
if(isalpha(c)) c = toupper(c);
fputc(c,f2);
}
return 0;
}
And your input.txt is
some text
Некоторый текст
Ъ - on this letter program will stop
in KOI8-R symbol Ъ have code 255 == -1 (when you use char).
That's why in case of using char instead of int will give your output.txt with only that text:
SOME TEXT
НЕКОТОРЫЙ ТЕКСТ
As for non-working code with parentheses: c=fgetc(f1)!=EOF could be denote by compiler as c = (fgetc(f1)!=EOF), that's why it's better always to add parentheses.
I recommend you to use flags -Wall -Werror when compiling your applications. In that case "forgetting" of parentheses would give you an error:
11.c: In function 'main':
11.c:9:2: error: suggest parentheses around assignment used as truth value [-Werror=parentheses]
cc1: all warnings being treated as errors