hopefully a simple oversight

**dopejack** · 10-16-2006

Why isn't this working?

All I want to do is take a character from one text file and copy it in a new text file...

Code:

int main(){

#include <stdio.h>

FILE *tf = fopen("./map", "r");
FILE *af = fopen("./array", "w");
unsigned char x;

while  ( ( x = fgetc( tf ) ) != EOF )
   {
          fprintf( af, "%c", x);
    }


return 0;
}

But I get the following warning on the while loop, and of course, an endless loop:

reader.c: In function `main':
reader.c:9: warning: comparison is always true due to limited range of data type

**Prelude** · 10-16-2006

EOF is a negative value and fgetc returns int.

**nadroj** · 10-16-2006

im not familiar with C (or overly familiar with C++ for that matter), but this is how i would change the code: (note i havent tested it but by reading it it seems it would work better)

Code:

#include <stdio.h> //these should definetly be before main not in a function!

int main(){

FILE *tf = fopen("./map", "r");
FILE *af = fopen("./array", "w");
unsigned char x;

x = fgetc( tf );
while  ( x != EOF )

return 0;
}

**fnoyan** · 10-16-2006

This code works...

Code:

#include <stdio.h>

int main(){

FILE *tf = fopen("./map.txt", "r");
FILE *af = fopen("./array.txt", "w");
char x;

while  ( ( x = fgetc( tf ) ) != EOF )
        fputc(x,af);

fclose(af);
fclose(tf);

return 0;
}

You said you want to read a single character. If so, why do you use a loop? Why unsigned char? And don't forget to close the file pointers!

And you cannot store the value you read if you don't write it into the file!

**dopejack** · 10-16-2006

Originally Posted by Prelude

EOF is a negative value and fgetc returns int.

Exactly right. Changing x to int solved the problem. Interestingly enough, this example was taken from the file i/o tutorial on cprogramming.com....

**Prelude** · 10-16-2006

>Interestingly enough, this example was taken from the file i/o tutorial on cprogramming.com....
Barring the forums and a lot of the FAQ, cprogramming.com is a below average resource for programming in my opinion.

**nadroj** · 10-16-2006

how come the example code here uses a char and compares that to EOF? a char is simply a int anyways so why shouldnt the above work? i cant try it as i dont have a c compiler.

edit: nevermind, i guess the solution isnt _exactly_ that it was a char not int, but that it was unsigned not signed. it can still be a char

**fnoyan** · 10-16-2006

The EOF is defined in my stdio.h like below (mingw32)

Code:

/* Returned by various functions on end of file condition or error. */
#define	EOF	(-1)

it is simply equal to signed int. But the code above includes something like "unsigned char". See the working example I sent.

**Prelude** · 10-16-2006

>See the working example I sent.
It only seems to work. char can be either signed or unsigned.

**fnoyan** · 10-16-2006

Originally Posted by Prelude

It only seems to work. char can be either signed or unsigned.

Actually, I don't know how they are stored in memory. But, this is the point that makes the compiler to produce "comparison is always true due to limited range of data type" error.

**jafet** · 10-18-2006

The Standard states that char may be implemented in terms of signed or unsigned char. And even then a library would be completely free to define EOF as ((int)-0xdeadbeef), which would be way outside the range of chars on most platforms.

The compiler nicely warns the programmer on such usage of an unsigned variable, but I doubt it would warn the user who's using a regular implicitly signed char and implicitly casting the return value from fgetc() (gcc doesn't seem to even with -Wall).

By the way, dopejack, you might want to check if the files were opened correctly before you try to use their handles. [ if(fileptr == NULL || ferror(fileptr)) printf("An error occured\n"); ]

**dwks** · 10-18-2006

http://faq.cprogramming.com/cgi-bin/...&id=1043284351

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