Why can't use "getc(f)) != EOF" to compare directly?

Question

I'm new to C language, and now get stuck with this kind of question: why do I get a weird result if I use above expression to print string in file?

Here is the situation: I have a file(data.txt) with the following content:

"Hello Everyone!!"

And here is my code:

int main()
{
   FILE *ptr = fopen("data.txt", "r");

   if (ptr != NULL)
   {
      while (getc(ptr) != EOF)    //print all contents in data.txt
         printf("%c", getc(ptr));
   }
   else
      printf("Open file failed.");

   return 0;
}

The execution result is:

"el vroe!"

If I assign getc(ptr) to variable first and do comparing, everything goes fine.

What's the difference between this two methods?

Hint: "el vroe!" is every second character from "Hello Everyone!!". — Quentin, Mar 13 '20 at 14:07
Think about what `getc()` does: It reads (or "consumes") a character from the input. If you call it twice, as you do in your loop, it reads two chacacters. — M Oehm, Mar 13 '20 at 14:07
A really klunky way you could do it without storing to a variable: `while (ungetc(getc(ptr),ptr) != EOF)` — R.. GitHub STOP HELPING ICE, Mar 13 '20 at 14:09
If `getc()` was called `get_next_character()` (which is what it does), would it be more obvious? That is, every time you call it, the "next" character changes. — Arkku, Mar 13 '20 at 14:09

score 2 · Accepted Answer · edited Mar 13 '20 at 16:02

2

You extract first char in condition of while and then extract second char in printf. So you print only each second char in a loop.

If you want, do something like:

int c;

while ((c = getc(ptr)) != EOF) {
printf("%c", c);
}

edited Mar 13 '20 at 16:02

Jonathan Leffler

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answered Mar 13 '20 at 14:08

Dr. Andrey Belkin

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2

`char c;` is wrong and is a classic bug causing the byte 0xff to be wrongly interpreted as EOF. It needs to be `int c;` – R.. GitHub STOP HELPING ICE Mar 13 '20 at 14:11

score 1 · Answer 2 · answered Mar 13 '20 at 14:10

You can of course but you need to save the read char. If you dont it will be lost.

int main()
{
   FILE *ptr = fopen("data.txt", "r");

   if (ptr != NULL)
   {
      int c;
      while ((c = getc(ptr)) != EOF)    //print all contents in data.txt
         printf("%c", c);
   }
   else
      printf("Open file failed.");

   return 0;
}

Why can't use "getc(f)) != EOF" to compare directly?

2 Answers2