why getchar() function work different in a loop?

Question

#include <stdio.h>
int main()
{
    char c;

    while ((c=getchar()) != EOF)
        putchar(c);
}

why when i input text such as for example "omar"put char print "omar" back .Isn't it supposed to print 'o' repeatedly since getchar will take first character only.I know that I am wrong about something probably because i don't know how exactly getchar or putchar works ,so can somebody please explain how they works.Another question why getchar and putchar work normally without a while loop but inside while loop behavior is something different.

It takes the first char, and the 'o' is removed. So next time, 'm' is the first char. And so on. — Prof. Falken
getchar and putchar work just the same regardless of the loop, so it is unclear why you would think that they work differently. Perhaps you are confused about what the loop does: it calls both getchar and putchar repeatedly, so the first calls read and print o in your omar example, the second calls print m, etc. (Meanwhile change char c to int c and search for a FAQ to explain why.) — Arkku
when you type o getchar will receive the o it will be checked against EOF and putchar will print the o then the loop will return to getchar. This will then happen for m and a and r — liamcomp

John Bollinger John Bollinger · Accepted Answer · 2019-04-12T13:30:09

why getchar() function work different in a loop?

I take you to be asking why getchar() works differently than you expect, as opposed to differently than in other contexts. If you in fact meant the latter then the answer would be "it doesn't."

But of course reading a character from a stream, whether via getchar() or some other I/O function, removes it from the stream. getchar() would not be very useful if it did not do that. Therefore, if you call it repeatedly, you read (and remove) each character in turn until and unless all available characters are consumed. You can test this by replacing the loop in your program with several getchar() calls in row.

And, of course, your loop does call it repeatedly. The loop-control expression, (c=getchar()) != EOF, is evaluated before each iteration of the loop, and that involves calling getchar() (as opposed to using a value previously returned by that function).

On a completely separate note, do be aware that getchar() returns a result of type int, exactly so that it can return at least one value, EOF, that is outside the range of type unsigned char. If you convert the result to type char then either there is one real input value that you will mistake for EOF, or you will never detect EOF, depending on whether char is signed or unsigned. To reliably and portably detect the end of the file, you must handle that return value as an int, not a char.

why getchar() function work different in a loop?

1 Answers