getchar example from K&R C does not work properly

Question

I was wondering what's happening in this code from K&R C:

#include <stdio.h>

int main() {
    double nc;

    for (nc = 0; getchar(); ++nc) {
        ;
    }
    printf("%.0f\n", nc);
}

When I run the code (OS X El Capitan), the for loop doesn't finish and I can't get to the printf statement.

check out [`getchar`](http://en.cppreference.com/w/c/io/getchar) — Sander De Dycker, Jun 29 '15 at 14:44
To clarify, your program is waiting for input. If it is running with its stdin connected to a terminal, that means it is waiting for you to type something and close the stream (on *nix, with ctrl-d). Try redirecting input from a file. `./a.out < file.txt` — William Pursell, Jun 29 '15 at 14:46
`enter` does not close the stream, it just writes a newline. — William Pursell, Jun 29 '15 at 14:46
@WilliamPursell : that won't work - `getchar` will return `EOF` when reaching the end of the stream, which is negative and won't end the loop. You need to have a `'\0'` character on the stream to end the loop. — Sander De Dycker, Jun 29 '15 at 14:51
Is that the only solution? What happened on the bigger programs of the industry that use C? — egarro, Jun 29 '15 at 14:51
That code is not from K&R - the original example uses `long nc` and tests for `EOF`. — Paul R, Jun 29 '15 at 14:52
@egarro: you can check all the open-source projects. note: reading a *single* character is quite rare in practice. — Karoly Horvath, Jun 29 '15 at 14:54
possible duplicate of [Problem with example 1.5.2 in K&R book on C](http://stackoverflow.com/questions/3646255/problem-with-example-1-5-2-in-kr-book-on-c) — David, Jun 29 '15 at 14:55
@Paul R This code is from: C The Programming Language second edition — egarro, Jun 29 '15 at 14:58
@egarro: you combine a posted answer with the comments above, and you have a solution... — Karoly Horvath, Jun 29 '15 at 14:59

score 4 · Accepted Answer · edited May 23 '17 at 12:30

4

The getchar() returns the obtained character on success or EOF on failure. Reference here.

Try this

for (nc = 0; getchar() != EOF; ++nc) { }

Note that, Enter doesn't means EOF, it means newline or '\n'.

Look at the header stdio.h, says,

#define EOF (-1)

Here you can see how to simulate an EOF.

Windows: Ctrl+Z
Unix :   Ctrl+D

So long story short, you can put any condition in the for loop, like

for (nc = 0; getchar() != '\n'; ++nc) { } // break the loop if enter pressed
for (nc = 0; getchar() != 'c'; ++nc) { } // break the loop if c pressed
.
.
.

edited May 23 '17 at 12:30

Community

1
1

answered Jun 29 '15 at 14:46

mazhar islam

5,561
3
20
41

This needs a semicolon or an empty block like `for (nc = 0; getchar() != EOF; ++nc) {}` which is actually better than the semicolon. – Iharob Al Asimi Jun 29 '15 at 14:55
@egarro, I added some more information for you, take a look. – mazhar islam Jun 29 '15 at 15:04
@rakeb.void: Thanks a lot! – egarro Jun 29 '15 at 15:18

Gopi · Answer 2 · 2015-06-29T14:49:51.143

1

getchar() waits on the input from stdin and even if you enter any character it would be read by getchar() but that doesn't make you get out of the loop.

So you need to add some condition on the return value of getchar() to gracefully exit the loop.

For Eg:

for(;getchar() != EOF;)
{
}

If you want to return on newline character then check should be getchar() != '\n'

PS: I have just shown how to check the return value , for me the exit condition is EOF it might vary for you.

edited Jun 29 '15 at 14:49

answered Jun 29 '15 at 14:46

Gopi

19,784
4
24
36

@KarolyHorvath To avoid confusion I got rid of it – Gopi Jun 29 '15 at 14:50
2

egarro If it still "is not working", you are probably ignoring the advice to close the input stream. – William Pursell Jun 29 '15 at 14:57
What's the difference between EOF and "\n"? – egarro Jun 29 '15 at 15:01
@egarro: what's the difference between the end of the file and end of the line? pretty much everything... – Karoly Horvath Jun 29 '15 at 15:01

Vlad from Moscow · Answer 3 · 2015-06-29T15:13:52.380

1

This loop

for (nc = 0; getchar(); ++nc) {
        ;

will iterate until getchar() returns 0. To achieve this without rewritting the program as others are suggesting you should for example in Windows press key combination Ctrl + @ and then press Enter. The value of key combination Ctrl + @ is equal to 0.

edited Jun 29 '15 at 15:13

answered Jun 29 '15 at 15:07

Vlad from Moscow

301,070
26
186
335

or eg. `echo -e "foo\0" | ./a.out` on linux – Sander De Dycker Jun 29 '15 at 15:11

David · Answer 4 · 2015-06-29T16:00:36.360

0

If you revise your code to make it:

#include <stdio.h>

int main() {
    double nc;

    while (getchar() != EOF) {
        ++nc;
    }

    printf("\n%.0f\n", nc);
}

Then if you run it in a terminal window and enter the following:

abcCtrl-D

you will get the output:

abc
3

(The Ctrl-D is the EOF character)

edited Jun 29 '15 at 16:00

answered Jun 29 '15 at 15:08

David

14,047
24
80
101

"The Ctrl-D is the EOF character" .. for Unix. Not for Windows. Why type twice? Why did you change the type of nc? – too honest for this site Jun 29 '15 at 15:22
Olaf - the questioner is using OS X, I assume in a terminal window, but I admit I might be wrong about that. – David Jun 29 '15 at 15:53
It is just if Windows-unsers read that; this might trigger another unecessary complaint. However, I removed the downvote, as my major concern was the changed type - I completely understand, actually; had to check myself first if that is actually allowed for float types. – too honest for this site Jun 29 '15 at 16:41

getchar example from K&R C does not work properly

4 Answers4