Why getchar() works int these two cases differently?

Question

I am writing little program for encoding characters and I have a problem with reading NUL characters maybe also with another ones. My problem is that getchar in this code somehow ignores the NUL characters entered by CTRL + @ .

while ((numberOfRead < 40) && (character = getchar()) && (character != EOF) && (character != '\n')) { ... }

But this code works correctly and reads the NUL characters properly.

character = getchar();
while ((numberOfRead < 40) && (character != EOF) && (character != '\n')) {
    numberOfChars++;
    ...
    character = getchar();
}

What is the difference causing the problem? Thanks for any explanation of this behaviour. Code was tested on Windows 8.1, under gcc 4.7.1

This is why you should avoid assignment inside conditions, makes it very easy to write bugs. — Lundin, Mar 07 '16 at 10:55
^^^ what @Lundin says. I looks clever and cool, but doesn't actually work:( Simple, one step at a time code, seemingly avoidable intermediate temp vars etc. result in working. or at least debuggable, code. Complex, compound boolean expressions result in posts to SO:( — Martin James, Mar 07 '16 at 11:02
I like the idea of using this assignment in the conditions due to many examples in the K&R, which I had read recently. :) — c0ntrol, Mar 07 '16 at 18:01

score 1 · Accepted Answer · answered Mar 07 '16 at 10:29

1

Here:

    while (... && (character = getchar()) && ...) ...

you use the value of character as truth value. When the read character is the null character with the value 0, this condition is false.

You can do something like:

    while (numberOfRead < 40
       && (character = getchar()) != EOF && character != '\n') { ... }

but I find your second variant with the getchar outside the condition more readable.

answered Mar 07 '16 at 10:29

M Oehm

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I had absolutely forgotten about the evaluating the assignment. :) – c0ntrol Mar 07 '16 at 17:58
Yes, it's easy to miss in compound conditions. (That's why I prefer your second variant where it's clear where the `getchar` is called.) – M Oehm Mar 07 '16 at 18:18

Why getchar() works int these two cases differently?

1 Answers1