Are both these two cleaning buffer methods equivalent?

Question

I was wondering:

do they do exactly the same thing? calling c = getchar on expression is the same as doing it with a do...while loop?

void clrbuf(void)
{
    int c;
    while ((c = getchar()) != '\n' && c != EOF);
}

void clrbuf(void)
{
    int c;
    do c = getchar(); while (c != '\n' && c != EOF);
}

edit: c was once typed char, but folks told me int was the appropriate type for it

Both better as `int c;` to distinguish and cope with the typical 257 different responses from `getchar()`. — chux - Reinstate Monica, Jun 10 '21 at 14:28
C++: `stream_name.ignore(std::numeric_limits::max(), '\n');` — NathanOliver, Jun 10 '21 at 14:32
@Yksisarvinen C. I didn't know it could be different in C++. I'll delete the tag. thanks — Gustavo Duarte, Jun 10 '21 at 14:35
would simply `int c` instead of `char c` make it better, then? — Gustavo Duarte, Jun 10 '21 at 14:38
Also: `while (1) { int c = getchar(); if (c == '\n' || c == EOF) return; } } ` — ikegami, Jun 10 '21 at 15:04

Vlad from Moscow · Answer 1 · 2021-06-10T22:24:17.050

For starters the variable c should be declared like

int c;

because if the type char behaves as the type unsigned char then this condition

c != EOF

will be always true.

According to the C Standard (7.21 Input/output <stdio.h>)

EOF

which expands to an integer constant expression, with type int and a negative value, that is returned by several functions to indicate end-of-file, that is, no more input from a stream;

So if the type char behaves as the type unsigned char (this depends on compiler options) then the value stored in the variable c after the integer promotion to the type int will be still a non-negative value.

The first while loop

while ((c = getchar()) != '\n' && c != EOF);

may be rewritten using the comma operator like

while ( c = getchar(), c != '\n' && c != EOF );

that is in fact it consists of two parts: the assignment expression c = getchar() and the condition c != '\n' && c != EOF.

As you can see it is equivalent to the do-while statement

do c = getchar(); while (c != '\n' && c != EOF);

However the first while loop

while ((c = getchar()) != '\n' && c != EOF);

is more expressive and clear.

The first loop is more idiomatic C (and that helps make it more expressive and clear). — Jonathan Leffler, Jun 10 '21 at 17:28

Are both these two cleaning buffer methods equivalent?

I was wondering:

1 Answers1