Very elementary C++ question

**Dondrei** · 07-20-2008

Okay, this is a bit basic but I'm having a hard time Googling it so what the hey...

Does C++ accept logic tests like:

Code:

if (a <= x <= y) {}

Or do I have to keep writing it out as:

Code:

if (a <= x && x <= y) {}

?

**Salem** · 07-20-2008

Yes it 'accepts' it (it's valid syntax), but it doesn't do what you want.

If the latter is what you want, then that's the way to write it.

**rudyman** · 07-20-2008

It's similar to saying:

Code:

   if (int(a <= b) <= c)

which is probably never what you intend to say.

So, yes, you have to separate the expressions with &&.

**Dondrei** · 07-24-2008

Oh, okay. So what does the <= operator do in that context?

**matsp** · 07-24-2008

Originally Posted by Dondrei

Oh, okay. So what does the <= operator do in that context?

Exactly what you would expect from that expression : compares if the result of (a <= b) [which is either 0 or 1] is greater or equal to c.

The problem is that you didn't want to compare if the boolean value of (a <= b) is greater or equal to c, you wanted to compare if c is greater or equal to b as well as a <= b.

--
Mats

**dwks** · 07-24-2008

If you're doing this sort of thing often, you could make a function out of it.

Code:

bool in_range(bool min, bool num, bool max) {
    return min <= num && num <= max;
}

**cpjust** · 07-24-2008

Originally Posted by Dondrei

Code:

if (a <= x && x <= y) {}

I always put parenthesis around my conditions to make is more clear as to what I intended to compare and also so I don't have to keep looking up the operator precedence chart all the time.

Code:

if ( (a <= x) && (x <= y) ) {}

**dwks** · 07-24-2008

I don't.

Code:

if (a <= x && x <= y) {}

is perfectly readable as far as I am concerned . . . but if you don't think so, then by all means use parentheses.

It's a common enough idiom.

**cpjust** · 07-24-2008

Originally Posted by dwks

I don't.

Code:

if (a <= x && x <= y) {}

is perfectly readable as far as I am concerned . . . but if you don't think so, then by all means use parentheses.

It's a common enough idiom.

Yes, that particular example is fine, but it you start getting into more complex expressions and start using different operators, you might run into problems, especially where precedence is concerned. So I choose to always be explicit with parentheses so I don't have to remember when it's safe not to use them...

I've seen some code that did something like this:

Code:

if ( ptr = func() == NULL )

and from the context it looks like the person who wrote it intended it to mean:

Code:

if ( (ptr = func()) == NULL )

but because of precedence it actually meant:

Code:

if ( ptr = (func() == NULL) )

**dwks** · 07-24-2008

I know what you mean; I always use parentheses when combining || and &&, for example, even though I know which one comes first in the precedence table . . . .

That reminds me. Sorry for the change of topic here, but is there anything wrong in using the assignment operator inside if/while statements/loops?

Code:

if(c = read())
while(c = something)

Without double-parentheses, that is?

Code:

if((c = read()))
while((c = something))

Turbo C (I know) gives me an error, and GCC a warning (I think), but that's probably just because the compilers thought I meant ==, right?

**cpjust** · 07-24-2008

Originally Posted by dwks

I know what you mean; I always use parentheses when combining || and &&, for example, even though I know which one comes first in the precedence table . . . .

That reminds me. Sorry for the change of topic here, but is there anything wrong in using the assignment operator inside if/while statements/loops?

Code:

if(c = read())
while(c = something)

Without double-parentheses, that is?

Code:

if((c = read()))
while((c = something))

Turbo C (I know) gives me an error, and GCC a warning (I think), but that's probably just because the compilers thought I meant ==, right?

I don't think there is anything wrong with it, but I'm pretty sure VC++ gives you a "assignment in conditional" warning because it's so easy to accidentally type = when you really mean ==
For that I usually get rid of the warning by doing:

Code:

if ( (ptr = func()) != NULL )

**dwks** · 07-24-2008

Yeah, that's what I thought. I usually just put double-parentheses.

Code:

if((c = something())

Of course, I try not to do that very often.

Anyway, now that we've successfully derailed the thread . . . if you created your own class, I think you could probably overload the <= operator in such a way that "a <= x <= y" works as expected.

[edit] All I have at the moment is Dinkumware, so this hasn't been tested, but here's sort of how you could do it. I think.

Code:

#include <iostream>

class type {
private:
    int x;

public:
    type(int x) : x(x) {}
    int get() { return x; }
};

type *operator <= (type *left, type &right) {
    return left->get() <= right.get() ? left : 0;
}

type *operator <= (type &left, type &right) {
    return left.get() <= right.get() ? &left : 0;
}

int main() {
    type one(1), two(2), three(3);

    if(one <= two <= three) {
        std::cout << "true" << std::endl;
    }
}

[/edit]

**Dondrei** · 07-26-2008

I think I'll just separate with &&s, probably the simplest way.

Originally Posted by matsp

Exactly what you would expect from that expression : compares if the result of (a <= b) [which is either 0 or 1] is greater or equal to c.

The problem is that you didn't want to compare if the boolean value of (a <= b) is greater or equal to c, you wanted to compare if c is greater or equal to b as well as a <= b.

--
Mats

Ah, that makes sense.

**Sander** · 07-28-2008

Originally Posted by dwks

Anyway, now that we've successfully derailed the thread . . . if you created your own class, I think you could probably overload the <= operator in such a way that "a <= x <= y" works as expected.

You can indeed. See Syntactic Aspartame: Recreational Operator Overloading which was published in the Feb 2006 issue of C/C++ User's Journal (the last issue before it's demise).

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