This is a really simple question, but I'm afraid it's a bit tricky.
Which of the following symbols has the highest precedence? (C++)
a. %
b. -
c. ||
d. &&
I know it sounds easy, but I'm really wondering if whether the answer is % or -. The symbol - can be seen as either a unary minus (which is higher than %) or subtraction (which is lower than %).
What do you think?
A google search for c++ Operator Precedence yielded this:
http://en.cppreference.com/w/cpp/language/operator_precedence
According to the link, Multiplication, division, and remainder come before Addition and subtraction and further down the list comes the Bitwise operations found in row 5,6,13, and 14 respectively.
I would assume the person who made this question wanted you to choose the operator that has the highest possible precedence.
I would choose b.
Let's have two lines of code:
&car->speed
&(car->speed)
Are these two lines equivalent? Will I get in both cases address to the speed?
If they are equivalents, what is better to choose as coding convention?
Are these two lines equivalent? Will I get in both cases address to the speed?
Yes. -> has higher precedence than that of unary &, therefore &car->speed and &(car->speed) are equivalent.
If they are equivalents, what is better to choose as coding convention?
Go with second as it shows the intended behaviour that you are interested in the address of speed.
This question already asked here several times. Postfix expression operators have higher priority than unary operators. So these two expressions
&car->speed
&(car->speed)
are equivalent.
Or another similar example with other unary operator !
!car->speed
!(car->speed)
As for coding convention I would prefer
&car->speed
and if you want to ampersandify car alone, use this:
( &car )->speed
I see a lot of programmers using brackets around an expression, e.g. :
&(tab[i]) /* I use `&tab[i]`. */
I think it isn't necessary, because the [] operator has a greater priority than & operator. So, why do they use brackets ?
For sake of clarity. Not everyone has all the operator precedences memorized.
I'd say there are basically two reasons:
The programmer isn't sure about operator precedence and codes defensively
To make absolutely clear for every reader (who may not have the precendence memorized) what is going on
It is indeed not necessary because postfix operators always have higher precedence than unary operators.
The people that use parens in &(tab[i]) use them for the same reason they use it in the expression (8 * 5) / 2.
In a recent question, someone had to decypher code like --p---> x < 0. Just like your code, that code is unambiguous as per C++ parsing rules.
However, humans don't always remember all of these complex rules, so many programmers make it a habit to use parens in situations that might look not totally clear to others (or to themselves). It is documenting the real intention of the code.
This is a Good Thing To Do™
Because they find it clearer? Especially for something like this:
int *a[];
Is that a pointer to an array of ints or an array of pointers to int?
This came up in a conversation with a friend of my nephew who is home from uni. Personally beside the most frequently used ones I never found it all that important and just use parenthesis. But I don't have to take his exam. Anything clever out there?
Edit Well, a rare moment of near unanimity on SO! If only professors listened.
I know this probably doesn't help you in the context of your exam, but I'll answer the question for anyone else who might stumble upon this:
Don't try to memorize operator precedence, beyond the "common cases" e.g. arithmetic. If your statement is unclear, either split it into multiple statements or toss in parenthesis.
Given that there are 16 levels of precedence, I don't think there's an easy trick to memorizing them (no "Roy G Biv" or other mnemonic that I know of).
IMO, the important ones to remember are postfix > unary (i.e., *p++ == *(p++)), unary > arithmetic (~a+b == (~a)+b), bitwise > logical (a|b&&c == (a|b)&&c)), and that conditional, assignment, and comma operators make up the bottom 3 in that order (a=b,c == (a=b),c).
This is why reference manuals were invented. That doesn't help during an exam, of course.
What I do to remember operator precedence (other than arithmetic, which works the same as arithmetic on paper) is that the unary operator always has precedence. Beyond that, I look it up and use parenthesis, as Steven suggests.
Or are we all sticking to our taught "&&, ||, !" way?
Any thoughts in why we should use one or the other?
I'm just wondering because several answers state thate code should be as natural as possible, but I haven't seen a lot of code with "and, or, not" while this is more natural.
I like the idea of the not operator because it is more visible than the ! operator. For example:
if (!foo.bar()) { ... }
if (not foo.bar()) { ... }
I suggest that the second one is more visible and readable. I don't think the same argument necessarily applies to the and and or forms, though.
"What's in a name? That which we call &&, || or !
By any other name would smell as sweet."
In other words, natural depends on what you are used to.
Those were not supported in the old days. And even now you need to give a special switch to some compilers to enable these keywords. That's probably because old code base may have had some functions || variables named "and" "or" "not".
One problem with using them (for me anyway) is that in MSVC you have to include iso646.h or use the (mostly unusable) /Za switch.
The main problem I have with them is the Catch-22 that they're not commonly used, so they require my brain to actively process the meaning, where the old-fashioned operators are more or less ingrained (kind of like the difference between reading a learned language vs. your native language).
Though I'm sure I'd overcome that issue if their use became more universal. If that happened, then I'd have the problem that some boolean operators have keywords while others don't, so if alternate keywords were used, you might see expressions like:
if ((x not_eq y) and (y == z) or (z <= something)) {...}
when it seems to me they should have alternate tokens for all the (at least comparison) operators:
if ((x not_eq y) and (y eq z) or (z lt_eq something)) {...}
This is because the reason the alternate keywords (and digraphs and trigraphs) were provided was not to make the expressions more readable - it was because historically there have been (and maybe still are) keyboards and/or codepages in some localities that do not have certain punctuation characters. For example, the invariant part of the ISO 646 codepage (surprise) is missing the '|', '^' and '~' characters among others.
Although I've been programming C++ from quite some time, I did not know that the keywords "and" "or" and "not" were allowed, and I've never seen it used.
I searched through my C++ book, and I found a small section mentioning alternative representation for the normal operators "&&", "||" and "!", where it explains those are available for people with non-standard keyboards that do not have the "&!|" symbols.
A bit like trigraphs in C.
Basically, I would be confused by their use, and I think I would not be the only one.
Using a representation which is non-standard, should really have a good reason to be used.
And if used, it should be used consistently in the code, and described in the coding standard.
The digraph and trigraph operators were actually designed more for systems that didn't carry the standard ASCII character set - such as IBM mainframes (which use EBCDIC). In the olden days of mechanical printers, there was this thing called a "48-character print chain" which, as its name implied, only carried 48 characters. A-Z (uppercase), 0-9 and a handful of symbols. Since one of the missing symbols was an underscore (which rendered as a space), this could make working with languages like C and PL/1 a real fun activity (is this 2 words or one word with an underscore???).
Conventional C/C++ is coded with the symbols and not the digraphs. Although I have been known to #define "NOT", since it makes the meaning of a boolean expression more obvious, and it's visually harder to miss than a skinny little "!".
I wish I could use || and && in normal speech. People try very hard to misunderstand when I say "and" or "or"...
I personally like operators to look like operators. It's all maths, and unless you start using "add" and "subtract" operators too it starts to look a little inconsistent.
I think some languages suit the word-style and some suit the symbols if only because it's what people are used to and it works. If it ain't broke, don't fix it.
There is also the question of precedence, which seems to be one of the reasons for introducing the new operators, but who can be bothered to learn more rules than they need to?
In cases where I program with names directly mapped to the real world, I tend to use 'and' and 'or', for example:
if(isMale or isBoy and age < 40){}
It's nice to use 'em in Eclipse+gcc, as they are highlighted. But then, the code doesn't compile with some compilers :-(
Using these operators is harmful. Notice that and and or are logical operators whereas the similar-looking xor is a bitwise operator. Thus, arguments to and and or are normalized to 0 and 1, whereas those to xor aren't.
Imagine something like
char *p, *q; // Set somehow
if(p and q) { ... } // Both non-NULL
if(p or q) { ... } // At least one non-NULL
if(p xor q) { ... } // Exactly one non-NULL
Bzzzt, you have a bug. In the last case you're testing whether at least one of the bits in the pointers is different, which probably isn't what you thought you were doing because then you would have written p != q.
This example is not hypothetical. I was working together with a student one time and he was fond of these literate operators. His code failed every now and then for reasons that he couldn't explain. When he asked me, I could zero in on the problem because I knew that C++ doesn't have a logical xor operator, and that line struck me as very odd.
BTW the way to write a logical xor in C++ is
!a != !b
I like the idea, but don't use them. I'm so used to the old way that it provides no advantage to me doing it either way. Same holds true for the rest of our group, however, I do have concerns that we might want to switch to help avoid future programmers from stumbling over the old symbols.
So to summarize: it's not used a lot because of following combination
old code where it was not used
habit (more standard)
taste (more math-like)
Thanks for your thoughts