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Dennis Ritchie on the priorities of && || vs. == etc. (1982)

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Re: Dennis Ritchie on the priorities of && || vs. == etc. (1982)

#21

Post by pcwalton »

tom_ wrote: post #17
o11c wrote: post #8

Earlier quoted context omitted.

Convert them to arithmetic. If you ignore the casting, a && b is just a * b a || b is just a + b Now you remember the precedence between them (except in broken languages, of which the only notable one is shell).

1+1=1? My maths education was a long time ago, but I triple checked with my calculator, and I'm uncertain this is quite right. My own mnemonic: SAXO. Shift, And, Xor, Or. (Like a real Saxo, it's fun to go a bit faster like this, but you do have to trust everybody involved, because any accident is probably going to end up badly for you.) And now you know the bitwise precedence as well! And this ordering actually works…

> (Main annoying thing: a lot of compilers, even popular ones such as clang and gcc, don't actually seem to know what the precedence rules actually are, and generate warnings asking you to clarify. Presumably the authors didn't realise that C has an ISO standard, that can be consulted to answer this question? Very surprising.)

The compilers do know what the precedence rules are, but they know that programmers don't routinely consult the ISO standard, so they emit those warnings to reduce the chance of error. Compilers are tools that are designed to help programmers avoid bugs. If they don't help programmers, they aren't doing their job.

Re: Dennis Ritchie on the priorities of && || vs. == etc. (1982)

#22

Post by [deleted] »

o11c wrote: post #8
Sohcahtoa82 wrote: post #2

I don't even know the priority between && and ||. If I'm using both, I use parentheses just so it's more explicit. "a && b || c" should flag a linter, IMO, with "(a && b) || c" or "a && (b || c)" being required.

Convert them to arithmetic. If you ignore the casting, a && b is just a * b a || b is just a + b Now you remember the precedence between them (except in broken languages, of which the only notable one is shell).

[deleted]

Re: Dennis Ritchie on the priorities of && || vs. == etc. (1982)

#23

Post by threatofrain »

o11c wrote: post #8
Sohcahtoa82 wrote: post #2

I don't even know the priority between && and ||. If I'm using both, I use parentheses just so it's more explicit. "a && b || c" should flag a linter, IMO, with "(a && b) || c" or "a && (b || c)" being required.

Convert them to arithmetic. If you ignore the casting, a && b is just a * b a || b is just a + b Now you remember the precedence between them (except in broken languages, of which the only notable one is shell).

Isn't it x ^ y which is like x + y?

Re: Dennis Ritchie on the priorities of && || vs. == etc. (1982)

#24

Post by jovial_cavalier »

tom_ wrote: post #17
o11c wrote: post #8

Earlier quoted context omitted.

Convert them to arithmetic. If you ignore the casting, a && b is just a * b a || b is just a + b Now you remember the precedence between them (except in broken languages, of which the only notable one is shell).

1+1=1? My maths education was a long time ago, but I triple checked with my calculator, and I'm uncertain this is quite right. My own mnemonic: SAXO. Shift, And, Xor, Or. (Like a real Saxo, it's fun to go a bit faster like this, but you do have to trust everybody involved, because any accident is probably going to end up badly for you.) And now you know the bitwise precedence as well! And this ordering actually works…

The analogy between logical AND and multiplication, and logical OR and addition is called Boolean Algebra, and it's very well known.

The analogy is that when you set 1 to true and 0 to false, 1 becomes the identity for AND, and 0 becomes the identity for OR. Just as 1 is the identity for multiplication and 0 is the identity for addition.

X * 0 = 0 | X ^ F = F

X * 1 = X | X ^ T = X

X + 0 = X | X V F = X

X + 1 = ? | X V T = T With this, you can turn any logical expression into something that looks and feels like normal algebra, with the only weird exception being that both operators distribute over each other:

A * (B + C) = A*B + A*C

A + (B * C) = (A + B) * (A + C)

Re: Dennis Ritchie on the priorities of && || vs. == etc. (1982)

#25

Post by LordShredda »

tom_ wrote: post #17
o11c wrote: post #8

Earlier quoted context omitted.

Convert them to arithmetic. If you ignore the casting, a && b is just a * b a || b is just a + b Now you remember the precedence between them (except in broken languages, of which the only notable one is shell).

1+1=1? My maths education was a long time ago, but I triple checked with my calculator, and I'm uncertain this is quite right. My own mnemonic: SAXO. Shift, And, Xor, Or. (Like a real Saxo, it's fun to go a bit faster like this, but you do have to trust everybody involved, because any accident is probably going to end up badly for you.) And now you know the bitwise precedence as well! And this ordering actually works…

1+1 isn't zero, so it's one.

Re: Dennis Ritchie on the priorities of && || vs. == etc. (1982)

#26

Post by [deleted] »

LordShredda wrote: post #25
tom_ wrote: post #17

Earlier quoted context omitted.

1+1=1? My maths education was a long time ago, but I triple checked with my calculator, and I'm uncertain this is quite right. My own mnemonic: SAXO. Shift, And, Xor, Or. (Like a real Saxo, it's fun to go a bit faster like this, but you do have to trust everybody involved, because any accident is probably going to end up badly for you.) And now you know the bitwise precedence as well! And this ordering actually works…

1+1 isn't zero, so it's one.

[deleted]

Re: Dennis Ritchie on the priorities of && || vs. == etc. (1982)

#27

Post by nicklecompte »

dananabread wrote: post #10
o11c wrote: post #8

Earlier quoted context omitted.

Convert them to arithmetic. If you ignore the casting, a && b is just a * b a || b is just a + b Now you remember the precedence between them (except in broken languages, of which the only notable one is shell).

Is there an intuition for this correspondence otherwise I don't think it's very helpful

IMO the intuition is to not use any intuition at all: there aren't built-in booleans in C, true is a #define for 1 and false is a #define for 0. For C conditionals, 0 = false, nonzero = true. So

a+b != 0 a!=0 or b!=0

a*b != 0 a!=0 and b!=0

Of course this intuition also reveals the pitfall behind this correspondence! You'd better make sure those are unsigned ints or #defined booleans, so you're not using general C expressions. 1 || -1 is true but 1 + (-1) is false.

Edit: forgot to mention: INT_MAX || 1 is true, but what about INT_MAX + 1 :)

Re: Dennis Ritchie on the priorities of && || vs. == etc. (1982)

#28

Post by ant6n »

JonChesterfield wrote: post #3
Sohcahtoa82 wrote: post #2

I don't even know the priority between && and ||. If I'm using both, I use parentheses just so it's more explicit. "a && b || c" should flag a linter, IMO, with "(a && b) || c" or "a && (b || c)" being required.

Consider (&& a b) and (|| a b) for no uncertainty over operator precedence and easy variadic representation (&& a b c) at the cost of zero additional parentheses.

Prefix operators considered harmful for readability.

Re: Dennis Ritchie on the priorities of && || vs. == etc. (1982)

#30

Post by pcwalton »

threatofrain wrote: post #23
o11c wrote: post #8

Earlier quoted context omitted.

Convert them to arithmetic. If you ignore the casting, a && b is just a * b a || b is just a + b Now you remember the precedence between them (except in broken languages, of which the only notable one is shell).

Isn't it x ^ y which is like x + y?

Yeah, | is technically x + y + x*y over GF(2) (this is the Boolean ring to Boolean algebra relation [1]). The GP is still a good way to remember the precedence though.

[1]: https://en.wikipedia.org/wiki/Boolean_ring#Relation_to_Boole...

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