5 ms·

> most programming languages don’t have an explicit “logical operator” for it I guess the author never taught of <boolean> != <boolean>. The only thing to be c

by ambrop7 10y ago

> most programming languages don’t have an explicit “logical operator” for it I guess the author never taught of <boolean> != <boolean>. The only thing to be careful with is that it doesn't implicitly convert arguments to boolean, so expressions like "<boolean> != (flags & Flag)" will go wrong without an explicit conversion "bool(flags & Flag)" or equivalent expression like "((flags & Flag) != 0)". And let's not forget about the friend, ==. I've more than once seen code like "(a && b) || (!a && !b)". A similar interesting pattern many don't think of is "bool(a1) + ... + bool(aN) == M" (particularly with M==1) and instead we see unreadable monstrosities :)

kevinwang 10y ago
Is "bool(a1) + ... + bool(aN) == M" not just "a1 || a2...||an"?
- ambrop7 10y ago
  Obviously no, the first (for M==1) means exactly one is true, the latter at least one is true. But a1||...||aN is equivalent to bool(a1)+...+bool(aN)>0 (assuming no integer overflow :).
- kmill 10y ago
  No, it's true when exactly M are true. "a1 || a2 || ... || an" is true when at least one is true. The latter is the same as "a1 + ... + an == 1 || a1 + ... + an == 2 || ... || a1 + ... + an == n".
  - Ded7xSEoPKYNsDd 10y ago
    So your + operator implicitly casts booleans to an integer type, with false=0 and true=1?
    - simcop2387 10y ago
      In c++, yes. Not every language will do that though.
    - kmill 10y ago
      What you are saying is the only way I can imagine to interpret what I wrote, if one assumes I was intending to make any sense. By the way, in Python, bool is a subtype of int (i.e., instanceof(True,int) == True), and True + True == 2. C is the same, and in Javascript booleans are converted to integers for arithmetic operations.
      - OJFord 10y ago
        > What you are saying is the only way I can imagine to interpret what I wrote One might imagine it (without experience, or not thinking, of any particular programming language) as being a logical OR on Booleans - which in EE at least is frequently written '+'.
TazeTSchnitzel 10y ago
> The only thing to be careful with is that it doesn't implicitly convert arguments to boolean Well, you could also use bitwise XOR in that case.
Existenceblinks 10y ago
> "(a && b) || (!a && !b)" It is XNOR, not XOR (no pun intended) Boolean algebra laws excellently help if there are many input aN. I used to use it to simplify legacy ruby codes written by c-style programmer (you can imagine how the conditions look like)
im3w1l 10y ago
If you only want to convert the second argument you can use the ==! operator ;)
- wruza 10y ago
  C has so many wonderful operators, why don't people use them? while (x --> 0) x goes to zero; while (0 <---- x) goes much faster on some compilers (ymmv);
andreareina 10y ago
!!a != !!b also works, but that's unwieldy. I think maybe part of the reason is that usually you're also doing something with the value of the lhs or rhs, so you end up having to separate the cases anyway: if (a && !b) { frob(a); } else if (b && !a) { twiddle(b); } else { panic(); }
- caf 10y ago
  You don't need to double-negate, !a != !b is sufficient.

by ambrop7 10y ago

kevinwang 10y ago

Is "bool(a1) + ... + bool(aN) == M" not just "a1 || a2...||an"?

ambrop7 10y ago
Obviously no, the first (for M==1) means exactly one is true, the latter at least one is true. But a1||...||aN is equivalent to bool(a1)+...+bool(aN)>0 (assuming no integer overflow :).
kmill 10y ago
No, it's true when exactly M are true. "a1 || a2 || ... || an" is true when at least one is true. The latter is the same as "a1 + ... + an == 1 || a1 + ... + an == 2 || ... || a1 + ... + an == n".
- Ded7xSEoPKYNsDd 10y ago
  So your + operator implicitly casts booleans to an integer type, with false=0 and true=1?
  - simcop2387 10y ago
    In c++, yes. Not every language will do that though.
  - kmill 10y ago
    What you are saying is the only way I can imagine to interpret what I wrote, if one assumes I was intending to make any sense. By the way, in Python, bool is a subtype of int (i.e., instanceof(True,int) == True), and True + True == 2. C is the same, and in Javascript booleans are converted to integers for arithmetic operations.
    - OJFord 10y ago
      > What you are saying is the only way I can imagine to interpret what I wrote One might imagine it (without experience, or not thinking, of any particular programming language) as being a logical OR on Booleans - which in EE at least is frequently written '+'.

TazeTSchnitzel 10y ago

> The only thing to be careful with is that it doesn't implicitly convert arguments to boolean Well, you could also use bitwise XOR in that case.

Existenceblinks 10y ago

> "(a && b) || (!a && !b)" It is XNOR, not XOR (no pun intended) Boolean algebra laws excellently help if there are many input aN. I used to use it to simplify legacy ruby codes written by c-style programmer (you can imagine how the conditions look like)

im3w1l 10y ago

If you only want to convert the second argument you can use the ==! operator ;)

wruza 10y ago
C has so many wonderful operators, why don't people use them? while (x --> 0) x goes to zero; while (0 <---- x) goes much faster on some compilers (ymmv);

andreareina 10y ago

!!a != !!b also works, but that's unwieldy. I think maybe part of the reason is that usually you're also doing something with the value of the lhs or rhs, so you end up having to separate the cases anyway: if (a && !b) { frob(a); } else if (b && !a) { twiddle(b); } else { panic(); }

caf 10y ago
You don't need to double-negate, !a != !b is sufficient.