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Bruce Salzman
Guest
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 Posted: Thu May 10, 2007 2:45 am Post subject: Weird + - behavior |
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Why doesn't this generate a syntax error in BDS2006?
#include <iostream>
#pragma argsused
int main(int argc, char* argv[])
{
int i = 1 + - - + - 1;
std::cout << i;
return 1;
}
(It prints 0, BTW).
--
Bruce |
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Alan Bellingham
Guest
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| Posted: Thu May 10, 2007 4:01 am Post subject: Re: Weird + - behavior |
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" Bruce Salzman" <bruce (AT) nospam (DOT) org> wrote:
| Quote: | Why doesn't this generate a syntax error in BDS2006?
#include <iostream
#pragma argsused
int main(int argc, char* argv[])
{
int i = 1 + - - + - 1;
std::cout << i;
return 1;
}
(It prints 0, BTW).
|
I guess you got the parser confused, and it keeps applying unary
operators to the following expression. i.e.
int i = 1 + (- (- (+ (- (1)))));
int i = 1 + (- (- (+ (-1))));
int i = 1 + (- (- (- 1)));
int i = 1 + (- (1));
int i = 1 + -1;
int i = 1 - 1;
int i = 0;
(which you reported.)
Alan Bellingham
--
Team Thai Kingdom
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Alex Bakaev [TeamB]
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| Posted: Thu May 10, 2007 4:57 am Post subject: Re: Weird + - behavior |
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Alan Bellingham wrote:
| Quote: | I guess you got the parser confused, and it keeps applying unary
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I don't think it's confused. At least it's as confused as Comeau online  |
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Eliot Frank
Guest
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| Posted: Thu May 10, 2007 6:08 pm Post subject: Re: Weird + - behavior |
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Bruce Salzman wrote:
| Quote: | Why doesn't this generate a syntax error in BDS2006?
int i = 1 + - - + - 1;
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In C++, plus ('+') is a unary operator just like minus ('-'). So you
have an expression of the form,
expr1 '+' expr2
where expr1 is 1 and expr2 is - - + - 1. Then expr2 is parsed as
unaryop- expr3
where unaryop- is - and expr3 is - + - 1. Etc etc etc.
-Eliot |
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Chris Uzdavinis (TeamB)
Guest
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| Posted: Thu May 10, 2007 6:21 pm Post subject: Re: Weird + - behavior |
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Alan Bellingham <alanb (AT) episys (DOT) com> writes:
| Quote: | " Bruce Salzman" <bruce (AT) nospam (DOT) org> wrote:
Why doesn't this generate a syntax error in BDS2006?
#include <iostream
#pragma argsused
int main(int argc, char* argv[])
{
int i = 1 + - - + - 1;
std::cout << i;
return 1;
}
(It prints 0, BTW).
I guess you got the parser confused, and it keeps applying unary
operators to the following expression. i.e.
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Do you mean by this that you think it is wrong? If so, which part of
the expression?
I think this is a valid program:
int main() {
- - - - - - 1;
}
Why shouldn't it be valid? Unary negation has right-to-left
associtivity.
--
Chris (TeamB); |
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Chris Uzdavinis (TeamB)
Guest
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| Posted: Thu May 10, 2007 6:25 pm Post subject: Re: Weird + - behavior |
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Eliot Frank <eliot_h_frank (AT) yahoosky (DOT) comski> writes:
| Quote: | Bruce Salzman wrote:
Why doesn't this generate a syntax error in BDS2006?
int i = 1 + - - + - 1;
In C++, plus ('+') is a unary operator just like minus ('-').
|
They are both unary and binary, depending on usage.
| Quote: | So you have an expression of the form,
expr1 '+' expr2
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Here + is binary.
| Quote: |
where expr1 is 1 and expr2 is - - + - 1. Then expr2 is parsed as
unaryop- expr3
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Here + is unary.
| Quote: | where unaryop- is - and expr3 is - + - 1. Etc etc etc.
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Exactly. This is simple right-to-left associtivity of the unary
operator+ and opeartor-.
Chris (TeamB); |
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Bruce Salzman
Guest
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| Posted: Thu May 10, 2007 7:53 pm Post subject: Re: Weird + - behavior |
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"Chris Uzdavinis (TeamB)" <chris (AT) uzdavinis (DOT) com> wrote in message
news:86r6postdk.fsf (AT) explicit (DOT) atdesk.com...
| Quote: | Exactly. This is simple right-to-left associtivity of the unary
operator+ and opeartor-.
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Yikes! I didn't realize that you could string unary operators together
that way. I stumbled over this by way of a typo:
+ +i
instead of
++i
An optional warning would be nice :^)
--
Bruce |
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Chris Uzdavinis (TeamB)
Guest
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| Posted: Thu May 10, 2007 7:57 pm Post subject: Re: Weird + - behavior |
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" Bruce Salzman" <bruce (AT) nospam (DOT) org> writes:
| Quote: | "Chris Uzdavinis (TeamB)" <chris (AT) uzdavinis (DOT) com> wrote in message
news:86r6postdk.fsf (AT) explicit (DOT) atdesk.com...
Exactly. This is simple right-to-left associtivity of the unary
operator+ and opeartor-.
Yikes! I didn't realize that you could string unary operators together
that way. I stumbled over this by way of a typo:
+ +i
instead of
++i
An optional warning would be nice :^)
|
Probably the biggest surprise is when we accidently insert a space;
for (int i = 10; i > 0; - -i)
{
std::cout << i << std::endl;
}
But mathematically it makes sense to be able to negate something, even
if that something is itself negating something-else, and so on.
Perhaps a warning would be useful, provided there was a syntax that
could indicate you meant to do this. Perhaps using parenthesis would
silence the warning:
x = -(-i); // ok, no warning
x = - -i ; // warning
But it still seems an argument in favor of fully parenthesizing every
expression, because someone may get confused. Basic math is simple,
but throw in the comma, some bit-shifts, a ternary ?: operator, some
address-of and dereferencing, array indexing, and suddenly it's not
clear at all.
This is one reason I like the syntax of scheme. There simply are no
precedence charts to remember, and there are never ambiguities like
this. The "problem" of lots of parenthesis was solved 40 years ago
with features in the editor: pretty-printing, paren balancing,
colorization, and "jump to corresponding delimiter" options. :)
--
Chris (TeamB); |
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