• qwerty <qw er.ty> Mar 18 2010
• bearophile <bearophileHUGS lycos.com> Mar 18 2010
• qwerty <qw er.ty> Mar 18 2010
• qwerty <qw er.ty> Mar 18 2010
• Don <nospam nospam.com> Mar 18 2010
• qwerty <qw er.ty> Mar 18 2010

qwerty <qw er.ty> writes:

If memory serves me right, module is undefined on negative integers.
int i = -2;
i%=10; // i=undefined?
What should I use instead to get i definitely equal to 7? 

On a sidenote, where can I read about operation order/definitions?
Like i++, ++i and i%10 not changing i etc. 

bearophile <bearophileHUGS lycos.com> writes:

qwerty:
 If memory serves me right, module is undefined on negative integers.


I think it's defined in D (but it's defined badly).


 int i = -2;
 i%=10; // i=undefined?
 What should I use instead to get i definitely equal to 7?


D outputs -2, Python outputs 8. It's not easy to find a language that outputs 7
there, maybe Malborge language?


 On a sidenote, where can I read about operation order/definitions?
 Like i++, ++i and i%10 not changing i etc. 


D acts like C, here. Even when C does something badly. So you can surely find
info about ++ and % in C.

Bye,
bearophile

qwerty <qw er.ty> writes:

bearophile Wrote:

 qwerty:
 If memory serves me right, module is undefined on negative integers.


 I think it's defined in D (but it's defined badly).


 
 
 int i = -2;
 i%=10; // i=undefined?
 What should I use instead to get i definitely equal to 7?


 D outputs -2, Python outputs 8. It's not easy to find a language that outputs
7 there, maybe Malborge language?


8 yeah :(somehow I did overcompensated for the max being 9 for %10)

 i      = -3 -2 -1  0  1  2
 i%3 =  7  8  9  0  1  2

So I should be safe with 
i % M;
if(i<0) i = M + i;

 
 On a sidenote, where can I read about operation order/definitions?
 Like i++, ++i and i%10 not changing i etc. 


 D acts like C, here. Even when C does something badly. So you can surely find
info about ++ and % in C.

qwerty <qw er.ty> writes:

qwerty Wrote:

 bearophile Wrote:
 
 qwerty:
 If memory serves me right, module is undefined on negative integers.


 I think it's defined in D (but it's defined badly).




http://www.digitalmars.com/d/1.0/expression.html#MulExpression

Don <nospam nospam.com> writes:

qwerty wrote:
 If memory serves me right, module is undefined on negative integers.
 int i = -2;
 i%=10; // i=undefined?


It's not undefined.  x%y always has the sign of x, so i will be -2.

This always holds:
x == y * (x/y) + (x%y);
And since D uses truncated division, the result follows.
This behaviour is inherited from C99.

 What should I use instead to get i definitely equal to 7? 


You mean 8?

i %= 10;
if (i<0) i += 10;

 
 On a sidenote, where can I read about operation order/definitions?
 Like i++, ++i and i%10 not changing i etc. 

qwerty <qw er.ty> writes:

Don Wrote:

 qwerty wrote:
 If memory serves me right, module is undefined on negative integers.
 int i = -2;
 i%=10; // i=undefined?


 It's not undefined.  x%y always has the sign of x, so i will be -2.
 
 This always holds:
 x == y * (x/y) + (x%y);
 And since D uses truncated division, the result follows.
 This behaviour is inherited from C99.


Yay,found the spec (mul expressions ?:):
"For integral operands of the / and % operators, the quotient rounds towards
zero and the remainder has the same sign as the dividend. If the divisor is
zero, an Exception is thrown. "
Personally I find your explanation a lot more clear :)

 
 What should I use instead to get i definitely equal to 7? 


 You mean 8?



 
 i %= 10;
 if (i<0) i += 10;
 
 
 On a sidenote, where can I read about operation order/definitions?
 Like i++, ++i and i%10 not changing i etc.