Writing a incrementing/decrementing function with roll-over

[pozz]

Hi all,

I need to write a simple incrementing/decrementing function like this:

unsigned char
change( unsigned char x, unsigned char min, unsigned char max, signed char d);

x is the value to increase/decrease
min is the minimum value that x can assume
max is the maximum value that x can assume
d is positive or negative and indicates how much x must be incremented or
decremented
Of course, the return value is the new value for x.

Some examples to clarify the question:
x=10,min=0,max= 20,d=5 --15
x=10,min=0,max= 20,d=-5 --5
x=18,min=0,max= 20,d=5 --2
x=18,min=5,max= 20,d=5 --7
x= 3,min=0,max=20, d=-5 --19
x=10,min=8,max= 20,d=-5 --18

I'd like a function that use only unsigned char...

Another similar question. I have a variabile x (unsigned char) and I must
increment it. The increment is stored in another variable, d (unsigned char).
x can't assume values greater than max, stored in another variable (unsigned
char).
I usually write:

if( x+d>max )
x = max;
else
x += d;

I think it's not correct because what happens when x=200, d=100 and max=220??
If I write:

if( x>max-d )
x = max;
else
x += d;

what happens when max=10 and d=20?? I need a variable greater than unsigned
char to do the temporary sum x+d? And if I use long, I need extra-long
variables?
Thank you very much for the help.

[pozz]

sp****@gmail.co m ha scritto:

pozz wrote:

goose ha scritto:

pozz wrote:
Another similar question. I have a variabile x (unsigned char) and I must
increment it. The increment is stored in another variable, d (unsigned char).
x can't assume values greater than max, stored in another variable (unsigned
char).
I usually write:

>
You can normally do this:
x = (x + d) % (max+1);

This is my attempt for the change function with roll-over:

unsigned char
change_roll( unsigned char x, unsigned char min, unsigned char max,
signed char d )
{
if( d>0 ) {
x -= min;
max -= min;
return (x+d)%(max+1) + min;
} else {
/* ??? */
}
}

What do I write in the else branch when d is negative?

It's not correct I'm afraid. Consider the case where
min=0 , max=199 , UCHAR_MAX=255 , x=199 , d=57

Then your function ought to return 56 but it will return 0.

Yes, you are right. I must check if (x+d) generates an overflows
(x+d<x).

if( d>=0 ) {
x -= min;
max -= min;
if( x+d<x )
return d - (max-x) + min;
else
return (x+d)%(max+1) + min;
} else
????

But what should I write when d is negative?