When you work with unsigned types, modular arithmetic (also known as “wrap around” behavior) is taking place. To understand this modular arithmetic, just have a look at these clocks:
9 + 4 = 1 (13 mod 12), so to the other direction it is: 1 – 4 = 9 (-3 mod 12). The same principle is applied while working with unsigned types. If the result type is unsigned
, then modular arithmetic takes place.
Now look at the following operations storing the result as an unsigned int
:
unsigned int five = 5, seven = 7;
unsigned int a = five - seven; // a = (-2 % 2^32) = 4294967294
int one = 1, six = 6;
unsigned int b = one - six; // b = (-5 % 2^32) = 4294967291
When you want to make sure that the result is signed
, then stored it into signed
variable or cast it to signed
. When you want to get the difference between numbers and make sure that the modular arithmetic will not be applied, then you should consider using abs()
function defined in stdlib.h
:
int c = five - seven; // c = -2
int d = abs(five - seven); // d = 2
Be very careful, especially while writing conditions, because:
if (abs(five - seven) < seven) // = if (2 < 7)
// ...
if (five - seven < -1) // = if (-2 < -1)
// ...
if (one - six < 1) // = if (-5 < 1)
// ...
if ((int)(five - seven) < 1) // = if (-2 < 1)
// ...
but
if (five - seven < 1) // = if ((unsigned int)-2 < 1) = if (4294967294 < 1)
// ...
if (one - six < five) // = if ((unsigned int)-5 < 5) = if (4294967291 < 5)
// ...