c++ - Why use hex?

Question

Hey! I was looking at this code at http://www.gnu.org/software/m68hc11/examples/primes_8c-source.html

I noticed that in some situations they used hex numbers, like in line 134:

for (j = 1; val && j <= 0x80; j <<= 1, q++)

Now why would they use the 0x80? I am not that good with hex but I found an online hex to decimal and it gave me 128 for 0x80.

Also before line 134, on line 114 they have this:

small_n = (n & 0xffff0000) == 0;

The hex to decimal gave me 4294901760 for that hex number. So here in this line they are making a bit AND and comparing the result to 0??

Why not just use the number? Can anyone please explain and please do give examples of other situations.

Also I have seen large lines of code where it's just hex numbers and never really understood why :(

Answer 1

In both cases you cite, the bit pattern of the number is important, not the actual number.

For example, In the first case, j is going to be 1, then 2, 4, 8, 16, 32, 64 and finally 128 as the loop progresses.

In binary, that is,

0000:0001, 0000:0010, 0000:0100, 0000:1000, 0001:0000, 0010:0000, 0100:0000 and 1000:0000.

There's no option for binary constants in C (until C23) or C++ (until C++14), but it's a bit clearer in Hex: 0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 0x40, and 0x80.

In the second example, the goal was to remove the lower two bytes of the value. So given a value of 1,234,567,890 we want to end up with 1,234,567,168.
In hex, it's clearer: start with 0x4996:02d2, end with 0x4996:0000.