Tim - 3 months ago 38

C++ Question

Using integer math alone, I'd like to "safely" average two unsigned ints in C++.

What I mean by "safely" is avoiding overflows (and anything else that can be thought of).

**For instance, averaging 200 and 5000 is easy:**

unsigned int a = 200;

unsigned int b = 5000;

unsigned int average = (a + b) / 2; // Equals: 2600 as intended

unsigned int a = 4294967295;

unsigned int b = 5000;

unsigned int average = (a + b) / 2; // Equals: 2499 instead of 2147486147

unsigned int a = 4294967295;

unsigned int b = 5000;

unsigned int average = (a / 2) + (b / 2); // Equals: 2147486147 as expected

Are there better ways?

Answer

Your last approach seems promising. You can improve on that by manually considering the lowest bits of a and b:

```
unsigned int average = (a / 2) + (b / 2) + (a & b & 1);
```

This gives the correct results in case both a and b are odd.