Increase when smaller than, decrease when lower than

Question

Out of curiosity, is there a (language independent*) way to make these two generic statements into one statement?

if (a < b) a += x;
else if (a > b) a -= x;

May also be used as

if (abs(b - a) < x) a = b;
else if (a < b) a += x;
else if (a > b) a -= x;

Oh, now that I'm writing this I'm thinking of something like this:

if (a != b) a += x * (a < b ? 1 : -1);

But that doesn't really benefit the readability. Other ways perhaps?

• = Normal languages, not Brainfuck and the likes or really ancient ones

Answer 1

a += (b - a) / abs(b - a) * x

But this is not language independent and does not really help readiness if you ask me....

Answer 2

Language independent is a bit tricky, but if you have cmp or similar, you can use that:

a += x * cmp(b, a)

cmp(b, a) returns:

• 0 if b==a
• 1 if b > a
• -1 if a < b

Also, there is a bug in your suggested implementation:

a = 9;
b = 10;
x = 2;

if (a < b) a += x;
if (a > b) a -= x;

print a;

Output: 9 (expected 11)

You need an else to fix this. Alternatively, if you want to prevent a going past b, do this:

if (a < b)
{
   a = min(b, a + x);
}
else if (a > b)
{
   a = max(b, a - x);
}

If you want to do this in one expression you can:

a = (a < b) ? min(b, a + x) : max(b, a - x);

I think the first way is clearer though.

Answer 3

You should really wonder why you want this, the compiler will do fine optimizing and the given statement is very readable.

Answer 4

a += x * sgn(b-a)

Where sgn is the sign function:

sgn x | x > 0      = 1
      | x < 0      = -1
      | otherwise  = 0