[RESOLVED] math equations

Mgccl

In php, I found that math equations could be a difficult thing because you just can't write it out, like

$e1 = 1/(1+10^($p2-$p1)/400)

so, are there any script convert equations into PHP codes so it is a lot easier?
and if not... can anyone give me a lesson on how to write math equations in PHP.... like how to use a variable at a place, and all....

Installer

The way I read it, your formula would go like this:

$e1 = 1 / (1 + (pow(10, $p2 - $p1) / 400));
// Or maybe you mean:
$e1 = 1 / ((1 + pow(10, $p2 - $p1)) / 400);

You do it in PHP pretty much the same way as on paper. Operator precedence will determine where you need parentheses, and you may add more to make reading easier (as in the second divisor term above).

laserlight

hmm... your equation could mean many things, but based on the 400 and other contextual clues, am I right to say that you are looking to use the win expectancy formula in an Elo rating system?

If so, the PHP code would be:

$e1 = 1 / (1 + pow(10, ($p2 - $p1) / 400));

Installer

$e1 = 1 / (1 + pow(10, -($p2 - $p1) / 400));

Elo football rating

laserlight

hmm... why would one do that? It seems like just writing:

$e1 = 1 / (1 + pow(10, ($p1 - $p2) / 400));

would be simpler. Not only that, but if one wanted to find the win expectancy of the opponent, it would be easier and more efficient to just write:

$e2 = 1 - $e1;

Installer

I was just showing a minus sign that you missed, that's all.

$e1 = 1 / (1 + pow(10, ($p2 - $p1) / 400));

may seem to be just like writing

$e1 = 1 / (1 + pow(10, -($p2 - $p1) / 400));

but it's not. The two give different results. The second "seems" to be the correct one, but if you say it isn't I won't disagree.

As for the rest: I'm sure the original poster will be glad to hear it.

laserlight

I was just showing a minus sign that you missed, that's all.

No, I didnt miss that out.

The two give different results. The second "seems" to be the correct one, but if you say it isn't I won't disagree.

Of course, they gave different results (except where $p1 = $p2). You will find that for the same input values one the result of one added to the result of the other will give 1. This because one gives the win expectancy of one player, the other gives the win expectancy of the other player. In that sense, both are correct, so we should choose the simpler one.

Installer

Sure, laserlight. They're both the same.