Basic math - the concept of log

In machine learning we will use mathematical formulas to give precise definitions for some methods.

The following is a reminder of the concept of "log".

"Log" is the exponent you put on - It's that simple.

Let's look at some examples:
(log base  2)

log of a fraction is negative

For example, in a decision tree we use entropy calculation:
$entropy = \sum _i-p_{i\times Lo{g}_{}}\left(p_i\right)$

pi is the proportion of the data labeled for each class.
pi is a fraction.entropy is the value between 0 and 1.
0 means no entropy and all data is the same.