找到位置的因素

Question

I know question title seems quite 'un-understandable', but I don't know how to write question title for this particular question.

Question: I want to find factor for position.

Let me clear you with an example.

Value          Factor
[Available]    [Have to find out] 
----------------------------------
1               10
3               10
9               10
10              10
11              10
25              10
50              10
75              10
99              10

100             100
101             100
105             100
111             100
127             100
389             100
692             100
905             100
999             100

1000            1000
1099            1000
1111            1000
4500            1000
6825            1000
7789            1000
9999            1000

10000           10000
10099           10000
51234           10000
98524           10000
99999           10000

100000          100000

and so on.

I hope you understand what I mean to get.

Answer 1

Assuming that the first three values should be 1 (as noted by Asaph), then you just need to use all that logarithm stuff you learned in school:

pow(10, floor(log10($n)))

So, how does this work? The base-10 logarithm of a number x is the y such that 10^y = x (where ^ stands for exponentiation). This gives us the following:

log(  1) 0
log( 10) 1
log(100) 2
...

So the log10 of a number between 1 and 10 will be between 0 and 1, the log10 of a number between 10 and 100 will be between 1 and 2, etc. The floor function will give you the integer part of the logarithm (we're only dealing with non-negative values here so there's no need to worry about which direction floor goes with negative values) so floor(log10()) will be 0 for for anything between 1 and 10, 1 for anything between 10 and 100, etc. Now we have how many factors of ten we need so a simple pow(10, ...) gives us the final result.

References:

• log10
• floor
• pow