How do I calculate a logarithm of a variable base?

186 次查看（过去 30 天）
Tzahi Shukrun
Tzahi Shukrun 2021-10-2
评论： Tzahi Shukrun 2021-10-2
Hi!
I tried to calculate a log of a varying base - no success. I saw that calculating log of the base of 10 or e is possible.
for an example: after typing log (a,b) - I recieve an ERROR message.
I'd like to use "a" as a varying number and change it using a loop until I'll get a good result. "a" can be for an example - 1.05, 1.003....
Here it is:
>> a=1.005;
>> log(a,2)
Error using log
Too many input arguments.
Can you please guide me how doing it?

请先登录，再回答此问题。

回答（3 个）

John D'Errico
John D'Errico 2021-10-2
编辑：John D'Errico 2021-10-2
Ran in:
Just write your own function. Save it on your search path. Now you have it.
[log(10),logb(10)] % natural log
ans = 1×2
2.3026 2.3026
[log10(5),logb(5,10)] % log(10) to base 5
ans = 1×2
0.6990 0.6990
[log2(3),logb(3,2)] % log(2) to base 3
ans = 1×2
1.5850 1.5850
logb(16,[2 3 4 5 16]) % log(16) to the bases [2,3,4,5,16]
ans = 1×5
4.0000 2.5237 2.0000 1.7227 1.0000
Simple enough.
function L = logb(X,Base)
% log(X) to the base Base
% If Base is not supplied, then the natural log is presumed.
if (nargin < 2) || isempty(Base)
% default case is the natural log
L = log(X);
else
% test for an invalid base
if any(Base<=0) || any(Base == 1)
error('Base must be > 0, and ~= 1')
end
L = log(X)./log(Base);
end
end
DGM
DGM 2021-10-2
Nowhere in the synopsis for log() does it say that it accepts two arguments.
For base 2, you can just use log2()
More generally, just remember the properties of logarithms and use either log(), log10(), or log2() to compute the arbitrary base-n logarithm.
https://www.mathworks.com/matlabcentral/answers/44770-logarithm-of-base-other-than-e-10-and-2
Star Strider
Star Strider 2021-10-2
Ran in:
The general approach is:
So:
a = 64
a = 64
log(a)/log(2)
ans = 6
2^(log(a)/log(2))
ans = 64
See the Wikipedia article on List of logarithmic identities for an extended discussion.
.
  1 个评论
Tzahi Shukrun
Tzahi Shukrun 2021-10-2
First, thank you all for your answers. I wrote it and calculated it that way. Now Im sure that matlab dont have a direct command for this action
I did it using excel and wondered about this possibility using matlab.
Again, thank you very much!

请先登录，再进行评论。

类别

在 Help Center 和 File Exchange 中查找有关 Axis Labels 的更多信息

标签

产品


版本

R2018a

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by