How do I calculate a logarithm of a variable base?

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?

Answer 1

[John D'Errico]

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

Answer 2

[DGM]

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

Answer 3

[Star Strider]

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.

.

[Tzahi Shukrun]

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!