Using Log in Matlab

Something strange seems to be happening in Matlab and i dont understand why

if im calculating a matrix called M, using actual numbers lets call them A,B and reading from another matrix say M2;

M = 5*log10(A) - B + M2;

i should if i set A to be 1000 get 3 minus two numbers right? but i dont, however i do get the expected answer if i hard code the number A in at 1000 just not with the variable, what is the reason for this? im guessing it involves the matrix M2 or something?

Answer 1

[Thomas]

I get the correct result both times

A=1000;
B=3;
M2=[1 2;3 4]

M=5*log10(A)-B+M2
M =
         13.00         14.00
         15.00         16.00

Now by hard coding the values

M=5*log10(1000)-3+[1 2;3 4]
M =
         13.00         14.00
         15.00         16.00

Answer 2

[Wayne King]

Can you be more specific with a simple example, you won't get 3, you'll get 15 since you are doing

5*log10(A)

So if A = 1000, the 5*log10(A) is 15. You get 15 - the sum of the two matrices B and M2

   B = ones(5,5);
   M2 = 2*ones(5,5);
   A = 1000;
   M = 5*log10(A) - B + M2;

So for the above you get 15- a matrix of ones, that gives you a matrix of 14's. And then you add a matrix of 2's so you get a matrix of 16's.

Not that is different than

M = 5*log10(A) - (B + M2);

In the above you first add the matrix of ones to the matrix of 2's and then subtract that matrix of 3's from 15 to get a matrix of 12's.