# I Keep Getting Error "Matrix dimensions must agree"

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Alexandra Keith on 11 Dec 2019
Commented: Adam on 11 Dec 2019
I keep getting error like
Error using .*
Matrix dimensions must agree.
How Can I fix it ? Please help me. Any help given will be much appreciated. Here is the line of code

#### 1 Comment

You can fix it by ensuring the two sides of the multiplication are the same size. It's really that simple! We have no idea what size af is from the code you showed. You can see trivially in your workspace though so if you give all the information it is a lot easier for people to help.

Nicolas B. on 11 Dec 2019
If you are using *, than it is a matrix multiplication and the size of your matrices must be compatible.
If you want to multiply element per element, uses .* .
Examples:
a = [1 2 3];
b = [1 2 3; ...
4 5 6; ...
7 8 9];
c = [9 8 7; ...
6 5 4; ...
3 2 1];
a * b % result -> [30 36 42]
a * c % result -> [30 24 18]
a .* b % result -> [1 4 9
% 4 10 18
% 7 16 26] each value of a multiply all values of the same column in b
b .* c % result -> [ 9 16 21
% 24 25 24
% 21 16 9] multiply element per element

Alexandra Keith on 11 Dec 2019
But I have using .* and still error.
>> lp=butterlp(a,15,1);
>> af=fftshift(fft2(a));
>> aflp=af.*lp;
Nicolas B. on 11 Dec 2019
what are the size of lp and af?