"Error using == Matrix dimensions must agree" Finding max point on a plot?

My code is:

%Plotting the frequency spectrum.
%Frequency domain.
f = -Fs/2:Fs/(n-1):Fs/2;
z = fftshift(fft(A3));
%Magnitude graph.
a = abs(z);
figure(2)
plot(f,a,'b')

%Finding the maximum frequency.
indexmax = find(max(a) == a);
amax = a(indexmax); %Doesn't work.
fmax = f(indexmax);

And it's saying the problem is at "indexmax = find(max(a) == a);"

The dimensions of a are 440574x2 and the dimensions of max(a) are 1x2.

I'm relatively new to Matlab so I'm guessing it's something obvious but I've tried everything I can think of and I can't figure out what's going on.

Thanks :)

[Adam]

What are you trying to find with your max? Do you want to find the max, ignoring the structure of your code containing 2 columns or do you want the max per column?

In the first case use

max( a(:) )

In the second case the 1x2 result array that you get will contain the max from each column which will likely be on different rows to each other so this cannot then be compared against the original 'a' matrix.

Answer 1

[KSSV]

Use simply max() to find the position of maximum of array.

Eg:

k = rand(100,1) ;
[value,index] = max(k) ;

Answer 2

[Azzi Abdelmalek]

Maybe you want

indexmax = find(a==max(a))

Which can be done otherwise

[~,indexmax]=max(a)