How to calculate SNR using Matlab?

Hi. I want to calculate SNR of the noisy signal and the filtered signal. I used this following code but I am not sure whether the result is correct or not. Can somebody help me?

%generate the noisy signal which will be filtered
x= cos(2*pi*12*[0:0.001:1.23]);
x(end) = [];
[b,a] = butter(2,[0.6 0.7],'bandpass');
filtered_noise = filter(b,a,randn(1, length(x)*2));
y = (x + 0.5*filtered_noise(500:500+length(x)-1))/length(x)*2;

%Use matlabs built-in buttord function to get the optimum order to meet a specification
[N,Wn] = buttord(0.1, 0.5, 5, 40)

%use the N and Wn values obtained above to design the filter in the usual way
[b,a] = butter(N, Wn, 'low');

%filter the signal and plot the ouput of the filter
Y_filtered = filter(b,a,y);

%calculation of snr before filtering
snr_before = mean(x.^2) / mean(filtered_noise.^2);
snr_before_db = 10 * log10(snr_before) % in dB

%calculation of snr after filtering
residual_noise = x - Y_filtered; 
snr_after = mean(x.^2) / mean(residual_noise.^2); 
snr_after_db = 10 * log10(snr_after)

Result that I got is:

snr_before_db =

      6.8725

snr_after_db =

      0.0132

[PRANAV BIBRA]

good