Where is the data?

howdy again,

I am plotting the fourier amplitude spectrum for the mauna loa data yet when I run the code, the plot is empty. is it an axes problem? and if so, how do I fix it so that the x axis is still in 10^nth hertz.

% Load the data
filename = 'maunaloa_weekly.csv';
data = readtable(filename, 'ReadVariableNames', false, 'Delimiter', ',');
variableNames = {'Date', 'Values'};

% Assign variable names to the table
data.Properties.VariableNames(1:2) = variableNames;

% Convert the dates to datetime format
dates = datetime(data.Date, 'Format', 'dd/MM/yyyy');
Values = data.Values;

% Create a time vector using datetime
time_vector = dates;

% Plot the data
figure;
subplot(2, 1, 1);
plot(dates, Values, 'LineWidth', 2);
title('Maunaloa Weekly Data');
xlabel('Time');
ylabel('Floating Point Values');

% Fourier analysis plot
subplot(2, 1, 2);

% Time interval between data points
dt = days(7); % Weekly data

% Perform FFT
N = length(Values);
Fs = 1/days(dt); % Sampling frequency
frequencies = Fs*(0:(N/2))/N;
fft_values = fft(Values);
amplitudes = 2/N * abs(fft_values(1:N/2+1));

% Plot the amplitude spectrum with log scale on x-axis
semilogx(frequencies, amplitudes, 'LineWidth', 2);
title('Fourier Amplitude Spectrum');
xlabel('Frequency (Hz)');
ylabel('Amplitude');

[the cyclist]

Can you upload the maunaloa_weekly.csv file? You can use the paper clip icon in the INSERT section of the toolbar.

[Kenzie]

just did!

Answer 1

[Star Strider]

Use fillmissing to interpolate the NaN elements in ‘Values’ —

% Load the data
filename = 'maunaloa_weekly.csv';
data = readtable(filename, 'ReadVariableNames', false, 'Delimiter', ',');
variableNames = {'Date', 'Values'};

% Assign variable names to the table
data.Properties.VariableNames(1:2) = variableNames;

% Convert the dates to datetime format
dates = datetime(data.Date, 'Format', 'dd/MM/yyyy');
Values = data.Values;

% Create a time vector using datetime
time_vector = dates;

% Plot the data
figure;
subplot(2, 1, 1);
plot(dates, Values, 'LineWidth', 2);
title('Maunaloa Weekly Data');
xlabel('Time');
ylabel('Floating Point Values');

% Fourier analysis plot
subplot(2, 1, 2);

% Time interval between data points
dt = days(7); % Weekly data

% Perform FFT
fprintf('There are %d NaN elements in ''Values''.\n',nnz(isnan(Values)))

There are 44 NaN elements in 'Values'.

Values = fillmissing(Values,'linear');
N = length(Values);
Fs = 1/days(dt); % Sampling frequency
frequencies = Fs*(0:(N/2))/N;
fft_values = fft(Values);
amplitudes = 2/N * abs(fft_values(1:N/2+1));

% Plot the amplitude spectrum with log scale on x-axis
semilogx(frequencies, amplitudes, 'LineWidth', 2);
title('Fourier Amplitude Spectrum');
xlabel('Frequency (Hz)');
ylabel('Amplitude');

Make appropriate changes to get the result you want.

.