K-Means on temporal dataset

I have a temporal dataset(1000000x70) consisting of info about the activities of 20 subjects. I need to apply subsampling to the dataset as it has more than a million rows. How to select a set of observations of each subject ideally from it? Later, I need to apply PCA and K-means on it. Kindly help me with the steps to be followed. I'm working in MATLAB.

I'm not really clear on what you're looking for. If you just want to subsample a matrix on matlab, here is a way to do it:

myData;                      % 70 x 1000000  data
nbDataPts = size(myData, 2); % Get the number of points in the data

subsampleRatio = 0.1;        % Ratio of data you want to keep
nbSamples = round(subsampleRatio * nbDataPts);  % How many points to keep
sampleIdx = round(linspace(1, nbDataPts, nbSamples)); % Evenly space indices of the points to keep

sampledData = myData(:, sampleIdx);  % Sampling data

Then if you want to apply PCA and K means I suggest you take a look at the relevant documentation:

• PCA
• K means

Try to work with it, and open a new question if a specific problem arises.