Probability of a sample?

How can I find probability of a sample in a dataset?

%# load data
load carsmall
%# construct predicting attributes and target class
vars = {'MPG' 'Cylinders' 'Horsepower' 'Model_Year'};
x = [MPG Cylinders Horsepower Model_Year];
y = cellstr(Origin);
%# train classification decision tree
t = classregtree(x, y, 'method','classification', 'names',vars, ...
                'categorical', [2 4], 'prune','off');

After this I want to find the probability of only one sample e.g. sample number 35?

[Walter Roberson]

Probability? As in "What is the probability that the 35'th die rolled is a '4'?" ?

When you have a finite set of occurrences, then one would speak about "frequency" rather than "probability". "Probability" requires an assumption of an infinite number of occurrences (or at the very least, as the asymptope as the number of occurrences approaches infinity.)