Separate matrix of column vectors into training and testing data in MATLAB

Question

I'm currently doing a project in MATLAB using the MNIST data set. I have a training data set of n = 50000, represented by a matrix of 784 x 50000 (50000 column vectors of size 784).

I am trying to separate my training and testing data (70-30, respectively), but the method I am using is a bit wordy and brute force for my liking. Being that this is MATLAB, I'm sure there has got to be a better way. The code I have been using is listed below. I'm brand new to MATLAB so please help! Thanks :)

% MNIST  - data loads into trn and trnAns, representing
% the input vectors and the desired output vectors, respectively
load('Data/mnistTrn.mat');

mnist_train = zeros(784, 35000);
mnist_train_ans = zeros(10, 35000);

mnist_test = zeros(784, 15000);
mnist_test_ans = zeros(10, 15000);

indexes = zeros(1,50000);
for i = 1:50000
    indexes(i) = i;
end
indexes(randperm(length(indexes)));

for i = 1:50000
  if i <= 35000
      mnist_train (:,i) = trn(:,indexes(i));
      mnist_train_ans(:,i) = trnAns(:,indexes(i));
    else
      mnist_test(:,i-35000) = trn(:,indexes(i));
      mnist_test_ans(:,i-35000) = trnAns(:,indexes(i));
    end
end

Answer 1

I hope this works:

% MNIST  - data loads into trn and trnAns, representing
% the input vectors and the desired output vectors, respectively
load('Data/mnistTrn.mat');

% Generating a random permutation for both trn and trnAns:
perm = randperm(50000);

% Shuffling both trn and trnAns columns using a single random permutations: 
trn  = trn(:, perm);
trnAns = trnAns(:, perm);


mnist_train = trn(:, 1:35000);
mnist_train_ans = trnAns(:, 1:35000);

mnist_test = trn(:, 35001:50000);
mnist_test_ans = trnAns(:, 35001:50000);