Principal component analysis pca

HI ALL, i need to help me when i run this code of pca i find this error please help me its urgent Thanks.

this is my error:

Undefined variable data.

Error in new_pca (line 35)

Training ( size ( Training ,1) +1 ,:)= data( counter ,:) ;

clc
clear all
% The total number of samples in each class is denoted by NSamples
NSamples =3;
 % The number of classes
NClasses =30;
counter =1;
% Read the data
S=[];% S will store all the images
figure(1);
for i=1:NClasses
str = strcat('D:\eigenfaces\s\', int2str(i));
str = strcat(str, '.jpg');
eval('I=imread(str);');
I = rgb2gray(I);
I = imresize(I, [50,50]);
[m n]=size(I);
subplot(ceil(sqrt(NClasses)),ceil(sqrt(NClasses)),i)
%imshow(I)
if j==NSamples
title('TrainingDatasets','fontsize',18)
 % Each image is represented by one column
          data( counter ,:)=I (:) ;
          % Y is the class label matrix
          Y( counter ,1)=NClasses;
          counter = counter +1;
end
drawnow;
end
NTrainingSamples =2;
counter =1;
Training =[]; Testing =[]; TrLabels =[]; TestLabels =[];
for i=1: NClasses
    for j=1: NTrainingSamples
        Training ( size ( Training ,1) +1 ,:)= data( counter ,:) ;
        TrLabels ( size ( TrLabels ,1) +1 ,1)= Y( counter ,1) ;
        counter = counter +1;
    end
        for j= NTrainingSamples +1: NSamples
            Testing ( size ( Testing ,1) +1 ,:)= data( counter ,:);
            TestLabels ( size ( TestLabels ,1) +1 ,:)= Y( counter ,1);
            counter = counter +1;
        end
end
clear data Y I;
tic

Answer 1

[sarine_nassima]

i have error in tow lines if any one correcte me probleme please .thank you

% Each image is represented by one column

data( counter ,:)=I (:) ;

% Y is the class label matrix

Y( counter ,1)=NClasses;

Answer 2

[Walter Roberson]

if j==NSamples

You did not assign to j by that point in the code, so it has its default value of sqrt(-1) which does not equal NSamples, so you never assign to data