array values are overwritten

Hello,

I am working with for loop in my code. I am getting the answer but after 8 steps i.e. from the step 9, the value of array is overwritten.

means The value I was getting at column 2 in the first 8 steps , I am getting that value in the column 5 when i start with step 9.

please help me for the same.

[KALYAN ACHARJYA]

Please share the code, if possible?

[Nicolas B.]

Do you have any example code to illustrate your problem?

[swati mane]

imgFiles=dir('*.jpg');
numFiles=length(imgFiles);
valueOfA=zeros(1,numFiles);
for k=1:1:numFiles
n=k+1;
if n<=numFiles
img=imread(imgFiles(k).name);
compareimg=imread(imgFiles(n).name);
P=imabsdiff(img,compareimg);
figure
imshow(P,[]);
N=nnz(P);
valueOfA(k)=N;
else
end
end

This is my code. When I read 8 images I am getting right answers. But when I work with images more than 8 the array values are overwritten.

means The value I was getting at column 2 in the first 8 images , I am getting that value in the column 5 when i start with image 9.

[madhan ravi]

Swati learn to format your code properly by selecting the code and pressing the code button.

[Jan]

@swati mane: Please post explicitly, which variables are affected. Column 2 of what?

A simplification of your code:

imgFiles = dir('*.jpg');
numFiles = numel(imgFiles);  % NUMEL is safer than LENGTH
valueOfA = zeros(1, numFiles);
for k = 1:numFiles - 1
  img = imread(imgFiles(k).name);
  compareimg = imread(imgFiles(k + 1).name);
  P   = imabsdiff(img,compareimg);
  figure;
  imshow(P,[]);
  valueOfA(k) = nnz(P);
end

[swati mane]

Thank you for your help. But not getting the result.

my question is when i take 8 frames using this code i got correct answer. But when i include the frames from 9 to 12. When i observe column 1...2..3..The values in the columns of array A are changed.

As i am taking the difference of two frames.The difference should be constant although i am adding more and more frames in the folder.

[Guillaume]

There is no array A in your code. Perhaps you mean the vector ValueofA. ValueofA(k) is the count of different pixels between imgFiles(k) and imgFiles(k+1). I don't see why that number should be constant.

[Joel Handy]

It sounds like maybe you are getting the same or simiiar output, just the order of the values is changing.

Look at the order of imgFiles when you run with 8 images and when you run with 9 images. My guess is that the order of the file list that the call to dir gives you changes. What are the names of these image files?

[swati mane]

Sir ..thanks for your reply. I understood that it is a vector.

When i take difference of pixels of frame 1 and frame 2 ..i got some value. And i think that value should be constant. When i have 9 frames in my folder i have constant value of difference. But when i add frame 10 in my folder then the difference of frame 1 and frame 2 changed. Why it is so?

[swati mane]

@joel handy...the file names are 1..2...3..4..and so on

[Joel Handy]

Because the file list returned by "dir" is sorted more or less alphabetically (it does capital letters before lower case ones). If I have 9 images:Img1.jpg, Img2.jpg, Img3.jpg, etc, dir will return:

Img1.jpg, Img2.jpg, Img3.jpg, Img4.jpg, Img5.jpg, Img6.jpg, Img7.jpg, Img8.jpg, Img9.jpg

If I add an new file with name Img10.jpg, it will return:

Img1.jpg, Img10.jpg, Img2.jpg, Img3.jpg, Img4.jpg, Img5.jpg, Img6.jpg, Img7.jpg, Img8.jpg, Img9.jpg

[swati mane]

@joel handy ...now I got what happend with my code.

Thank you so much.

[swati mane]

If i want to take files 1 to 500 ..in the sequence ..instead of dir ..what should be used?

[Joel Handy]

One option is to rename your files. If you named them something like Img001, img002, .., Img498, Img499, Img500, dir would return your file names in the right order.

You might be able to sort your images by date. The file info that dir gives you has the files' datestamp. That can be unreliable though if people are copying images around or opening and closing them. Anything that would update the files metadata.

The most reliable but most complicated option would be to parse you file names to extract the file number and sort that way.

imgFiles=dir('*.jpg');
fileNames = {imgFiles.name}'
fileNumStr = regexp(fileNames, '[0-9]*', 'match');
fileNum = str2double([fileNumStr{:}]);
[~,sortIdx] = sort(fileNum);
imgFiles = imgFiles(sortIdx);

[swati mane]

@joel handy

Thanks a ton ...its working perfectly.

Thanks

[swati mane]

Hello sir..

Now i have frames named as 000 to 505 .

I want to take these frames as 000 to 025 then 026 to 050 then 051 to 075 and so on.

Per second i should have 25 frames. Total time is 20 seconds. Thanks in advance.

Answer 1

[Jan]

Prefer file names including leading zeros: image0001.jpg, image 0002.jpg etc. Then the numeical order equals the alphabetical order.

If you have the less useful naming scheme already, use https://www.mathworks.com/matlabcentral/fileexchange/47434-natural-order-filename-sort to get it in order.