Error using imwrite with variable filename in for loop

I'm trying to make a fairly simple programme to convert a batch of images to grayscale and save them in my current folder.

Convert = fullfile('225C');
imds = imageDatastore(Convert);

for i = 1:149
    img = readimage(imds,i);
    img = rgb2gray(img);
    imwrite(img,[i,'.png']);
end

However, when I run this code I get the error 'Unable to open file ".png" for writing. You might not have write permission.'

Through other forumposts I was lead to believe this should be the correct syntax for a variable filename. I just want 149 files numbered 1-149. Any help appreciated.

[Stephen23]

Rather than concatenating strings, SPRINTF gives more control and makes the intent clear:

sprintf('%d.png',i)

Answer 1

[KSSV]

Replace the line:

imwrite(img,[i,'.png']);

with

imwrite(img,[num2str(i),'.png']);

[Zane Peterkovic]

Works perfectly, thank you!