This should be a start
sourcefolder = 'E:\input';
filepattern = fullfile(sourcefolder, '*.png');
% this shouldn't be in the loop
% the subdirectory needs to exist before you use it
outputfolder = './hist/img';
mkdir(outputfolder);
srcFiles = dir(filepattern);
numImages = length(srcFiles);
for k = 1 : numImages
% build full file path+name
% use sprintf and zero-padded numbers (this assumes 3 digits is enough)
inputfilename = fullfile(sourcefolder, srcFiles(k).name);
outputfilename = fullfile(outputfolder, sprintf('histo_%03d.png',k));
I = imread(inputfilename);
imhist(I);
saveas(gcf,outputfilename)
end
