I suspect you want to use the dir command to get a list of files in the directory, remove ".json" from the end of the file names, then search that list for your target user IDs.
Something like this perhaps:
userIDs = readmatrix('userids.txt');
jsonFiles = dir('*.json');
jsonFileNames = string({jsonFiles.name});
jsonFileUserIDs = extractBefore({jsonFiles.name},".json");
fileExistsForUserID = ismember(string(userIDs), jsonFileUserIDs);
filesWithUserIDInList = ismember(jsonFileUserIDs, string(userIDs));
jsonFilesFromList = jsonFileNames(filesWithUserIDInList);
for f = 1:numel(jsonFilesFromList)
fileName = jsonFilesFromList(f);
data = readstruct(fileName);
% Do whatever you want with the data here.
end
