removing non prime number function

I am trying to create a function that removes nonprime numbers from a file. My file is

integersB.txt:

11
2
3
709
303
288
15

1625
987

and here is my function:

function rid_of_nonprimes
fid = fopen('integersB.txt', 'r');
wtf = [];
if fid == -1
    disp('File not opened successfully')
else
    numgroup = textscan(fid, '%d');
    numgroup = cell2mat(numgroup);
    for i = 1:length(numgroup)
        if isprime(numgroup(i))
            wtf = [wtf;numgroup(i)]; 
        end
    end
end
    closer = fclose(fid);
    if closer == 0
        disp('File successfully closed')
    else
        disp('File NOT closed')
    end
 fid2 = fopen('integersB.txt', 'w');
    for j = 1:length(wtf)
        fprintf(fid2, '%d\n', wtf(j));
    end
    closer2 = fclose(fid2);
    if closer2 == 0
        disp('File successfully closed')
    else
        disp('File NOT closed')
    end 
end 

When I go to the command window:

>> fid = fopen('integersB.txt', 'r')
fid =
    11
>> fid2 = fopen('integersB.txt', 'w')
fid2 =
    12
>> rid_of_nonprimes
File successfully closed
File successfully closed

When I run the function in the command window, all I get is 2 "file successfully closed."

Answer 1

[Bhaskar R]

In short your function, assuming you have the data format as you mentioned

data = textread(' integersB.txt', '%d');
prime_numbers = data(isprime(data));

Answer 2

[Stephen23]

str = fileread('temp.txt');
vec = str2double(regexp(str,'\d+','match'))
[fid,msg] = fopen('output.txt','wt');
assert(fid>=3,msg)
fprintf(fid,'%d\n',vec(isprime(vec)));
fclose(fid);