Error opening a text file using fopen

I have this for loop running to read a series of text files

STA_NAME = CODE2(j);
    filename = [STA_NAME day_num '-' date_str1 '.Cmn'];
    Cmn_File = [Cmn_Dir '\' filename];
    fid  = fopen(Cmn_File);

CODE2(j) is fine. Cmn_Dir is also fine. But why it is showing an error in fopen.

Error using fopen, First input must be a file name of type char, or a file identifier of type double. ?? Can anyone solve this ?

[Stephen23]

It is better to use sprintf to generate the filename and fullfile to join the the path parts together:

    fnm = sprintf('%s%s-%s.Cnm', STA_NAME, day_num, date_str1);
    pth = fullfile(Cmn_Dir, fnm);
    fid = fopen(pth);

This will also generate clearer warnings when some parts are not compatible types.

[Add]

I tried your suggestion but then it says

Error using sprintf
Function is not defined for 'cell' inputs.

Answer 1

[Star Strider]

‘Can anyone solve this ?’

Please post the string that is assigned to ‘Cmn_File’. (Remove the semicolon from the end of that line, and copy the result from the Command Window and paste it here.) We have no idea what the problem may be without seeing it.

[Add]

The string assigned to Cmn_File is as follows

Cmn_File = [Cmn_Dir '\' filename];

And the result from the command window is as below

'D:\Adarsh\PhD\Da...'    '\'    'hyde'    '274'    '-'    '2013-10-01'    '.Cmn'

[Walter Roberson]

One or more of your components for Cmn_Dir or filename is a cell array of strings instead of being a plain string. The result of the [] then becomes a cell array of strings, and fopen() cannot process that.

While you are cleaning up the code, I recommend you switch to using fullfile()