how to overwrite a certain value in input file(inp)

I would like to replace the value of roughness of a certain pipe in inp. file. I tried the following but it didn't work.

replace=[PIPES];ID1_roughness;
myformat='%5d \n';
newData=[160];
fid=fopen('Y:\MOD1.inp','r+');
for k=1:(replace-1);
fget1(fid);
end

second question: the command replaceLine how do I combine line and row? for instance the 126th Line and the third row.

Thanks in advance. Wei

[Wei Tai]

This want I want to do: change the roughness of a certain pipe(Number 153). How do i write a command to do that?

Answer 1

[dpb]

The short answer is "you can't" -- sequential files are, well, "sequential"; whatever you write other than appending to the end of an existing file truncates the file at that point and rewrites from that point on.

To do what you want, simply read the file in its entirety; make the modification in the location in memory that you wish and write it back out.

Alternatively, you can use the " fgetl in a loop" strategy but you'll have to have a subsequent second file that copies the input from the first to the second going along with it.

I can find no reference to any such function as replaceLine using TMW doc search.

[Walter Roberson]

No, if you write when you are not positioned at the end of a file (and it is not set up to automatically convert the write to an append) then you overwrite whatever is there but the file does not get any longer or shorter. If you overwrite with exactly the same length of data then it works. But you cannot replace by line, only byte-for-byte.

[dpb]

Strictly speaking, yes, Walter, but in general practice one who has to ask the question will end up closing the file and the OS will write the EOF...