How to use a user input to open a file and use values within that file?

I have a number of files that contain similar information. I am trying to write a program to open each file on user input and do its thing. But I am struggling to code it as a complete newbie;

line=input('Which DASI line do you want to organise?(Type 1 for Line 1) ','s'); navfile=strcat('D',line,'_positions'); load (strcat(navfile,'.txt'))

e.g load D1_positions.txt

I want to be able to do this: T=[D1_positions(:,3),D1_positions(:,2),D1_positions(:,1),D1_positions(:,4),D1_positions(:,5),D1_positions(:,6)];

The value D1_positions need to change depending on the user input.

How do I do this?

[per isakson]

If D1_positions.txt contains at least six columns of of numbers (and no letters) your code should work. What's the problem?