How can I get Octave to change the variables in my input files?

Question

I am trying to get Octave to change variables in my input files like it would if I were using the command line in Ubuntu. I've stripped this back to a simple case where I get it to change '1111' to the string Naer_str and '2222' to the string sigma_str; printing the result to a new file. 1111 and 2222 are both present in the first file. Here is the entire code I am running below.

 Naer_str= num2str(1000)

 disp(Naer_str)

 sigma_str = num2str(0.491)

 eval(['system(''sed -e "s~1111~${Naer_str}~; s~2222~${sigma_str}~;" OctaveChangeVarTestFile.IN > OctaveChangeVarTestFile_out.IN'');']);

The new input file is made, but rather than seeing the values 1000 and 0.491 as I anticipated the places where 0.491 and 1000 should be are blank. Running it in debug mode showed the follow error message (which I've seen a lot since I started to use Octave a few days ago, but I still don't really grasp what it means).

 error: invalid use of script in index expression

I have tried just inputting stuff into the Linux terminal like so:

 Naer_str=1000
sigma_Str=0.491
 sed -e "s/1111/${Naer_str}/; s/2222/${sigma_str}/;" OctaveChangeVarTestFile.IN > OctaveChangeVarTestFile_out.IN

This appears to work fine, so I assume that I must have made a grammar error either using system or eval, but I can't see it.

What is causing this error message? Why am I not seeing my anticipated output?

Answer 1

I don't know why specifically you're getting this error, but I'm 90% sure this won't do what you want:

eval(['system(''sed -e "s~1111~${Naer_str}~; s~2222~${sigma_str}~;" OctaveChangeVarTestFile.IN > OctaveChangeVarTestFile_out.IN'');']);

In bash, ${Naer_str} inside a double-quoted string is a variable substitution: it replaces the ${Naer_str} with the value of the Naer_str variable in the string, before passing it along to the command. So, sedwill get s/1111/1000/; as one of its commands.

But I can't find any reference to such a feature in octave's Variables, Manipulating Strings, Strings, or elsewhere. Those characters are just going to be treated as literal characters. So, sed will get s~1000~${Naer_str}~; as an argument, which means it's going to produce a garbage script. It looks like the way you do this kind of thing in octave is by using printf-style formatting (as described all over the Manipulating Strings page).

Answer 2

1. It looks like all you have is a system call with a computed command. You can pass a computation result as the parameter of system() already, you don't need, and should not use, eval() here.

2. The way to concatenate strings (character arrays) in octave is just horizontal matrix concatenation. So instead of 's~1111~${Naer_str}~' (which as abarnert points out, gets interpolated by the shell which has no concept of octave variables) you can write ['s~1111~' Naer_str] and octave will concatenate.

3. You don't need to reach outside octave to sed via system() for text replacement. strrep and regexprep can do replacement on character arrays right inside octave (or Matlab).