Call variables in for loop

Hi to all,

L1=4 L2=3 ... L30=12

for i=1:30 P(:,i)=sprintf(L%1d,i) - sprintf(L%1d,i+1) end

The result should be P(1)=L1-L2, P(2)=L2-L3, P(3)=L3-L4, P(4)=L4-L5 but it's not :) What should I use instead sprintf?

Thank you for you answers!!!

Answer 1

[Kevin Holst]

To do this like you're attempting requires the use of the eval function, but be very careful in its use. Google 'eval matlab' for more information on why. And actually, the way you've got it set up wouldn't work. Your for loop needs to run from 1:29 because there's no L31.

for i = 1:29
  eval(['P(' num2str(i) ')=L' num2str(i) '-L' num2str(i+1) ';'])
end

Is there any reason why your L variables aren't in a single L vector?

[Thomas]

Yes, Be very careful on using the 'eval' command.. I almost got labelled a 'troll' for suggesting it on the forum :)..

http://www.mathworks.com/support/tech-notes/1100/1103.html

[Kevin Holst]

Haha, I've suggested it a couple of times now. I always try to point out that it's dangerous, but it's not like eval = Voldemort (or the dark side, pick your metaphor).

[Oleg Komarov]

Eval:

- is not necessary in 99.99% of the cases.

- it is not a feasible solution to scale the dimensionality of your project.

- it is obscure

- it can be dangerous if used maliciously

I dare you execute:

eval(eval(eval('char([102 108 105 112 108 114 40 39 116 105 120 101 39 41])')))

- it can generate errors hard to debug (thus slowing your progress significanlty).

An example: say you have 10 filters and you use

eval([sprintf('filter%d',1:10),'=rand(10)'])

Now, while filter1 is not a matlab function filter2 is.

Suppose you need to use filter2 and try to call it, while having a variable in the editor with the same name...

Although sounds trivial to solve, when your project grows rapidly and you keep generating variables like crazy you don't even see them in your workspace anymore. This nightmare took away a week of debugging (true story).

[Kevin Holst]

I couldn't agree more, hence the question about why L1,2,3,4 isn't just a vector. But when it's the only option, you need to be able to use it properly and recognize possible pitfalls, like you filter example.

[Oleg Komarov]

@Kevin have you tried eval(eval(eval... @trollface :)

I only use evalin (not eval) when I have to swap variables to the 'base' workspace.

I only used to use eval when I needed to convert '2' to 2 (str2num), but then came str2double.

[Kevin Holst]

@Oleg, I haven't tried the eval(eval(eval(... suggestion, but I did evaluate char([102 108 105 112 108 114 40 39 116 105 120 101 39 41]) in my workspace... very sneaky! ;) That's the nice way to show a problem with eval. The mean way would be to "accidentally" have someone recursively remove all of the contents of their hard drive. That'd be a bad day.

[Oleg Komarov]

Should have concealed it better...