Water Filtration Problem for a class

I need to find the minimum number of stages that are needed to remove 99% of the initial impurity level of water.

I have a FOR loop set up to due the filtration like so:

for n = 1:21
    n_stage(n) = n-1:
    y(n) = (1/3)^(n-1);
    if (y(n) <= 0.01)                 Here's where my issues start.
        n_1p(n) = n_stage(n);
        y_1p(n) = y(n);
    end
end
fprintf('Minimum number of stage for 99%% filtration = %d, impurity level = %f\n',n_1p,y_1p);

#1: I'm getting two many answers because I'm not sure how to get the IF statement to stop.

#2: When it prints, it doesn't print the filtration stage (n_1p) nor the impurity level (y_1p) as would expect.

My programming skills are kinda lackluster. This is for a class I'm taking to help with said skills. Any help would be greatly appreciated.

[Troy Brown]

Follow up question: should I move the IF statement out of the FOR loop?

[Walter Roberson]

You should use

break

to exit a loop early.

[Guillaume]

or use a while loop.

[Joel Handy]

In regards to your second question, what doesnt print as expected? Is it more than the fact that you have multiple n_1p and y_1 values?

In regards to the first, since this is homework specifically to work on improving coding skills, I dont want to just give you the answer (and I won't mention that you can do this without a loop at all in 3 lines of code). However, if computing all values is actually important, I will point out two things.

1) there is no reason your loop can't count down, i.e. for n = 21:1

2) You don't need to save more than one n_1p or y_1 (i.e. you don't need to index into them). You just want the values associated with the smallest value of n that meets conditions. So why not just overwrite them each time you find a better answer?

If you just need to find the desired values of n_1p and y_1 and dont care about computing all the other values, then yeah just use a break statement or a while loop instead of a for loop as suggested.

[Walter Roberson]

Note: the syntax for counting down would be 21:-1:1

[Troy Brown]

Thanks for the help folks. In the end, one of you said something that sparked some recollection back to the lecture notes. I ended up using "find".

ind = find(y <= 0.01);

n_1p = ind(1,1)-1;

I then altered my "fprintf" line a bit and everything worked like a charm.

Thanks again!!