differencing a time series

I want to difference a time series till it becomes stationary.how to code it using matlab.i am writing my code below.please help me to continue it acf=autocorr of time series

l(k)=lower limit
u(k)=upper limit

if ((acf(k+1)<l(k)) || (acf(k+1)>u(k))),a time series is non-stationary.now i have to difference a time series till it becomes stationary.how to write using a single expression.

[dpb]

What's k supposed to represent--the k-th lag of the acf series or the acf after the k-th difference of x?

Maybe my utility function can help some w/ the complexity -- it's just syntactic sugar but by putting the logic expressions at a lower level it can often lead to seeing how to reduce expressions...

while ~all(iswithin(acf,l,u))
  ... do another difference here
end

where acf is presumed to be the acf at the kth iteration

iswithin is

function flg=iswithin(x,lo,hi)
% returns T for values within range of input
% SYNTAX:
%  [log] = iswithin(x,lo,hi)
%      returns T for x between lo and hi values, inclusive

flg= (x>=lo) & (x<=hi);

[Chithralekha]

k is the kth lag of time series.

Answer 1

[dpb]

OK, presuming you have a vector of given length and the limits of the same, then

(acf(k+1)<l(k)) || (acf(k+1)>u(k)))

is

acf(2:end)<l || acf(2:end)>u

if do want the short-circuiting operator here.