Question

I have a vector containing a time series with different values and some missing values inbetween that are set to zero:

X=[0,0,2,0,5,0,0,0,4,0];

I want to create a new vector where the missing values (zeros) are populated by the previous value if one exist so that I get a new vector looking like:

Z=[0,0,2,2,5,5,5,5,4,4];

I have been browsing through the Matlab help and forums like this to find a neat and suitable function that would solve this for me with a one line solution or similar, but I have failed to do so. I can solve the problem through a few different steps according to below but I am guessing that there must be a better and easier solution available?

Current solution:

X=[0,0,2,0,5,0,0,0,4,0];
ix=logical(X);
Y = X(ix);
ixc=cumsum(ix);
Z=[zeros(1,sum(~logical(ixc))) Y(ixc(logical(ixc)))];

This does the trick, but it seems like an overly complicated solution to a simple problem, so can anyone help me with a better one? Thanks.

Answer 1

Here’s a somewhat simpler version using cumsum:

X=[0,0,2,0,5,0,0,0,4,0];

%# find the entries where X is different from zero
id = find(X); 

%# If we want to run cumsum on X directly, we'd 
%# have the problem that the non-zero entry to the left
%# be added to subsequent non-zero entries. Thus, 
%# subtract the non-zero entries from their neighbor 
%# to the right 
X(id(2:end)) = X(id(2:end)) - X(id(1:end-1));

%# run cumsum to fill in values from the left
Y = cumsum(X)

Y =
     0     0     2     2     5     5     5     5     4     4