If they just need to be indices, you can use ceil, floor, fix or round. eg
x = fix(x)
If you want to convert them to ACTUAL integer types, you can use:
x = uint32(x);
[EDIT] As the question changes...
As Walter pointed out, you are overwriting Z every loop. Perhaps you wanted Z indexed by i and j... In which case:
Z(i,j) = SPI(precip30y_model1(i,j),12,12);
But since you said that Z is empty after iteration, is it possible that SPI() can return an empty set instead of a scalar? In that case:
Z = nan(144,91);
for i=1:size(Z,1)
for j=1:size(Z,2)
s = SPI(precip30y_model1(i,j),12,12);
if ~isempty(s)
Z(i,j) = s;
end
end
end
That uses NaN to represent "no data".
[EDIT 2012-05-23]
Okay, from your comments I think you want this:
Z = nan(144,91);
for i=1:size(Z,1)
for j=1:size(Z,2)
Z(i,j) = SPI(squeeze(precip30y_model1(i,j,:)),12,12);
end
end
