Calling multiple outputs from a function in a loop

I am trying to call multiple outputs from a function in a for loop. The code I tried initially is below

for i =1:tsteps
[tnewfin1 (:,i+1),crfin1(1,i)] = newtstep (tnewfin1 (:,i));
end

However I constantly received the error "Invalid use of operator".

I changed it to the following,

for i =1:tsteps
[tnewfin,crfin] = newtstep (tnewfin1 (:,i));
tnewfin1 (:,i+1) = tnewfin
crfin1(1,i) = crfin
end

which worked, despite not really changing what is happening.

I thought the first one would work because initially, when I just wanted one output from the function, the following code ran succesfully

for i =1:tsteps
tnewfin1 (:,i+1) = newtstep (tnewfin1 (:,i));
end

Why would it work to call one output but not two, or why is the first scenario an "invalid use of operator"?

[madhan ravi]

please upload the function and the datas to run your code