Recursion in Array SAS - arrays

Question

I have an existing collection of variables a_0,...,a_45 where a_i represents the amount of stuff I have on day i. I'd like to create a new collection of variables b_0,...,b_45 to represent the incremental change in stuff I have on day i (i.e. b_k=a_k-a_(k-1) ). My approach:
data test;
set dataset;
array a a_0-a_45;
array b b_0-b_45;
b(1)=a(1);
do i=2 to 45;
b(i)=a(i)-a(i-1);
end;
run;
However my b variables just come out missing.

Answer 1

What initial values do you have for a_1 to a_45 before you start the loop? As you are not intialising them (except for a_0 ≡ a(1)), every b(i) term will be a difference of 2 a terms, of which at least one will be missing, unless these variables are populated in your input dataset.

Answer 2

Here is some sample code showing that the delta computation is correct when the variable names in the data set align with the variables named in the array statement in the data step.
Sample data
data have(keep=product_id note a_:);
do product_id = 1 to 100;
length note $15;
array amount a_0-a_45;
call missing(of amount(*));
if (ranuni(123) < 0.5) then do;
note = 'static deltas';
static_delta = ceil(5 * ranuni(123));
amount(1) = static_delta;
do inventory_day = 2 to dim(amount);
amount(inventory_day) = amount(inventory_day-1) + static_delta;
end;
end;
else do;
note = 'random deltas';
amount(1) = ceil(5 * ranuni(123));
do inventory_day = 2 to dim(amount);
amount(inventory_day) = max ( 0, amount(inventory_day-1) + floor(10 * ranuni(123)) - 5 );
end;
end;
OUTPUT;
end;
run;
Compute deltas
data want;
set have;
array amount a_0-a_45;
array delta b_0-b_45;
delta(1) = amount(1);
do i=2 to dim(amount);
delta(i) = amount(i) - amount(i-1);
end;
drop i;
format a_: b_: 4.;
run;

Answer 3

As Richard has already suggested in his comment while I was working on writing the code...Basically the only error that you have in your code is that your code should loop from 2 to 46 because there are 46 elements in the array. below code should work for you.
%macro f();
data dataset;
%do i = 0 %to 45;
a_&i. = ranuni(2);
%end;
run;
%mend;
%f();
data test;
set dataset;
array a1 a_0-a_45;
array b1 b_0-b_45;
/* This line will help in avoiding b_0 to have a missing value */
b1(1)=a1(1);
do i=2 to 46;
b1(i)=a1(i)-a1(i-1);
end;
run;