create data step variables using a dynamic macro-variable

You are misusing the macro system.

The ampersand (&) introducer in source code tells SAS to resolve the following symbol and place it into the code submission stream. Thus, the resolved &MOB1_acct. can not be changed in the running DATA Step. In other words, a running step can not change it's source code -- The resolved macro variable will be the same for all implicit iterations of the step because its value became part of the source code of the step.

You can use SYMPUT() and SYMGET() functions to move strings out of and into a DATA Step. But that is still the wrong approach for your problem.

The most straight forward technique could be

• use of a retained variable
• mod (_n_, 36) computation to determine every 36^th row. (_n_ is a proxy for row number in a simple step with a single SET.)

Example:

data a;
  set a;

  retain mob1_accounts;

  * every 36 rows change the value, otherwise the value is retained;
  if mod(_n_,36) = 1 then mob1_accounts = accounts;
run;

You didn't show any data, so the actual program statements you need might be slightly different.

Contrasting SYMPUT/SYMGET with RETAIN

As stated, SYMPUT/SYMGET is a possible way to retain values by off storing them in the macro symbol table. There is a penalty though. The SYM* requires a function call and whatever machinations/blackbox goings on are happening to store/retrieve a symbol value, and possibly additional conversions between character and numeric.

Example:

1,000,000 rows read. DATA _null_ steps to avoid writing overhead as part of contrast.

data have;
  do rownum = 1 to 1e6;
    mob + 1;
    accounts = sum(accounts, rand('integer', 1,50) - 10);
    if mob > 36 then mob = 1;
    output;
  end;
run;

data _null_;
  set have;

  if mob = 1 then call symput ('mob1_accounts', cats(accounts));

  mob1_accounts = symgetn('mob1_accounts');
run;

data _null_;
  set have;
  retain mob1_accounts;

  if mob = 1 then mob1_accounts = accounts;
run;

On my system logs

142  data _null_;
143    set have;
144
145    if mob = 1 then call symput ('mob1_accounts', cats(accounts));
146
147    mob1_accounts = symgetn('mob1_accounts');
148  run;

NOTE: There were 1000000 observations read from the data set WORK.HAVE.
NOTE: DATA statement used (Total process time):
      real time           0.34 seconds
      cpu time            0.34 seconds


149
150  data _null_;
151    set have;
152    retain mob1_accounts;
153
154    if mob = 1 then mob1_accounts = accounts;
155  run;

NOTE: There were 1000000 observations read from the data set WORK.HAVE.
NOTE: DATA statement used (Total process time):
      real time           0.04 seconds
      cpu time            0.03 seconds

Or

   way          real   cpu
------------- ------  ----
SYMPUT/SYMGET   0.34  0.34
    RETAIN      0.04  0.03