Check how many observations (rows) of a dataset verify a given condition.

%obs_count(dsn, _ans_=, where=, pct=yes, distinct=no, lib=WORK);

dsn : a dataset, for which the condition has to be verified;
where : (option) SAS expression used to further refine the selection (WHERE option); should be passed with %str; default: empty;
pct : (option) a boolean flag (yes/no) set to return the result as a percentage of the total observations in dsn that verify the condition cond above; default: pct=yes, i.e. result is returned as a percentage [0,100] of the total numbers of observations;
distinct : (option) boolean flag (yes/no) set to count only distinct values; in practice, runs a SQL SELECT DISTINCT process instead of a simple SELECT; default: no, i.e. all values are counted;
lib : (option) the library in which the dataset dsn is stored.

_ans_ : name of the macro variable used to store the (quantitative) output of the test, which is, depending on the value of the flag pct:

n, the number of observations that verify the condition cond when pct=yes;
100*n/N, where N is the total number of observations in the dataset dsn, and n is like above;

hence the nul result corresponds to the situation n=0. where no observation in the dataset verifies the input condition.

Let's perform some test on the values of test datatest #1000 (with 1000 observations sequentially enumerated), e.g.:

%_dstest1000;
%let ans=;
%let cond=%quote(i le 0);
%obs_count(_dstest1000, where=&cond, _ans_=ans);

returns ans=0, while:

%let cond=%quote(i gt 0);

%obs_count(_dstest1000, where=&cond, _ans_=ans);

returns ans=100, and:

%let cond=%quote(i lt 400);

%obs_count(_dstest1000, where=&cond, _ans_=ans);

returns ans=40.

Run %_example_obs_count for more examples.

For very large tables, the accuracy of the result returned when pct=yes is relative to the precision of your machine. In practice, for tables with more than 1E9 observations where all but 1 verify the condition cond, the percentage calculated may still be equal to 100 (instead of a value<100). In that case, it is preferred to set the flag pct to no (see %_example_obs_count).
Note in general the use of %str (or %quote) so as to express a condition.