5.10 Declaring multiple variables at once
Stan will interpret multiple comma-separated variable names following a single type as declaring multiple new variables. This is available for all variable declarations in all blocks.
Types for multiple declarations
real x, y;
is equivalent to
real x; real y;
As a result, all declarations on the same line must be of the same type.
Combining with other features
The ability to declare multiple variables can be combined with assignments whenever a declare-define is valid, as documented in the section introducing compound declarations and definitions :
real x = 3, y = 5.6;
Constrained data types can also be declared together, so long as the constraint for each variable is the same:
real<lower=0> x, y;