110  Pascal User Manual  4. Variable Parameters.  Procedllre  MinMax  shows the case ofthe  variable parameter.  The actual parameter  must be a variable;  the corresponding formal pw'ameter must be preceded by the  symbol  var and becomes a ,ynonym for this actual variable  during the entire execution  If  the procedure. Any operation  involving the formal paran leter is then performed directly  upon  the  actual  parameter,  Use  variable  parameters  to  represent the  results  of a pn Icedure - as is the case for  Min  and  Max  in Program 11.2. J urthermore, if  Xl ... Xn  are the  actual  variables  that  com~pond  to  the  formal  variable  parameters  VI ... Vn,  then,  1 ... Xn  should  be  distinct  variables. All address calcuLltions are done at the time of the  procedure activation. Hence,  I f  a variable is a component of an  array, its index expression is ,'valuated when the procedure is  activated. Note that a compolllcnt of a packed structure or a tag  field in a variant record must not appear as an actual variable  parameter,  thus  avoiding  implementation  problems  for  calculating addresses.  When no symbol heads the paumeter section, the parameter(s) of  this section are said to be  value parameter(s).  In this case the actual  parameter  must be an expression  (of which a variable is a simple case).  The corresponding formal parameter represents a local variable in the  activated procedure. As its initial  value, this variable receives the  current value of the corresponding actual parameter (i.e., the value of  the expression at the time of the prlCedure activation). The procedure  may then change the value of this \ .lriable through an assignment; this  cannot, however, affect the value  ()f the actual parameter. Hence, a  value parameter can never represen. a result of a computation. Note that  file parameters or structured variahles with files as components may  not be specified as actual value par.  I  meters, as this would constitute an  assignment.  The difference in the effects  01  value and variable parameters is  shown in Program 11.3.  The following table summarize', the correct kinds of parameters for  formal and actual parameter lists. 