CHAPTER 4  The Concept of Action  4.A.  The Assignment Statement and Expressions  28  Essential to a computer program is action. That is, a program must do  something with its data - even if that action is the choice of doing  nothing!  Statements  describe these  actions.  Statements  are  either  simple  (e.g., the assignment statement) or  structured.  See the syntax  diagram for  Statement  (Figure 4.a).  The most fundamental of statements is the  assignment statement.  It  specifies that a newly computed value, specified by an expression, be  assigned to a variable. Assignment statements have the form shown in  Figure 4.b. The  :  =  symbol denotes  assignment  and is  not to be  confused with the relational operator  =. The statement "A : = 5" is read  "the current value of  A  is replaced with the value 5," or simply, "A  becomes 5."  An  expression  consists of operators and operands. An operand may  be a constant, variable, array-parameter bound (discussed in Chapter  A  variable  (see Figure 4.c) may be an  entire variable  representing  all the data storage for a simple, structured, or pointer type.  In  the case  of structured types (see Chapter:; 6 through 9), a variable may be a  component variable  or a  buffer variable  representing one component of  the data storage. For pointer types, a  variable may be an  identified  variable  representing data  storag\~  indirectly referenced by a pointer. 