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Why is There an Expression System?
Figure 238: Expression List
Window: Accessing Array
Elements
Looking up array data is slightly more complicated. For example, if the program
wants my_var[9]—this chapter will most often use C and C++ notation
rather than Fortran—it looks up the array’s starting address, then
applies an offset to locate the array’s 10 element. In this case, if each array
element uses 32 bits, my_var[9] is located 9 times 32 bits away.
In a similar fashion, your program obtains information about variables
stored in structures and arrays of structures.
Structures complicate matters slightly. For example ptr->my_var requires
three operations: extract the data contained within address of the my_var
variable, use this information to access the data at the address being
pointed to, then display the data according to the variable’s datatype.
Accessing an array element such as my_var[9] where the array index is an
integer constant is rare in most programs. In most cases, your program
uses variables or expressions as array indices; for example, my_var[cntr] or
my_var[cntr+3]. In the later case, TotalView must determine the value of
cntr+3 before it can access an array element.
Using variables and expressions as array indices are common. However, the
array index can be (and often is) an integer returned by a function. For
example:
my_var[access_func(first_var, second_var)+2]
In this example, a function with two arguments returns a value. That
returned value is incremented by two, and the resulting value becomes the
array index. Here is an illustration showing TotalView accessing the my_var
array in the four ways discussed in this section:
In Fortran and C, access to data is usually through variables with some sort
of simple evaluation or a function. Access to variable information can be
the same in C++ as it is in these languages. However, accessing private
variables within a class almost always uses a method. For example:
myDataStructureList.get_current_place()
TotalView built-in expression evaluation system is able to understand your
class inheritance structure in addition to following C++ rules for method
382 Chapter 17: Evaluating Expressions