You can also read values from an array the same way you would read the value of a variable. To do so, include the

*array_name*[*index_expression*]

statement as an element in a mathematical expression. For example:
This example assumes that the array *reference point*, to be used in computing for

`foo`

was built using the construct in Example 3.14, “Associating Timestamps to Process Names” (from Section 3.5.1, “Assigning an Associated Value”). This sets a timestamp that will serve as a `delta`

.
The construct in Example 3.15, “Using Array Values in Simple Computations” computes a value for the variable *reading* the value of

`delta`

by subtracting the associated value of the key `tid()`

from the current `gettimeofday_s()`

. The construct does this by `tid()`

from the array. This particular construct is useful for determining the time between two events, such as the start and completion of a read operation.
**Note**

If the

*index_expression*

cannot find the unique key, it returns a value of 0 (for numerical operations, such as Example 3.15, “Using Array Values in Simple Computations”) or a null/empty string value (for string operations) by default.