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### ⁠3.5.2. Reading Values From Arrays

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:
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Example 3.15. Using Array Values in Simple Computations

    delta = gettimeofday_s() - foo[tid()]

This example assumes that the array `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 reference point, to be used in computing for `delta`.
The construct in Example 3.15, “Using Array Values in Simple Computations” computes a value for the variable `delta` by subtracting the associated value of the key `tid()` from the current `gettimeofday_s()`. The construct does this by reading the value of `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.

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