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A constant value is defined once and can be referenced many times throughout a program. Using a constant instead of specifying the same value multiple times can simplify code maintenance (as in don't repeat yourself) and can be self documenting by supplying a meaningful name for a value, for instance, PI instead of 3.1415926.
While a constant does not change its value while the program is running, an object declared const may indeed change its value while the program is running. A common example are read only registers within embedded systems like the current state of a digital input. The data registers for digital inputs are often declared as const and volatile.
For example, the constant π may be defined as the ratio of the length of a circle's circumference to its diameter. The following list includes a decimal expansion and set containing each number, ordered by year of discovery. The column headings may be clicked to sort the table alphabetically, by decimal value, or by set.
In computer programming, a magic number is any of the following: A unique value with unexplained meaning or multiple occurrences which could (preferably) be replaced with a named constant; A constant numerical or text value used to identify a file format or protocol (for files, see List of file signatures)
An example of a constant function is y(x) = 4, because the value of y(x) is 4 regardless of the input value x. As a real-valued function of a real-valued argument, a constant function has the general form y(x) = c or just y = c. For example, the function y(x) = 4 is the specific constant function where the output value is c = 4.
A sample program is provided below to get an idea of what a SIC program might look like. In the code below, there are three columns. The first column represents a forwarded symbol that will store its location in memory. The second column denotes either a SIC instruction (opcode) or a constant value (BYTE or WORD).
The constant e also has applications to probability theory, where it arises in a way not obviously related to exponential growth. As an example, suppose that a slot machine with a one in n probability of winning is played n times, then for large n (e.g., one million), the probability that nothing will be won will tend to 1/e as n tends to infinity.
Constant folding is the process of recognizing and evaluating constant expressions at compile time rather than computing them at runtime. Terms in constant expressions are typically simple literals, such as the integer literal 2, but they may also be variables whose values are known at compile time.